O Vernet-Garnier

Delphine Bajolet

Lebrun

Sadr

Gautier Julien

Maud Francois

Véronique Vernet-Garnier

Odile Bajolet

Delphine Lebrun

Maxime Hentzien

Xavier Ohl

Saidou Diallo

Firouzé Bani- Sadr
email: fbanisadr@chu-reims.fr

Prof Firouzé Bani-Sadr

therapy in diabetic foot patients with total resection of osteomyelitis

Keywords: diabetic foot osteomyelitis, diabetes, osteomyelitis, antibiotic duration

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Professional societies for infectious diseases in America and France recommend a short duration of post-amputation antibiotic therapy (2-5 days) in patients with diabetic foot osteomyelitis after total resection of infected bone tissue [START_REF] Lipsky | Infectious Diseases Society of America Clinical Practice Guideline for the Diagnosis and Treatment of Diabetic Foot Infections[END_REF][START_REF]Société de Pathologie Infectieuse de Langue Française. [Management of diabetic foot infections. Long text[END_REF]. In case of residual infected bone, they recommend prolongation of antibiotic therapy (4-6 weeks) [START_REF] Lipsky | Infectious Diseases Society of America Clinical Practice Guideline for the Diagnosis and Treatment of Diabetic Foot Infections[END_REF][START_REF]Société de Pathologie Infectieuse de Langue Française. [Management of diabetic foot infections. Long text[END_REF]. However, these recommendations have a low level of evidence, because of the limited number of clinical studies [START_REF] Lipsky | Infectious Diseases Society of America Clinical Practice Guideline for the Diagnosis and Treatment of Diabetic Foot Infections[END_REF][START_REF]Société de Pathologie Infectieuse de Langue Française. [Management of diabetic foot infections. Long text[END_REF]. In addition, how residual osteomyelitis should be evaluated is not defined.

We conducted a prospective study to evaluate the long-term effectiveness of short-duration post-amputation antibiotic therapy in diabetic patients with total resection of osteomyelitis, as assessed by sterile bone bacteriological samples obtained from the resection margin.

METHODS

We performed a prospective, observational study between June 2015 and May 2017 at the university hospital of Reims, France. Inclusion criteria were adults (>18 years of age) with type 1 or type 2 diabetes who underwent surgical amputation for foot osteomyelitis. Bone biopsies in a "macroscopically" healthy zone had to be performed and bacterial cultures had to be sterile on the fifth day. Foot osteomyelitis was determined by positive probe to bone test and imaging data on plain X-rays and/or bone scans.

All patients' observations were discussed at a multidisciplinary meeting on bone and joint infection at Reims university hospital and the indication for both surgical resection and antibiotic therapy was jointly decided.

The endpoint was the absence of osteomyelitis relapse at 6 months, defined as recurrence of osteomyelitis with the need for surgical revision and/or new bone antibiotic therapy.

Patients were systematically seen in consultation at 1, 3, and 6 months postoperatively, with clinical reassessment of the foot, and x-ray. All events occurring during follow-up, particularly infections, were recorded.

Demographic variables, and data on all diabetes-related disorders were collected, including duration of diabetes, presence of hypertension, chronic renal failure, occlusive arterial disease of the lower limb, and peripheral neuropathy. Doppler ultrasound explorations with transcutaneous measurement of oxygen pressure (TcPO2) were performed in case of clinical abnormalities. Revascularization procedures were performed before amputation if necessary.

Microbiological data was also recorded.

Qualitative variables are described as number (percentage) and quantitative variables as mean ± standard deviation.

RESULTS

Between June 2015 and May 2017, a total of 50 patients with type 1 or type 2 diabetes underwent surgical amputation for foot osteomyelitis in our hospital. Among these, a total of 15 patients (30%) with sterile biopsies in macroscopically healthy zone were included. The characteristics of the study population are reported in Table 1. Mean age was 68±11 years.

Most were men (73%) and had type 2 diabetes (93%). The mean duration of diabetes was 14.9±9.8 years. Comorbidities included hypertension (60%), chronic renal failure with glomerular filtration rate between 30 and 59 mL/min/1.73m² (40%), and peripheral neuropathy (80%). Occlusive arterial disease of the lower limb was present in eight patients (53%), two of whom had already been treated by surgery. Previous lower limb amputation was reported in eight patients (53%); for osteomyelitis in seven cases (47%) and for occlusive arterial disease in one case.

Osteomyelitis was located in the forefoot (metatarsal and/or toe involvement) in 13 cases (87%), extended to the tarsal bones in one case and was located only in the calcaneus bone in one case. Vascular investigations were necessary in 11 patients (73%). Doppler ultrasound was performed in 10 patients (67%) and TcPO2 measurement in 10 patients (67%). Two patients (20%) had a pathological TcPO2 measurement (<35 mmHg), but only one patient required thromboendarterectomy and stenting prior to orthopedic surgery. Before amputation, antibiotic therapy was initiated for concomitant skin and soft infections in 12 patients (80%), averaging 9.7 ±5.4 days before surgery. Most patients (n=9) received amoxicillin-clavulanic acid. Two patients discontinued the antibiotic therapy on respectively Day 8 and 28 before surgery.

Nine patients (60%) required transmetatarsal resection and five patients (33%) minor resection (arthroplasty resection). One patient had major amputation (transtibial amputation).

The mean number of bone biopsies in macroscopically healthy zones was 1.6 ±0.83. On average, the duration of postoperative antibiotic therapy was 8.3 ±5.9 days.

A bacterial culture of resected bone was available in seven patients, which was positive in all cases. The median number of bacteria types was three (range 1-3). Among the eight other patients, the bacterial culture of deep tissue sample obtained before intervention was positive for Staphylococcus aureus in six cases, streptococci in two cases, Proteus mirabilis or vulgaris in three cases, and anaerobes in two cases. The median number of bacteria types was two (range 1-4).

Twelve patients (80%) did not experience recurrence of osteomyelitis during the six-month postoperative follow-up. Among the remaining three patients, one required antibiotic therapy to be resumed in the 3 rd month due to soft tissue infection, and subsequently required surgical revision at 6 months due to radiological osteomyelitis in relation to the adjacent bone tissues.

The second patient was successfully treated one month postoperatively with two weeks of antibiotic therapy alone for persisting purulent flow from the scar with positive microbiological samples for the same pathogens that caused the osteomyelitis. The third patient was lost to follow-up from the 3 rd month of follow-up, and outcome at six months was unknown, although it should be noted that up to one month of follow-up, no recurrence of osteomyelitis was observed.

DISCUSSION

To the best of our knowledge, this is the first prospective study aimed at evaluating the recommendations of a short duration of antibiotic therapy in patients with diabetic foot osteomyelitis and no residual infected bone after amputation. The absence of residual bone infection was defined as sterile bone biopsies (after a culture of five days) obtained from the resection margin. A high rate of residual osteomyelitis, ranging from 35% to 63% based on bone margin culture, has indeed been reported after amputation in the literature [START_REF] Kowalski | The Effect of Residual Osteomyelitis at the Resection Margin in Patients with Surgically Treated Diabetic Foot Infection[END_REF][START_REF] Atway | Rate of Residual Osteomyelitis after Partial Foot Amputation in Diabetic Patients: A Standardized Method for Evaluating Bone Margins with Intraoperative Culture[END_REF][START_REF] Vaznaisiene | Major amputation of lower extremity: Prognostic value of positive bone biopsy cultures[END_REF].

Residual osteomyelitis was also observed in 70% of cases over the study period. Furthermore, bacteriological culture of bone biopsy from resection margins, in patients with total surgical resection of infected bone tissue, has been shown to be predictive of osteomyelitis recurrence [START_REF] Kowalski | The Effect of Residual Osteomyelitis at the Resection Margin in Patients with Surgically Treated Diabetic Foot Infection[END_REF][START_REF] Atway | Rate of Residual Osteomyelitis after Partial Foot Amputation in Diabetic Patients: A Standardized Method for Evaluating Bone Margins with Intraoperative Culture[END_REF]. In a retrospective cohort of 111 patients with surgically treated diabetic foot osteomyelitis, 39 (35.1%) had pathologically confirmed positive margins for residual osteomyelitis. The median duration of antibiotic therapy was 19 days in patients with positive margins and 14 days (2-63) in patients with negative margins. Patients with positive margins had a significantly higher rate of re-amputation compared with patients with negative margins (43.6% vs 15.3%) [START_REF] Kowalski | The Effect of Residual Osteomyelitis at the Resection Margin in Patients with Surgically Treated Diabetic Foot Infection[END_REF]. In another study including 27 diabetic patients who underwent partial foot amputation, patients with a positive bone margin had a significantly higher rate of poor outcome than patients with negative bone margin (82% versus 25%; p=0.0063) [START_REF] Atway | Rate of Residual Osteomyelitis after Partial Foot Amputation in Diabetic Patients: A Standardized Method for Evaluating Bone Margins with Intraoperative Culture[END_REF].

The number of bone biopsies which had to be performed in macroscopically healthy zones to eliminate the risk of residual infection is not defined. In our study, the mean number of bone biopsies in macroscopically healthy zones was 1.6 ±0.8, because it is usually difficult to perform more than one bone biopsy in patients with minor resection (arthroplasty resection).

Most of our patients (80%) had received preoperative antibiotics for concomitant skin and soft tissue infections for a mean of 9.7 ±5.4 days. Although the Infectious Diseases Society of America (IDSA) recommends a 2-week antibiotic-free period to avoid false-negative cultures in the absence of data in patients with diabetic foot osteomyelitis, only two of our patients had discontinued antibiotics on Day 28 and 8 before surgery, while three patients did not receive any antibiotics before surgery [START_REF] Lipsky | Infectious Diseases Society of America Clinical Practice Guideline for the Diagnosis and Treatment of Diabetic Foot Infections[END_REF]. We cannot exclude false-negative culture results in other patients, but it has been shown that culture of bone specimens remains largely positive despite antibiotic therapy [START_REF] Lesens | Culture of perwound bone specimens: a simplified approach for the medical management of diabetic foot osteomyelitis[END_REF][START_REF] Lázaro-Martínez | Histopathologic Characteristics of Bone Infection Complicating Foot Ulcers in Diabetic Patients[END_REF][START_REF] Marschall | The Impact of Prebiopsy Antibiotics on Pathogen Recovery in Hematogenous Vertebral Osteomyelitis[END_REF]. In a study of 80 patients with diabetic foot osteomyelitis, culture of per-wound bone specimens was positive in 96% of patients, although half of patients received a course of antimicrobials within 14 days of bone specimen collection [START_REF] Lesens | Culture of perwound bone specimens: a simplified approach for the medical management of diabetic foot osteomyelitis[END_REF]. In another cohort study of 165 patients with diabetic foot osteomyelitis, in whom 83% were given antibiotics (mean duration of exposure, 6.2 ±4.1 weeks; amoxicillin-clavulanic acid was the most frequently prescribed antibiotic), antibiotics did not have any negative impact on microbiologic culture of surgical bone biopsy results (87.8% in patients exposed to antibiotics versus 61.8% in antibiotic-naïve patients (p<.001) [START_REF] Lázaro-Martínez | Histopathologic Characteristics of Bone Infection Complicating Foot Ulcers in Diabetic Patients[END_REF]. In a cohort study of 60 adults with vertebral osteomyelitis who had biopsy performed because of previous antibiotic exposure ≤14 days (median duration 4 days; 1-37 days), only 17 (28%) had potentially false-negative culture results because of the empirical antibiotic treatment. The rate of negative culture was also very low in patients with open surgical bone biopsy compared with percutaneous bone biopsy (5% versus 41%) [START_REF] Marschall | The Impact of Prebiopsy Antibiotics on Pathogen Recovery in Hematogenous Vertebral Osteomyelitis[END_REF].

Among the 15 patients included in our study, 80% were cured without recurrence of osteomyelitis at 6 months with antibiotic therapy of a mean duration of 8.3 ±5.9 days postsurgery. The antibiotic therapy duration was slightly higher than the 3-5 days recommended because discontinuation of therapy had to be validated at our weekly multidisciplinary consultation meeting, and bone bacterial cultures had to be sterile on Day 5. Among the three patients who experienced treatment failures, one was lost to follow-up, although without recurrence during the first month of follow-up. The two others included one case of recurrent clinical and radiological osteomyelitis requiring surgical revision, and one case of persistent purulent flow from the scar successfully treated with two weeks of antibiotic treatment alone.

Our cure rate (80%), with less than 10 days of post-amputation antibiotic therapy and six months of follow-up, is comparable to literature data, although most other authors reported a longer duration of antibiotic therapy (above 10 days) and/or shorter follow-up [START_REF] Lipsky | Ertapenem versus piperacillin/tazobactam for diabetic foot infections (SIDESTEP): prospective, randomised, controlled, double-blinded, multicentre trial[END_REF][START_REF] Lipsky | Antibiotic therapy for diabetic foot infections: comparison of two parenteral-to-oral regimens[END_REF][START_REF] Lipsky | the Linezolid Diabetic Foot Infections Study Group. Treating Foot Infections in Diabetic Patients: A Randomized, Multicenter, Open-Label Trial of Linezolid versus Ampicillin-Sulbactam/Amoxicillin-Clavulanate[END_REF][START_REF] Saltoglu | Piperacillin/tazobactam versus imipenem/cilastatin for severe diabetic foot infections: a prospective, randomized clinical trial in a university hospital[END_REF][START_REF] Lázaro-Martínez | Antibiotics Versus Conservative Surgery for Treating Diabetic Foot Osteomyelitis: A Randomized Comparative Trial[END_REF][START_REF] Grayson | Use of ampicillin/sulbactam versus imipenem/cilastatin in the treatment of limbthreatening foot infections in diabetic patients[END_REF].

However, some of them included patients with surgical debridement but not amputation [START_REF] Lázaro-Martínez | Antibiotics Versus Conservative Surgery for Treating Diabetic Foot Osteomyelitis: A Randomized Comparative Trial[END_REF].

In addition, the duration of antibiotic therapy was not determined by the results of bone biopsies from resection margins. Furthermore, the primary objective of these studies was the comparison of antibiotics [START_REF] Lipsky | Ertapenem versus piperacillin/tazobactam for diabetic foot infections (SIDESTEP): prospective, randomised, controlled, double-blinded, multicentre trial[END_REF][START_REF] Lipsky | Antibiotic therapy for diabetic foot infections: comparison of two parenteral-to-oral regimens[END_REF][START_REF] Saltoglu | Piperacillin/tazobactam versus imipenem/cilastatin for severe diabetic foot infections: a prospective, randomized clinical trial in a university hospital[END_REF][START_REF] Grayson | Use of ampicillin/sulbactam versus imipenem/cilastatin in the treatment of limbthreatening foot infections in diabetic patients[END_REF].

Conclusion

In a small, real-life population of diabetic patients undergoing surgical amputation for osteomyelitis, our study shows that a short duration of post-amputation antibiotic therapy in patients with sterile bacteriological samples on Day 5 obtained from resection margins, is effective and supports the French and American guidelines. This approach should be validated in a larger cohort.

Acknowledgements

We would like to thank Fiona Ecarnot (EA3920, University Hospital Besancon, France) for editorial assistance.

Disclosure of interest

The authors declare no conflict of interest.

Source of funding: none

ETHICAL STATEMENT

The work has not been submitted elsewhere for publication, in whole or in part. The article is not currently being considered for publication in another journal. All the authors listed have approved the article that is enclosed.

Authors' contributions

All authors cared for the patients. FBS, MF, and SD were responsible for the overall supervision of the study. JG, SD, and FBS wrote the first draft of the article. All authors critically reviewed the article and gave final approval.


F Binda

G Tebano

M C Kallen

J Ten Oever

Mejl Hulscher

J A Schouten

Pulcini

Nationwide survey of hospital antibiotic stewardship programs in France Enquête nationale sur les programmes de bon usage des antibiotiques dans les établissements de santé en France

Keywords: antibiotic stewardship, implementation, hospital, quality indicators bon usage des antibiotiques, hôpitaux, implémentation, indicateurs qualité

Objective. We aimed to evaluate the current state of antibiotic stewardship (ABS) in French public and private acute care hospitals.

Methods. We conducted a cross-sectional online questionnaire survey. The selection of participating hospitals was performed through a stratified random sampling procedure among all French public and private hospitals with acute care beds.

Results

. 97/215 (45%) hospitals participated. A formal ABS program was implemented in 84% (80/95) of hospitals. A person officially in charge of this program (i.e., ABS program leader) was present in almost all participating hospitals (99%, 95/96) and s/he coordinated a multidisciplinary ABS team in 42% (40/96) of cases. The median time spent on ABS activities was 1.7, 1.6, and 0.8 hours/week/100 acute care beds for infectious disease (ID) specialists, pharmacists, and microbiologists respectively; 27% (7/26) of ID specialists/other clinicians, 58% (15/26) of pharmacists, and 80% (16/20) of microbiologists received no salary support for the stewardship activities conducted as part of the team. Local guidelines (94%, 88/94), electronic medical records (85%, 80/94), and an antibiotic restriction policy (92%, 82/89) were implemented in almost all hospitals. Reports on antibiotic consumption and local resistance rates were available in 100% (91/91) and 84% (76/91) of hospitals, respectively. Conclusion. Despite the existence of national requirements, hospital ABS programs are not fully implemented in France, mainly because of inadequate institutional support and funding. Résumé Objectif. L'objectif était d'évaluer la mise en place des programmes de bon usage des antibiotiques (PBUA) dans les établissements de santé (ES) français. Méthodes. Nous avons conduit une enquête avec un questionnaire en ligne. La sélection des ES a été effectuée après tirage au sort aléatoire stratifié sur le type d'établissement et sur le nombre de lits MCO. Résultats. 97/215 (45 %) ES ont participé. Un PBUA était mis en place dans 84 % (80/95) des ES. Un référent en antibiothérapie était présent dans presque tous les établissements (99 %, 95/96) et il/elle coordonnait une équipe multidisciplinaire dans 42 % (40/96) des cas. Le temps médian consacré au PBUA par l'infectiologue, le pharmacien et le microbiologiste au sein de l'équipe multidisciplinaire était de 1,7, 1,6 et 0,8 heures/semaine/100 lits ; 27 % (7/26) des infectiologues, 58 % (15/26) des pharmaciens et 80 % (16/20) des microbiologistes (membres d'une équipe) ne recevaient aucun financement spécifique par l'établissement pour les activités du programme. Dans la grande majorité des ES, des recommandations (94 %, 88/94), un dossier informatisé (85 %, 80/94) et une restriction de la prescription de certains antibiotiques (92 %, 82/29) étaient en place. Les consommations d'antibiotiques et des données de résistance bactérienne étaient disponibles dans 100 % (91/91) et 84 % (76/91). Conclusion. Les programmes de bon usage des antibiotiques ne sont pas encore déployés de manière optimale dans les ES français, en grande partie du fait d'un soutien institutionnel et de moyens insuffisants.

Introduction

The World Health Organization described antimicrobial resistance as a "problem so serious that it threatens the achievements of modern medicine" [START_REF] Who | Antimicrobial resistance global report on surveillance: 2014 summary[END_REF]. A recent study conducted by the French public health agency (Santé Publique France) estimated that multidrug-resistant bacteria affect 158,000 persons annually in France, of whom 12,500 individuals die from such infections [START_REF] Colomb-Cotinat | Morbidité et mortalité des infections à bactéries multi-résistantes aux antibiotiques en France en 2012. Étude Burden BMR, rapport -Juin 2015[END_REF].

The fight against antimicrobial resistance demands a multidisciplinary "One health" approach [START_REF]National Action Plan for Combating Antibiotic-resistant Bacteria[END_REF],

and three strategies must be simultaneously implemented: antimicrobial stewardship, infection prevention and control (including vaccination), and measures to reduce the risk of bacterial spread in the environment [START_REF] Pulcini | Antimicrobial stewardship: an international emergency[END_REF].

Antibiotic stewardship (ABS) has been described as a coherent set of actions which promote the responsible use of antibiotics at the patient and population levels [START_REF] Dyar | ESCMID Study Group for Antimicrobial stewardshiP). What is antimicrobial stewardship?[END_REF]. It has been demonstrated that hospital ABS programs improve patients' outcomes while reducing antibiotic use and bacterial resistance [START_REF] Hulscher | Antibiotic stewardship: does it work in hospital practice? A review of the evidence base[END_REF][START_REF] Davey | Interventions to improve antibiotic prescribing practices for hospital inpatients[END_REF][START_REF] Schuts | Current evidence on hospital antimicrobial stewardship objectives: a systematic review and meta-analysis[END_REF].

In France effort has been made over the last two decades to fight bacterial resistance. National guidance for antibiotic stewardship programs in hospitals were issued by the French National Authority for Health (French acronym HAS) in 2008 [START_REF]Stratégie d'antibiothérapie et prévention des résistances bactériennes en établissement de santé[END_REF], and since 2001 a national action plan aiming to curb antimicrobial resistance and to promote responsible use of antibiotics has been led by the Ministry of Health [START_REF]Plan national d'alerte sur les antibiotiques[END_REF].

One of the cornerstones of these strategies was the mandatory identification of an ABS program leader in each hospital, who both leads the program and can be consulted by clinicians to guide and optimize antibiotic prescriptions. S/he can lead a multidisciplinary antibiotic stewardship team including various professionals (usually infectious disease [ID] specialists, pharmacists, and microbiologists). The presence of an ABS program leader is mandatory in every French hospital and an ABS team is highly recommended by national authorities [START_REF] Carlet | Tous ensemble, sauvons les antibiotiques -Rapport du groupe de travail spécial pour la préservation des antibiotiques[END_REF].

To facilitate and to monitor the implementation of ABS programs in hospitals, a composite indicator (ICATB combining organization, process, and outcome measures) was introduced a decade ago. An updated and expanded version of this indicator (ICATB2) is currently in place and is part of the mandatory certification of hospitals [START_REF]Haute autorité de Santé[END_REF]. ICATB2 scores are however highly variable among hospitals, suggesting that ABS programs are still not optimally and uniformly implemented in France [START_REF]Haute autorité de Santé. Indicateurs de qualité et de sécurité des soins -Infections Associées aux Soins (ex-Tableau de Bord des Infections Nosocomiales) -Résultats nationaux de la campagne 2016 -Données[END_REF].

The aim of our study was to evaluate the current state of ABS programs in French public and private acute care hospitals and to make an inventory of the organizational characteristics and activities of these programs.

Methods

A systematically developed questionnaire to evaluate the current state of antimicrobial stewardship in hospitals was developed and administered in the Netherlands [START_REF] Kallen | A survey on antimicrobial stewardship prerequisites, objectives and improvement strategies: systematic development and nationwide assessment in Dutch acute care hospitals[END_REF]. This project was transferred to ESGAP (ESCMID Study Group for Antimicrobial stewardshiP) that explored the implementation of ABS programs in four countries: France, Italy, Slovenia, and the Netherlands.

We focus here on the cross-sectional survey conducted in France, which was also supported by the French Infectious Diseases Society (French acronym SPILF).

Study design

ABS program leaders from a random sample of French hospitals were invited by email to participate in a cross-sectional online questionnaire survey. The questionnaire was available from November 2017 to February 2018 and was hosted on a survey tool platform (LimeSurvey, Version 2.6.4). Up to two reminders were sent in case of absence of response. Participation was not compensated, and anonymity was guaranteed during analysis and reporting of results. Ethical approval was not required according to French legislation.

Selection of hospitals

Participating hospitals were selected using a stratified random sampling procedure among all French public and private hospitals with acute care beds. The complete list of hospitals was obtained consulting various websites of the HAS [START_REF]Haute autorité de Santé[END_REF]15] and the French National Hospital Federation (French acronym FHF) [15], and using the database kindly provided by CPIAS Bretagne (Regional Infection prevention and control network). Hospitals with less than 50 acute care beds were considered not eligible, resulting in a total of 860 eligible hospitals.

Stratification was based on 1) the type of hospital (teaching public hospitals; non-teaching public hospitals; cancer institutes; not-for-profit private hospitals; for-profit private hospitals); 2) the number of acute care beds (≥ or < median number of beds, calculated for public hospitals, not-forprofit hospitals, and for-profit hospitals).

A stratified random sampling procedure (https://www.randomizer.org/) selected 25% of hospitals for each category, resulting in 215 hospitals. The procedure is detailed in Figure 1.

Questionnaire development

The questionnaire included 46 questions, Supplementary material 1.docx grouped into four sections: hospital characteristics, organization and institutional support of ABS program, hospital resources, and ABS activities. The questionnaire was developed by a multidisciplinary group of experts (n=5, infectious disease specialists, intensivists, and epidemiologists) following a literature review; it included organization, process, and outcome indicators such as those developed by the Transatlantic Taskforce on Antimicrobial Resistance (TATFAR). It was first used for the aforementioned nationwide survey among Dutch hospitals [START_REF] Kallen | A survey on antimicrobial stewardship prerequisites, objectives and improvement strategies: systematic development and nationwide assessment in Dutch acute care hospitals[END_REF]; then it was translated into French, with some adaptation to national specificities concerning organization of healthcare facilities and ABS programs. This French questionnaire was pilot tested among six selected ESGAP and SPILF members, to check for clarity and conciseness.

Evaluation of salary support

For the evaluation of salary support of ABS teams and members, two standards currently available in France were considered: i) 0.3 full time equivalent (FTE)/400 acute care beds for the ABS program leader; ii) 2.7 FTE/400 acute care beds for the whole team, divided as follows:

• 1.4 FTE/400 acute care beds for infection specialists (ID specialists or other clinicians);

• 1 FTE/400 acute care beds for pharmacists;

• 0.24 FTE/400 acute care beds for microbiologists.

The first standard was derived from the ICATB2 [START_REF]Haute autorité de Santé[END_REF] and the second from a nationwide survey conducted in 2015 by the Task force on antimicrobial resistance coordinated by the ministry of Health [START_REF] Coz | Human resources needed to perform antimicrobial stewardship teams' activities in French hospitals[END_REF].

Statistical analysis

Descriptive analyses presenting frequencies, percentages, medians, and means were performed using IBMSPSS software (version 23.0, Chicago, IL, USA). Differences between groups of hospitals were explored using the Chi-square test.

Results

Participants and response rate

Ninety-seven hospitals participated in the survey, with an overall response rate of 45% (97/215).

Ten out of 15 (67%) teaching public hospitals, 50/87 (57%) non-teaching public hospitals, 14/20 (70%) not-for-profit and 23/93 (25%) for-profit private hospitals participated in the survey. The median number of acute care beds among participating hospitals was 250 (IQR 110-372).

The response rate of public hospitals (59%, 60/102) was significantly higher than the response rate of private ones (33%, 37/113) (p<0.001). Among the respondents, 89% (86/97) completed the whole questionnaire while 11% (11/97) partially replied. The characteristics of participating hospitals are detailed in Table 1.

Organization, accountability, and institutional support of ABS program (Table 2)

A formal ABS program was implemented in 84% (80/95) of hospitals; in 68% (65/95) an annual report on ABS activities was produced. A committee in charge of the ABS program (e.g., drug and therapeutic committee) was present in 90% (85/95) of hospitals. Funding for ABS activities (other than for human resources) was provided by the hospital board in 18% (17/96) of cases. In 12% (11/94) of hospitals, a senior executive administrative officer with accountability for ABS leadership was officially named by the hospital board. When implicated in the ABS teams, the median time spent on ABS activities was 1.7, 1.6, and 0.8 hours/week/100 acute care beds for ID specialists, pharmacists, and microbiologists, respectively. This translated into 0.19, 0.18, and 0.09 FTE/400 acute care beds (annual paid hours for a full-time hospital consultant = 1,820 [35 hours x 52 calendar weeks]) [START_REF] Coz | Human resources needed to perform antimicrobial stewardship teams' activities in French hospitals[END_REF]. Physicians in charge of helping implementing ABS actions in their own wards (i.e., antibiotic champions), in close collaboration with the ABS program leader/team, were absent in most hospitals (77%, 72/94).

The
Twenty-seven per cent (7/26) of ID specialists/other clinicians, 58% (15/26) of pharmacists, and 80% (16/20) of microbiologists who were in an ABS team received no salary support from the institution for ABS activities. Four per cent (1/26) of ID specialists/other clinicians, 4% (1/26) of pharmacists, and 15% (3/20) of microbiologists had a salary support superior or equal to the corresponding staffing standard indicated by the Task Force (see methods for definitions). Thirtyone per cent (10/32) of ABS program leaders who were in an ABS team had no salary support and 50% (16/32) had a salary support superior or equal to the ICATB2 standard (Table 2).

Considering the salary support for the whole ABS team (Table 2), 21% (7/34) of ABS teams did not receive any salary support; 62% (21/34) of ABS teams had a salary support superior or equal to the standard required in ICATB2 (0.3 FTE/400 acute care beds); and 12% (4/34) had a salary support superior or equal to that recommended by the 2015 task force (2.7 FTE/400 acute care beds). 3 and4, and Supplementary material 2.docx, Supplementary material 3.docx) IT support (Table 3) Electronic medical records were implemented in most hospitals (85%, 80/94). Information technology (IT) personnel was specifically allocated to supporting ABS activities in 1/94 hospital (1%). IT service could be consulted for ABS activities if needed in 53% of cases (50/94).

Antibiotic stewardship resources and activities (Tables

Local guidelines (Table 3)

Local guidelines were available in 94% (88/94) of hospitals and were based on local antibiotic susceptibility data in 53% (47/88) of cases. Guidelines were mostly available on the Intranet (85%, 75/88) and/or through a printed pocket version (35%, 31/88).

Antibiotic formulary (Table 3)

A local antibiotic formulary was available in 98% (92/94) of hospitals and it could mainly be consulted on the intranet (76%, 70/92) and/or through a printed pocket version (42%, 39/82). A list of restricted antibiotics was available in 76% (70/92) of antibiotic formularies.

Antibiotic prescription policy (Table 4)

A restriction policy for selected antibiotics was implemented in 92% (82/89) of hospitals.

Monitoring was mainly performed using antibiotic order forms (76%, 62/82).

A written policy for documentation of indication of all antibiotic prescriptions was available in 45% (42/94) of hospitals. Daily care bundles and checklists for antibiotic use were used in 9% (8/88) of hospitals.

Bedside ID consultations (Table 4) Bedside ID consultations were performed in 54% (48/89) of hospitals. The most frequent indications were prosthetic joint infections (77%, 37/48) and infective endocarditis (65%, 31/48).

Quality indicators (e.g., performance of an echocardiography in case of Staphylococcus aureus bacteremia) for one or more of these conditions were monitored in 60% (29/48) of hospitals.

Quality indicators concerning the management of fungal invasive infections were monitored in 24% (7/29) of hospitals.

Monitoring and audits for ABS actions (Table 4) Systematic monitoring and audits mostly concerned surgical antibiotic prophylaxis (49%, 43/88), while actions aiming at optimizing diagnostic investigations, adaptation of dosing using therapeutic drug monitoring, IV-to-oral switch, and duration of treatment were present in 7% (6/88), 10% (9/88), 15% (13/88), and 16% (14/88) of cases, respectively.

Considering monitoring (the activity of repeated measurement and reporting to ensure that stewardship goals are met) and audits (a longitudinal measurement of the quality of antibiotic use) separately, audits were more frequently performed than monitoring for the majority of these ABS activities (Table 4).

The timing, frequency, and categories of patients involved in these activities are detailed in Supplementary material 2.docx et Supplementary material 3.docx

Reporting and feedback (Table 4)

Reports on consumption of antibiotics and local resistance rates were available in 100% (91/91) and 84% (76/91) of hospitals, respectively. Defined daily doses (DDD) were the most frequent unit of measure used to assess the quantity of prescribed antibiotics (91%, 83/91). Point-prevalence surveys assessing the quality of antibiotic use were conducted in 67% (58/87) of hospitals; pointprevalence surveys were conducted at least once a year in 53% (31/58) of cases. Feedback of point-prevalence surveys or results of audits were provided to prescribers in 82% (72/88) of cases.

Education (Table 5)

Educational activities on ABS were organized for junior physicians and senior physicians in 66% (58/88) and 40% (35/88) of cases, respectively. Education for residents was mandatory in 53% (31/58) of cases, whereas education for senior specialists was made available on a voluntary basis in 97% of cases (34/35). Topics included in these educational activities mainly focused on antibiotic streamlining, antimicrobial resistance, and specific infective syndromes.

Discussion

To the best of our knowledge this is the first survey systematically investigating all crucial aspects of ABS programs in a large representative nationwide sample of French public and private acute care hospitals.

The performance of several ABS measures in a sample of French hospitals was previously evaluated in a global survey in 2012 [START_REF] Howard | on Antimicrobial Stewardship. An international cross-sectional survey of antimicrobial stewardship programmes in hospitals[END_REF], but results were presented pooled for all European countries. A 2016 study involving 65 French hospitals investigated human resources dedicated to ABS and highlighted that these resources were not adequate, mainly due to a lack of institutional and national support [START_REF] Coz | Human resources needed to perform antimicrobial stewardship teams' activities in French hospitals[END_REF]. More recently, a large survey specifically explored the role of pharmacists in ABS in public and private hospitals in France [START_REF] Weier | Pharmacist participation in antimicrobial stewardship in Australian and French hospitals: a cross-sectional nationwide survey[END_REF]. Another recent study performed in the southwestern region of France (Nouvelle Aquitaine) evaluated the measures performed in 234 hospitals and the functioning of the multidisciplinary ABS team [START_REF] Coppry | Antimicrobial stewardship programs in hospitals: implementation of international structure and process indicators (TATFAR) in Southwestern France[END_REF]. All these studies confirmed the need for human resources to implement ABS programs in France.

Due to the stratified random sampling used to select participating hospitals, our sample included a wide range of healthcare facilities (public and private, teaching and non-teaching hospitals, notfor-profit and for-profit clinics). It is remarkable that a large proportion of small hospitals and nonteaching hospitals participated.

ABS programs and identification of an official ABS program leader in French hospitals are requirements for accreditation standards [START_REF] Carlet | Tous ensemble, sauvons les antibiotiques -Rapport du groupe de travail spécial pour la préservation des antibiotiques[END_REF]. Our results confirmed that the great majority of participating hospitals had both. Furthermore, monitoring of antibiotic consumption, microbiological susceptibility reports, and antibiotic formularies containing the list of restricted molecules were available in almost all hospitals. Guidelines for the treatment of common infections were also available in most hospitals, but they were based on local susceptibility data in only half of them. These results demonstrate that some core actions included in ICATB2 [START_REF]Haute autorité de Santé[END_REF] have been implemented in France (Table 2), suggesting that the implementation of ICATB2 had a strong and positive impact on the implementation of ABS programs in France.

Comparing our findings with the results of the global survey conducted in 2012 [START_REF] Coz | Human resources needed to perform antimicrobial stewardship teams' activities in French hospitals[END_REF] (even if sampling methodologies differed), the presence of an ABS program in France seems higher than in the pooled European countries (84% vs 66%). The presence of local guidelines, antibiotic formulary, and restriction policy was similar in both studies (98% in our survey vs 94% in the 2012 survey, 95% vs 94%, 88% vs 92%).

Nonetheless, considering the core elements and checklist items recommended for all hospital ABS programs worldwide [START_REF] Pulcini | Developing core elements and checklist items for global hospital antimicrobial stewardship programmes: a consensus approach[END_REF] and the indicators validated by the TATFAR [START_REF]TATFAR): summary the modified Delphi process for common structure and process indicators for hospital antimicrobial stewardship programs[END_REF], our results highlight that several actions not included in the ICATB2 score are still not fully implemented.

It has been shown that ABS teams significantly improve the quality of antibiotic prescriptions [START_REF] Howard | on Antimicrobial Stewardship. An international cross-sectional survey of antimicrobial stewardship programmes in hospitals[END_REF] and are cost effective [START_REF] Dumartin | Antibiotic usage in south-western French hospitals: trends and association with antibiotic stewardship measures[END_REF]. We observed that multidisciplinary ABS teams were present in less than half of surveyed hospitals, and this result is consistent with the survey conducted in southwestern France (multidisciplinary ABS team present in 50% of cases) [START_REF] Coppry | Antimicrobial stewardship programs in hospitals: implementation of international structure and process indicators (TATFAR) in Southwestern France[END_REF]. As advocated by the 2015 Task force [START_REF] Carlet | Tous ensemble, sauvons les antibiotiques -Rapport du groupe de travail spécial pour la préservation des antibiotiques[END_REF], it is now time to move from a single ABS program leader to a full multidisciplinary ABS team.

Another element identified as suboptimal in our survey is the support provided by the hospital board to the ABS program: a senior manager officially in charge of the ABS program or a budget specifically allocated to ABS activities (excluding human resources) was present in only a minority of hospitals. Other interesting findings concerned financial support. First of all, a considerable proportion of ABS team members (ranging from 21% to 44% for the various professional categories) were not able to quantify salary support or time spent in ABS activities; the majority of ABS team members, particularly pharmacists (58%) and microbiologists (80%), received no salary support for their time spent on ABS activities; salary support was equal or superior to the standard recommended in ICATB2 (0.3 FTE/400 acute care beds) for only 62% of ABS teams; and finally only a minority of hospitals (12%) achieved the staffing standard objectives suggested by the 2015 Task Force [START_REF] Coz | Human resources needed to perform antimicrobial stewardship teams' activities in French hospitals[END_REF].

As we had to use a questionnaire as similar as possible to the one used for the European survey, we unfortunately could not explore the time spent by each ABS program leader (in the absence of a multidisciplinary ABS team) on ABS activities and the salary support they received for these activities. Assuming that hospitals with multidisciplinary ABS teams are possibly the most involved in ABS activities, it is quite probable that an ABS program leader working on his/her own without an ABS team may have even less institutional support. Supporting this assumption, the survey performed in Southwestern France showed that 85% of ABS program leaders did not have any specific salary support [START_REF] Coppry | Antimicrobial stewardship programs in hospitals: implementation of international structure and process indicators (TATFAR) in Southwestern France[END_REF].

Considering the specific ABS activities, our results showed low levels of regular or continuous monitoring (15% or less for all stewardship activities). Occasional (i.e. audits) or incidental actions (i.e. request from the prescriber) were more frequent. The activities less frequently monitored are those targeting the optimization of diagnostic investigations and the adaptation of dosing based on therapeutic drug monitoring. This last result is consistent with a previous French survey which highlighted that therapeutic drug monitoring (TDM) for some molecules (e.g., beta-lactams) is only available in a minority of hospitals [START_REF] Charmillon | The ANTIBIOPERF study: a nationwide cross-sectional survey about practices for β-lactam administration and therapeutic drug monitoring among critically ill patients in France[END_REF]. Therefore, ABS programs need to facilitate the implementation of TDM and its correct use.

Other actions were not sufficiently performed: presence of a documentation policy (<50%); availability of bedside ID consultations (54%); education on ABS (mainly focused on residents and only available in some hospitals); presence of antibiotic champions in charge of helping ABS implementation in their wards (23%). There is clear room for improvement for all these elements.

Comparing our results with the nationwide Dutch survey included in the European project conducted by ESGAP [START_REF] Kallen | A survey on antimicrobial stewardship prerequisites, objectives and improvement strategies: systematic development and nationwide assessment in Dutch acute care hospitals[END_REF], antibiotic formularies and local guidelines, as well as a restriction policy are similarly present in both countries (93% in the Netherlands vs 98% in France, 93% vs 94%, 91% vs 92%). In contrast to France, ABS teams are present in almost all Dutch hospitals (94% vs 42%); activities targeting stewardship objectives such as timely IV to oral switch, discontinuation, adaptation of dosing after TDM, and optimization of diagnostic investigations are also more widespread in Dutch hospitals (55% vs 15%, 37% vs 16%, 65% vs 10%, 35% vs 7%).

Our study has several limitations. It is possible that our results give an optimistic view of ABS program implementation, as the response rate was 45% and participating hospitals were probably the ones most interested in ABS. Moreover, participation was significantly higher in public than in private hospitals. However, to reduce this selection bias we randomly included all categories of public and private acute care hospitals, with a stratification based on facility type and number of beds. The study design (cross-sectional survey) is at risk of social desirability bias, even though preserving anonymity of respondents might have limited such bias. Finally, our results present self-reported data, which might not always reflect real practices.

Conclusion

Despite all measures implemented at the national level to promote ABS in France, hospital ABS programs are not yet sufficiently implemented, mainly because of inadequate institutional support. The inclusion of new indicators in ICATB could be a way to strengthen ABS programs over the next years. Furthermore, as hospital accreditation is currently based on ICATB2 total score and as only this value is publicly reported, it would be useful to identify particularly essential indicators as mandatory for all hospitals (e.g., staffing standards for ABS teams), with external auditing of the declared score and public reporting of results, as already advised by the 2015 Task Force [START_REF] Carlet | Tous ensemble, sauvons les antibiotiques -Rapport du groupe de travail spécial pour la préservation des antibiotiques[END_REF].

A

  person formally in charge of the ABS program (i.e., ABS program leader) was present in almost all participating hospitals (99%, 95/96) and s/he coordinated a multidisciplinary ABS team in 42% (40/96) of cases. This team coordinated the ABS program at several locations within the hospital group in 63% (25/40) of cases. The hospital board officially endorsed the activities of the ABS team in 85% (34/40) of cases. The median year of creation of the ABS team was 2010 (range: 2000-2017). At least one ID specialist, one microbiologist, and one pharmacist were present in the ABS team in 68% (27/40), 80% (32/40), and 95% (38/40) of cases, respectively. At least one clinician (a medical physician with a clinical activity involving direct contact with patients) was present in 83% (33/40) of ABS teams.

Figure 1 . 1 .

 11 Figure 1. Methods for the selection of hospitals eligible to participate in the survey Figure 1. Méthodes pour la sélection des hôpitaux éligibles pour participer à l'enquête

  ABS program leader was an ID specialist, another clinician or a pharmacist in respectively 42% (39/94), 26% (24/94), and 16% (15/94) of cases. The ABS program leader was a clinician in 72% (68/94) of cases. The ABS program leader declared having received specialized training in antimicrobial stewardship in 80% (74/93) of cases; ABS activities were included in his/her job contract in 51% (47/93) of cases.

Table 1 .

 1 1,311 public and private hospitals with acute care beds Characteristics of participating hospitals Tableau 1. Caractéristiques des établissements de santé participant à l'enquête : e.g., Paris hospital group (French acronym AP-HP)

	Exclusion of healthcare institutions
	with <50 acute care beds
	855 hospitals (405 public, 450 private)
	Categorization (based on type and size of hospital)
	1. Teaching hospitals
	2. Public hospitals <200 acute care beds
	3. Public hospitals ≥200 acute care beds
	4. Cancer institutes
	5.

Not-for-profit private clinics <130 acute care beds 6. Not-for-profit private clinics ≥130 acute care beds 7. For-profit private clinics <110 acute care beds 8. For-profit private clinics ≥110 acute care beds *

Table 2 .

 2 Percentage of hospitals achieving the objectives of ABS programs Tableau 2. Pourcentage d'hôpitaux qui atteignent les objectifs des programmes de bon usage

			% of		% of
		ICATB2 indicators	hospitals achieving	Other indicators not included in the ICATB2	hospitals achieving
			objective		objective
		Formal ABS program (ATB01) 84	Report on ABS activities	68
		ABS program leader (ATB02-ATBM3)	99	ABS team	42
	Organization	Organizational structure of ABS program (ATB03)	90	Senior executive officer with accountability for ABS Clinician leader of ABS team ABS activities included in the leader contract of the ABS program	12 72 51

Acknowledgements

The authors would like to thank all participating hospitals for their contribution. A special acknowledgement to Dr Martine Aupee (CPIAS Bretagne) for providing the national hospital database.

Funding

The development of the antimicrobial stewardship survey was supported in part by a grant

Contribution of authors

All authors contributed to the study design. The original English questionnaire was designed by MK, JS, JtO, MH; the translation of the questionnaire from English into French and its adaptation to the French setting was performed by CP; random selection of participating centers was made by FB and GT; dissemination of the survey was led by CP; statistical analyses were performed by MK; the article was written by FB, GT, and CP. All authors were involved in data interpretation and in the final review of the article.

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Keywords: germ cell tumors, septum pellucidum, suprasellar, intracranial germinomas, bifocal germinomas

Introduction

Intracranial germinomas (IGs) are rare malignant tumors which constitute only 0.5-2% of all primary intracranial tumors. They are a type of germ cell tumor (GCT) and account for approximately 60-70% of the brain GCT (2016 WHO classification, 1).

They have a higher incidence in Asians. They usually occur in children and young adults (10-30 years old), with a male predominance. They affect in 90% of cases the pineal and sellar / suprasellar regions. In 10%-20% they are "bifocal", synchronously involving both regions. Sometimes they involve the corpus callosum (CC), septum pellucidum (SP), basal ganglia (BG), thalamus, corona radiata, frontal and temporal lobe, cerebello -pontine angle, medulla oblongata, fourth ventricle. These IGs, also named "ectopic" IGs, represent approximately 5% of all IGs.

We report a very unusual presentation of an IG of the SP with synchronous involvement of the sellar-supra-sellar region. We discuss the MRI characteristics and review the literature [START_REF] Kamiya | A case of malignant teratoma developing from the septum pellucidum 7 years after removal of a mature teratoma in the pineal region[END_REF][START_REF] Koyama | Choriocarcinoma of the septum pellucidum: case report[END_REF][START_REF] Sato | A case of germinoma in the septum pellucidum manifesting as amnesia and hemiparesis[END_REF][START_REF] Ks Eom | Mixed germ cell tumors in septum pellucidum after radiochemotherapy of suprasellar germinoma: de novo metachronous or recurrent neoplasms?[END_REF][START_REF] Park | Primary intracranial germ cell tumor originating from septum pellucidum that mimics central neurocytoma[END_REF]. This is the first documented report, to our knowledge, of a germinoma of the SP with a synchronous involvement of the sellar region.

Case:

A 28-year-old woman, without past medical history, presented with unsteady gait,

Discussion

The WHO classification divided GCTs into benign (mature teratoma) and malignant (germinoma and non-germinoma) tumors [START_REF] Louis | The 2016 World Health Organization Classification of Tumors of the Central Nervous System: a summary[END_REF]. The SP represents an excessively rare location of GCT with only 6 reported cases to date including ours, making difficult to describe their radiological features (Table 1) (2-6).

Three of them were mixed GCTs, and 2/3 were metachronous [START_REF] Kamiya | A case of malignant teratoma developing from the septum pellucidum 7 years after removal of a mature teratoma in the pineal region[END_REF][START_REF] Ks Eom | Mixed germ cell tumors in septum pellucidum after radiochemotherapy of suprasellar germinoma: de novo metachronous or recurrent neoplasms?[END_REF]. However, they share a few common imaging features with ectopic IGs, different from those originating from sellar/suprasellar and pineal regions: well-defined margins, isointense on T1 WI, iso/hypointense onT2 WI, contain multiple cysts which tend to be large because of the minimal or nonspecific clinical signs and symptoms. The solid part presents moderate or strong enhancement of the lesion and cysts-walls enhancement and mild peritumoral edema are present [START_REF] Shankar | Ectopic intracranial germinoma[END_REF][START_REF] Wei | Radiologic features of primary intracranial ectopic germinomas: Case reports and literature review[END_REF][START_REF] Wu | MRI features of pediatric intracranial germ cell tumor subtypes[END_REF]. Ectopic IGs have a more marked male predominance than midline IGs and present predominantly as a single tumor [START_REF] Wei | Radiologic features of primary intracranial ectopic germinomas: Case reports and literature review[END_REF]. The reported cases of SP germinomas (SPG) present numerous and large (50-60 mm) cysts, marked enhancement, without associated hemorrhage or calcifications as demonstrated by NCCT and T2* sequence. SPG share radiological features with CC germinomas, as shown in our literature review of 14 cases with enough radiological data. CC germinomas are often large (9/14: ≥ 30 mm), with strong enhancement (12/14), well-defined margins (9/14), and present rarely with edema (2/14). They are hypo-isointense on T1WI and isohyperintense on T2WI. Calcifications and haemorrhage are absent and in 5/14 cases.

However, none of the reported cases of CC germinomas presented with a synchronous lesion.

Nevertheless, our case presents many atypical features. First, the occurrence of synchronous lesions, a presentation not previously reported. Secondly the presence of a slight T1 WI hyperintensity of the solid parts which appear hyperdense on NCCT. These findings were not related to calcifications or hemorrhagic components. Slight restriction diffusion in our case was in concordance with the few reported cases [START_REF] Wei | Radiologic features of primary intracranial ectopic germinomas: Case reports and literature review[END_REF][START_REF] Tong | MRI and 1H-MRS on diagnosis of pineal region tumors[END_REF].

Biological markers, such as serum and CSF α-foetoprotein and β -hCG may be useful, but they are rarely considered in the work-up of hemispheric tumors.

In the literature' cases the SP lesion was isolated. However, the presence of a synchronous lesion in SP IG, as in our case, and as in case of other ectopic IG, is a clue for the presumptive diagnosis.

The differential diagnosis includes mainly gliomas and lymphomas, as these lesions share the same MRI signal characteristics: hypo/ isointense on T1 WI, iso/hyperintense on T2 WI, with strong, homogeneous or heterogeneous enhancement (in case of cystic component) on post-contrast T1 WI. Furthermore the MRI appearance of our case mimics central neurocytoma (CN), as Park's case, making the preoperative diagnosis challenging in case of isolated IG of the SP [START_REF] Park | Primary intracranial germ cell tumor originating from septum pellucidum that mimics central neurocytoma[END_REF].

Our report adds one more case to such unusual presentation. Nevertheless, marked restricted diffusion in case of CN may help to differentiate them. As ventricular seeding is common for IG, the radiotherapy field usually includes whole ventricles rather limited to tumoral mass. Craniospinal radiotherapy is usually underwent by patients with leptomeningeal seeding.

Conclusion

IG arising in the SP is rare but should be considered in the differential diagnosis, especially in young adult and particularly in male, presenting with multi-cystic lesion.

The presence of a synchronous mass is a clue for the presumptive diagnosis of germinoma.

These findings suggest that IG in SP may present as a large tumor with large cysts and may look as central neurocytoma. ADC value is helpful to differentiate them.

Figures Legend:

  mild left motor deficit, polydipsia and polyuria of two-month duration. Examination revealed a left hemiparesis. Non-contrast CT (NCCT) showed a cystic midline tumor consisting of solid areas slightly hyperdense extending to the CC and the frontal lobes (Fig 1-a). MRI revealed 2 lesions: one arising from the sella and extending in the pituitary stalk, and another one arising from the SP and extending in the frontocallosal areas. The 60-mm-sized lesion of the SP had a butterfly aspect. The larger 2 cysts had 19 mm in maximum diameter. Both enhancing masses were slightly hyperintense on T1-weighted images (T1 WI), T2 WI, fluid attenuation inversion recovery (FLAIR) and diffusion-WI (DWI), with apparent diffusion coefficient (ADC) restriction and thick edema (Fig 1-b-e). Spine MRI and spinal tapering did not reveal any concomitant lesion. Serum α-foetoprotein and β-human chorionic gonadotrophin (β -hCG) were within normal limits. A whole-body CT did not reveal any lesions. Considering the radiological features, a presumptive diagnosis of IG was made. A biopsy of the frontal lesion was obtained by navigation-guided. The pathological result was pure germinoma. Immunohistochemical staining showed tumor cells positivity for Placenta alkaline phosphatase (PLAP) and c-kit (CD117) (Fig 1 -f-g). The patient received radiation therapy: 16 Gy focal boots to the 2 lesions, and 24 Gy to the whole ventricle. MRI at 12 months after the completion of the treatment showed complete resolution of the tumors and no intraspinal seeding (Fig 1 -h). The patient is fully working without disturbance 8 years after treatment.

Figure

  Figure 1:

  Figure 1: a -Non-contrast Brain CT shows a midline cystic lesion invading the frontal lobes. The solid parts are hyperdense. b-c-d-e-MRI realized 2 days later shows 2 lesions. First a 60-mm-sized cystic lesion of the septum pellucidum which invades the white matter of frontal lobes and has an asymmetric growth. The mass has a butterfly aspect and a honeycomb-like

Acknowledgements

The authors would like to thank Dr Selma Elouahdani-Hamdi for performing the tumor biopsy

Funding This report was not supported by any funding.

Compliance with ethical standards

Conflict of interest All authors have no conflict of interest.

Informed consent

The reported patient was treated with standard of care therapy after informed consent was obtained.


A Briclot

J F Henry

C Popa

C T Nguyen

S Fohanno

Experimental investigation of the heat and fluid flow of an Al 2 O 3 -water nanofluid in the laminar-turbulent transition region

Keywords: nanofluid, laminar-turbulent transition, heat transfer coefficient, thermal performance

Nanofluids present enhanced thermal conductivity compared to their base fluid and then have become good candidates for heat transfer fluids. In this study, the convective heat transfer coefficient and the pressure drop are measured in a circular tube of inner diameter 4.4 mm and outer diameter 5 mm. The experiments were conducted from Reynolds number of 500 up to 4500. After making sure that the results for water were in good agreement with the literature, five concentrations of a γ-Al2O3-water nanofluid were tested. The results show that the concentration has no impact on the critical Reynolds number corresponding to the laminar-turbulent transition for this nanofluid up to 5w%. Moreover, a slight enhancement of the convective heat transfer coefficient is noted in laminar flow with increasing mass concentration while no significant amelioration is observed in turbulent regime compared to water. On the other hand, the pressure drop strongly increases with concentration whatever the flow regime. As a result, the thermal performance (Convective heat transfer coefficient at constant pumping power) of this nanofluid is lower than pure water.

Introduction

Since Maxwell [START_REF] Maxwell | A treatise on electricity and magnetism[END_REF], it is known that adding particles into a fluid increases the thermal conductivity of the mixture. With the development of nanotechnologies, side effects of the usage of millimetric and micrometric particles like erosion, sedimentation and an increase in pressure drop have attenuated. Choi and Eastman [START_REF] Choi | Enhancing Thermal Conductivity of Fluids With Nanoparticles[END_REF] coined the term nanofluid to describe a nanoparticle-laden fluid. Metallic or ceramic nanoparticles are often used due to their high thermal conductivity compared to base fluids like water or oil. Although many publications exclusively deal with the thermophysical properties of nanofluids [START_REF] Mondragón | Experimental characterization and modeling of thermophysical properties of nanofluids at high temperature conditions for heat transfer applications[END_REF][START_REF] Murshed | Investigations of thermal conductivity and viscosity of nanofluids[END_REF][START_REF] Sekhar | Study of viscosity and specific heat capacity characteristics of waterbased Al2O3 nanofluids at low particle concentrations[END_REF][START_REF] Turgut | Thermal conductivity and viscosity measurements of water-based TiO2 nanofluids[END_REF][START_REF] Barbés | Thermal conductivity and specific heat capacity measurements of CuO nanofluids[END_REF][START_REF] Popa | New specific heat data for Al2O3 and CuO nanoparticles in suspension in water and Ethylene Glycol[END_REF][START_REF] Hachey | Experimental investigation of Al2O3 nanofluids thermal properties and rheology -Effects of transient and steady-state heat exposure[END_REF][START_REF] Lai | Pressure-Drop Viscosity Measurements for γ-Al2O3 Nanoparticles in Water and PG-Water Mixtures (Nanofluids)[END_REF][START_REF] Zhou | Measurement of the specific heat capacity of water-based Al2O3 nanofluid[END_REF], the results are heterogeneous, especially for the thermal conductivity and the viscosity making the development of a single correlation difficult. Turgut et al. [START_REF] Turgut | Thermal conductivity and viscosity measurements of water-based TiO2 nanofluids[END_REF], Barbés et al. [START_REF] Barbés | Thermal conductivity and specific heat capacity measurements of CuO nanofluids[END_REF] and Yang et al. [START_REF] Yang | Characterization and convective heat transfer with nanofluids[END_REF] measured thermal conductivities of various nanofluids in good agreement with the model of Hamilton-Crosser (HC) [START_REF] Hamilton | Thermal conductivity of heterogeneous two-component systems[END_REF]. On the other hand, Murshed et al. [START_REF] Murshed | Investigations of thermal conductivity and viscosity of nanofluids[END_REF] reported increases in thermal conductivity of 18% and 45% for a particle volume concentration of 5% of Al and TiO2 in ethylene glycol which is much higher than the predicted values of HC model. They proposed a model to determine the relative thermal conductivity taking into account the particle size and the thickness Besides the positive effect of the nanoparticle on the thermal conductivity, their presence also increases the viscosity of the mixture. For very dilute suspensions, Einstein [START_REF] Einstein | Eine neue bestimmung der molekuldimensionen[END_REF] developed a theoretical model depending on the volume concentration only to predict the increase in viscosity. Later, Brinkman [START_REF] Brinkman | The viscosity of concentrated suspensions and solutions[END_REF] and Batchelor [START_REF] Batchelor | The effect of Brownian motion on the bulk stress in a suspension of spherical particles[END_REF] extended Einstein's model to higher concentrations. However, even at very low volume concentrations (φv < 1%), these three models often fail to predict correctly the evolution of viscosity for Al2O3-water [START_REF] Sekhar | Study of viscosity and specific heat capacity characteristics of waterbased Al2O3 nanofluids at low particle concentrations[END_REF][START_REF] Lai | Pressure-Drop Viscosity Measurements for γ-Al2O3 Nanoparticles in Water and PG-Water Mixtures (Nanofluids)[END_REF][START_REF] Yang | Characterization and convective heat transfer with nanofluids[END_REF][START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles[END_REF][START_REF] Lee | Effective viscosities and thermal conductivities of aqueous nanofluids containing low volume concentrations of Al2O3 nanoparticles[END_REF][START_REF] Hwang | Flow and convective heat transfer characteristics of water-based Al2O3 nanofluids in fully developed laminar flow regime[END_REF] and TiO2-water [START_REF] Turgut | Thermal conductivity and viscosity measurements of water-based TiO2 nanofluids[END_REF][START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles[END_REF] nanofluids. The discrepancies between the theoretical predictions and the experimental results can be attributed to the numerous parameters to take into account in the determination of the thermophysical properties of nanofluids, like the size of the nanoparticles, their shapes, the pH of the solution etc. As the theoretical models often fail to predict the change in the properties of nanofluids, several empirical correlations are available in the literature and compiled by Khanafer and Vafai [START_REF] Khanafer | A critical synthesis of thermophysical characteristics of nanofluids[END_REF].

Due to their enhanced thermal conductivity, nanofluids have become good candidates for heat transfer fluids. Kim et al. [START_REF] Kim | Convective heat transfer characteristics of nanofluids under laminar and turbulent flow conditions[END_REF] studied the flow of an Al2O3-water nanofluid in a pipe and reported an enhancement of 15% and 20% in the convective heat transfer coefficient in laminar and turbulent regimes respectively. At 0.3 v% of Al2O3 in water, Hwang et al. [START_REF] Hwang | Flow and convective heat transfer characteristics of water-based Al2O3 nanofluids in fully developed laminar flow regime[END_REF] observed an increase of 8% in the convective heat transfer coefficient in laminar flow that is higher than the enhancement of the thermal conductivity (+1.44%). The friction factor is unchanged with the increasing volume concentration up to 0.3% while the viscosity is 3% higher than pure water at the maximum volume concentration. Wen and Ding [START_REF] Wen | Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions[END_REF] showed that the thermal entrance length of nanofluids is greater than that of pure fluid in laminar flow. They suggested that the particle migration due to high velocity gradient in the entrance length could affect the development of the boundary layer and then can be responsible for the anomalous heat transfer enhancement. However, the increase in pressure drop is not always considered while it is an important feature of a heat exchanger. Pak and Cho [START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles[END_REF] already noted in 1998 that under the condition of constant velocity, the convective heat transfer coefficient is higher for water than for nanofluids because of the high increase in viscosity. This observation is confirmed by other studies [START_REF] Williams | Experimental Investigation of Turbulent Convective Heat Transfer and Pressure Loss of Alumina/Water and Zirconia/Water Nanoparticle Colloids (Nanofluids) in Horizontal Tubes[END_REF][START_REF] Sajadi | Investigation of turbulent convective heat transfer and pressure drop of TiO2/water nanofluid in circular tube[END_REF][START_REF] Martínez-Cuenca | Forced-convective heat-transfer coefficient and pressure drop of water-based nanofluids in a horizontal pipe[END_REF][START_REF] Ferrouillat | Hydraulic and heat transfer study of SiO2/water nanofluids in horizontal tubes with imposed wall temperature boundary conditions[END_REF] constituting a potential hindrance for the usage of nanofluids in industrial applications where the pumping power is important.

To our knowledge, only a few studies deal with the laminar-turbulent transition. Liu and Yu [START_REF] Liu | Single-Phase Thermal Transport of Nanofluids in a Minichannel[END_REF], Meyer et al. [START_REF] Meyer | The influence of multi-walled carbon nanotubes on single-phase heat transfer and pressure drop characteristics in the transitional flow regime of smooth tubes[END_REF] and Nikulin et al. [START_REF] Nikulin | International Journal of Heat and Mass Transfer Effect of Al2O3 nanoparticles on laminar , transient and turbulent flow of isopropyl alcohol[END_REF] studied the convective heat transfer and the pressure drop of a flow of a nanofluid in the laminar-turbulent transition region while Rudyak et al. [START_REF] Rudyak | On laminar-turbulent transition in nanofluid flows[END_REF] investigated the laminar-turbulent transition (1000 ≤ ≤ 9000) through the friction factor only. In [START_REF] Liu | Single-Phase Thermal Transport of Nanofluids in a Minichannel[END_REF], it has been shown that the presence of the nanoparticles of Al2O3 (40 nm nominal diameter) in water shifts the transition to higher Reynolds numbers in pipe flows and that nanofluids should be used in laminar or fully turbulent flows with sufficiently high Reynolds number to present an interest in cooling applications. On the other hand, in [START_REF] Meyer | The influence of multi-walled carbon nanotubes on single-phase heat transfer and pressure drop characteristics in the transitional flow regime of smooth tubes[END_REF], [START_REF] Nikulin | International Journal of Heat and Mass Transfer Effect of Al2O3 nanoparticles on laminar , transient and turbulent flow of isopropyl alcohol[END_REF] and [START_REF] Rudyak | On laminar-turbulent transition in nanofluid flows[END_REF], the authors noted that the laminar-turbulent transition occurred earlier for nanofluids of MWCNT-water (d NP = 20 nm× 20 µm), Al2O3-isopropanol (d NP = 8 nm as dry powder and 80 nm in solution) and SiO2-water (25 nm and 100 nm nominal diameter) respectively compared to base fluids alone. In this study, we conducted an experimental investigation of the convective heat transfer coefficient ℎ and the linear pressure drop ∆ of a nanofluid (10 nm γ-Al2O3 in deionized (DI) water) in a pipe flow for several mass concentrations (0.75% ≤ ≤ 5%) and Reynolds numbers from 500 up to 4500. The objectives of this work are to study the flow and heat transfer of this nanofluid in the laminar-turbulent region and to determine its thermal performance compared to that of water based on the recommendation of Yu et al. [START_REF] Yu | Thermophysical propertyrelated comparison criteria for nanofluid heat transfer enhancement in turbulent flow[END_REF].

Nanofluid characterisation

The nanofluids used in this work are suspended γ-Al2O3 nanoparticles of 10 nm mean diameter, dispersed in DI water without any surfactant. The stock solution is a 20w% solution supplied by the manufacturer Nanostructured & Anamorphous Materials, Inc. DI water is used to dilute the stock solution and to obtain mass concentrations of 0.75%, 1.5%, 2.5%, 3.5% and 5%.

The density and specific heat capacity of these nanofluids are determined by mixtures laws:

67 = 69 + ;1 -= >7 , (1) 67
,67 = 69 ,69 + ;1 -= >7 ,>7 . (2)
The density and the specific heat capacity of Al2O3 nanoparticles are respectively 3700 kg• m -3 and 773

J• kg -1 • K -1 .
The volume concentration is determined from the mass concentration and the densities of the base fluid and the nanoparticles:

= + @ 69 >7 A ;1 - = . (3)
The work of Minakov et al. [START_REF] Minakov | Rheological behavior of water and ethylene glycol based nanofluids containing oxide nanoparticles[END_REF] suggests that the Al2O3-water nanofluids are Newtonian up to 2v%. In the present study, the Newtonian behaviour has been verified with a rheometer Rheomat RM100 (Lamy Rheology) in the range of the studied concentrations. Therefore, the kinematic viscosity of our nanofluids ν has been measured with an Ubbelohde viscometer from 15°C to 30 °C. The determination of the kinematic viscosity is based on the flow time t of the fluid through a capillary tube of 0.47 mm diameter:

= ;' -=, (4)
where K and γ are respectively the instrument constant and the kinematic energy correction. To avoid any human error in the process, the flow time is automatically measured by a ViscoClock plus module from SI analytics. The experiment is repeated three times for each configuration and the present results are the arithmetic means. 1)) as a function the temperature. The measured viscosity of water is in very good accordance with the literature [START_REF] Huber | New International Formulation for the Viscosity of H2O[END_REF] as the maximum relative deviation is 0.84%. The viscosity of the nanofluid clearly increases with mass concentration and its evolution as a function of the temperature follows the same trend as water. Actually, the relative viscosity B = 67 / >7 is independent of the temperature and can be described by the following equation:

B = 1 + 16.54 + 1329 H (5)
Consequently, the properties of nanofluids are estimated at the mean temperature of the flow in the test section, their temperature dependence being implicitly taken into account through the base fluid properties. 5) and two models from the literature [START_REF] Einstein | Eine neue bestimmung der molekuldimensionen[END_REF][START_REF] Khanafer | A critical synthesis of thermophysical characteristics of nanofluids[END_REF].

It can be seen that the theoretical model of Einstein [START_REF] Einstein | Eine neue bestimmung der molekuldimensionen[END_REF] is not appropriated for this nanofluid while the correlation of Khanafer and Vafai [START_REF] Khanafer | A critical synthesis of thermophysical characteristics of nanofluids[END_REF] obtained from curve-fitting the measurements of Pak and Cho [START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles[END_REF] (Al2O3-water nanofluid, dNP = 13 nm) overestimates the present results. This discrepancy can be due to the difference of pH allowing a better dispersion of the nanoparticles in [START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles[END_REF], decreasing their effective diameter.

Experimental set-up

The flow loop is composed of a gear pump, a temperature controlled water bath, a 1.56 m long straight copper tube with inner diameter di = 4.4 mm and outer diameter de = 5.0 mm, an ultrasonic bath and a flowmeter (Figure 3). The copper tube is electrically heated by Joule heating on 41.4 cm after a 1 m entry length to avoid entry effect (L/di ≈ 220). It is covered with a matte black paint with an emissivity of 0.93±0.01 that allows the recording of the external wall temperature thanks to a thermal camera (CEDIP-FLIR Titanium, 640×512 pixels, Spectral range from 1.5 to 5.1 µm, NETD 20 mK at 20 °C). The measurement area represents 6×2.4 mm 2 . Voltage is directly recorded by a GRAPHTEC data logger while intensity is measured by a DC clamp meter. Likewise, volume flow rate and pressure losses are respectively measured by a MACNAUGHT MX06P flowmeter (0.5 to 100 L•h -1 with ± 0.5% accuracy) and a KELLER PD23 pressure transmitter with a measuring range of 0.5 bar, both connected to the data logger.

The convective heat transfer coefficient is determined from a thermal balance on the copper tube after an electrical excitation increasing the external wall temperature by 3°C. Once the system has reached the steady state, the recording of the thermograms starts for 40 seconds at a frequency of 5 Hz. After 16 seconds, voltage ) and intensity are cut and the external wall temperature decreases to reach the temperature of the fluid at the inlet of the test zone (Figure 4). This methodology was previously validated for periodic excitations [START_REF] Leblay | IR thermography measurement of convective coefficients in a pipe with periodic excitation[END_REF] while a Heaviside step function is used here. Moreover, it does not require thermocouples, thus avoiding flow disturbances. Due to the very small thickness of the tube (e = 0.3 mm) and the high thermal conductivity of copper (k = 400 W•m -1 •K -1 ), the internal and external wall temperatures are considered to be equal. Indeed, the maximum power Q used in this study of 70 W (12.2 kW•m -2 ) would result in a temperature difference ∆T of 9×10 -3 K between the internal and external walls according to the following equation: Moreover, the heat losses to the surroundings are estimated. The radiative heat transfer coefficient is obtained by linearizing Stefan-Boltzman law:

Δ% = I ln; L / M = 2N . ( 6
)
ℎ B = ;% L + % O = H ;% L + % O = (7)
where σ, Te and Ta denote the Stefan-Boltzmann constant (5.67 × 10-8 W•m-2•K-4), the external wall temperature and the temperature of surroundings.

The convective heat transfer coefficient to the ambient air is determined with the following equation for horizontal cylinders with a uniform heat flux density [START_REF] Churchill | Laminar free convection from a horizontal cylinder with a uniform heat flux density[END_REF]:

ℎ P = 0.579 O L Q/R S1 + T 0.442 U V QW X YR/V , (8)
where ka, Ra and Pr are the thermal conductivity of the air, the Rayleigh number and the Prandtl number respectively. The global heat transfer coefficient to the surroundings he is the sum of hr and hc and is about 10

W•m -2 •K -1 .
As a result, the heat losses account for 1% of the heat transferred to the fluid. The heat flux, generated by Joule heating writes as:

" = H N M = H N H M Z L H -M H [ , (9)
where R and ρ respectively denote the electrical resistance and resistivity of the copper tube. As the input current and the electrical resistivity are constant, the heat flux is also constant. Finally, the energy balance writes as:

" = ) N M = ℎ M ;+=Z% L ;+= -% 67 ;+=[ + ℎ L L M ;% L ;+= -% O =, (10)
where T NF (x) denotes the temperature of the nanofluid at the x location.

As the heat flux is constant, the local temperature of the fluid follows a linear profile:

% 67 ;+= = % 67,M\ + ) + , (11)
where m ˙ is the mass flow rate.

Combining equations ( 10) and ( 11), the convective heat transfer coefficient writes as:

ℎ M ;+= = ) -ℎ L $ L ;% L ;+= -% O = N M ]% L ;+= -% 67,M\ - ) + ^. ( 12
)
The absolute uncertainties are evaluated as described in Appendix A. Measurement uncertainties and the relative uncertainties are listed in Table 1. The relative uncertainties of the pumping power PP, the pressure drop ∆P and the friction factor λ are high at low Reynolds number and quickly decrease. This is due to the accuracy of the pressure transmitter that is a percentage of the full scale.

Quantity

Relative uncertainty [%] 0.5 -3.5 3 -58 Δ 2.6 -47 2.9 -48 ℎ M ;+= 3.4 -6.9

Validation case

In order to assess the accuracy of the present results, experiments were first conducted with deionized water as a function of the Reynolds number:

= * M . ( 13
)
As the temperature difference between the inlet and the outlet of the test section (eq. 11) is less than 2°C, the thermal properties of the fluid are considered constant and the measured pressure drop is then compared with the Darcy-Weisbasch equation:

Δ = * H 2 M , (14)
where the friction factor is determined by the well-known Poiseuille's law in laminar flow:

λ = 64 , (15)
In turbulent flow, the correlation of Filonenko [START_REF] Filonenko | Hydraulic resistance in pipes[END_REF] is used :

λ = `1.82 log; = -1.64d YH . ( 16
)
According to the flow regime maps developed by Everts and Meyer [START_REF] Everts | Flow regime maps for smooth horizontal tubes at a constant heat flux[END_REF], the flow is forced convection for all Reynolds numbers. Therefore, the convective heat transfer coefficient is compared with the theoretical solution given by Kays and Crawford [START_REF] Kays | Convective heat and mass transfer[END_REF] for forced convection flows in laminar regime accounting for the thermal entrance length effect:

e = f 11 48 - 1 2 g exp;-H + l = m R n oQ p YQ , (17)
where the eigenvalues and constants are given in [START_REF] Kays | Convective heat and mass transfer[END_REF] and + l = He/q r sL t 9B .

In turbulent regime, the present results are confronted with the recent correlation of Meyer et al. [START_REF] Meyer | Heat transfer coefficients of laminar, transitional, quasi-turbulent and turbulent flow in circular tubes[END_REF]:

= 0.0018 Yu.Hv ; -500= Q.uw u.RH T x U u.QQ . ( 18
)
Figure 5 shows the evolution of the pressure drop for water as a function of the Reynolds number. The experimental measurements are in good agreement with the theoretical results obtained from equations 15 and 16 as the theoretical results are within the error ranges of the data points in laminar and turbulent flows, which confirms the good accuracy and reliability of the experiments. Similarly, Figure 6 compares the present results of the convective heat transfer coefficient with equations 17 and 18 as a function of the Reynolds number. In laminar regime, the convective heat transfer coefficient increases with the Reynolds number, confirming that the flow is not thermally developed. Moreover, one can observe that our experimental measurements are in very good agreement with the theoretical results in both laminar and turbulent regimes.

Results and discussion

Convective heat transfer coefficient

Figure 7 presents the evolution of the convective heat transfer coefficient as a function of the Reynolds number for three mass concentrations and water. Low concentrations of nanofluids are not shown to improve readability. In the laminar region (Fig. 7b), the convective heat transfer coefficient increases with mass concentration to reach a 9% enhancement at 5w% compared to water. This enhancement can be due to the increased thermal conductivity or the increased entrance length due to the presence of the nanoparticles [START_REF] Wen | Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions[END_REF][START_REF] Lai | Convective heat transfer for water-based alumina nanofluids in a single 1.02-mm tube[END_REF]. In the entrance region, the thermal boundary layer is thin favoring heat transfer.

On the other hand, in turbulent flow up to Reynolds number of 4000, all the curves collapse together indicating that the nanoparticles do not impact the heat transfer (Fig. 7a). This indicates that the increase in convective heat transfer coefficient observed in laminar regime is not solely due to the increased thermal conductivity, otherwise hi would be greater in turbulent flow too. However, this is consistent with the hypothesis of a longer entrance length due to the presence of the nanoparticles.

Indeed, the thermal entrance length in turbulent flows is short ; e q r y 10= while it can become very long in laminar flows @ e q r y 0.05 A [START_REF] Incropera | Fundamentals of Heat and Mass Transfer[END_REF]. As a result, the entrance region is not sufficiently lengthened in turbulent flow to have an impact on the heat transfer. Furthermore, the mass concentration of this nanofluid has no impact on the onset of the laminarturbulent transition. Indeed, the critical Reynolds number for the onset of the transition is 2300 ± 50 for all tested fluids and the one for the establishment of fully turbulent flow is around 2600 ± 50. This suggests that this nanofluid behaves like a single-phase fluid with modified properties. This observation contrasts with the work of Liu and Yu [START_REF] Liu | Single-Phase Thermal Transport of Nanofluids in a Minichannel[END_REF] on an Al2O3-water nanofluid where the transition is retarded and the studies of Meyer et al. [START_REF] Meyer | The influence of multi-walled carbon nanotubes on single-phase heat transfer and pressure drop characteristics in the transitional flow regime of smooth tubes[END_REF] on a MWCNT-water nanofluid, Rudyak et al. [START_REF] Rudyak | On laminar-turbulent transition in nanofluid flows[END_REF] on a SiO2-water nanofluid and Nikulin et al. [START_REF] Nikulin | International Journal of Heat and Mass Transfer Effect of Al2O3 nanoparticles on laminar , transient and turbulent flow of isopropyl alcohol[END_REF] on an Al2O3-isopropanol nanofluid where the transition is accelerated. This discrepancy is potentially due to the difference in the nominal diameter of the nanoparticles in the studies affecting the viscosity of the nanofluid and then the Reynolds number. Figure 8 shows the pressure drop in the test section as a function of the Reynolds number. One can note that the pressure drop significantly increases with the mass concentration whatever the flow regime so that at 5w%, it is more than doubled compared to that of water. Even at a very low mass concentration of 0.75%, the observed increase in pressure drop reaches 7%, which is much higher than the intensification of the convective heat transfer coefficient. The increase in pressure drop is due to the higher viscosity of the nanofluid compared to that of water creating more friction losses at the pipe wall. As the volumetric flow rate and the pressure drop of this nanofluid are higher than these of water, a greater pumping power is required to reach the same Reynolds number. This means that it can be difficult to obtain a fully turbulent flow with high concentrations of nanofluids.

Figure of merit

To determine the performance of this nanofluid over the base fluid, several figures of merit (FOM) can be used but different conclusions can be drawn. To analyse the performance of heat transfer fluids, it is natural to compare heat transfer coefficients. However, the simple base of constant Reynolds number can lead to inappropriate conclusions. Indeed, for a constant Reynolds number, the mean velocity of the nanofluid would be higher than that of pure water due to the increased viscosity. This would means that the convective heat transfer coefficient increases with viscosity because of the velocity effect. To compare fluids unambiguously, Yu et al. [START_REF] Yu | Comparative review of turbulent heat transfer of nanofluids[END_REF] recommend to compare the convective heat transfer coefficient in a constant pumping power (PP) basis. Figure 10 depicts the evolution of the heat transfer coefficient as a function of the pumping power necessary to drive the nanofluid in the test section, corresponding to the product of the volumetric flow rate by the pressure drop. In laminar flow (Fig. 10b), low concentrations of nanofluid (0.75w% and 1.5w%) show no significant difference compared to water while higher concentrations clearly deteriorate the thermal performance. Moreover, as the transition occurs at a lower velocity for water (Fig. 10a), a better heat transfer coefficient is reached for the same pumping power. In turbulent flow, the thermal performance decreases with increasing concentration. This nanofluid could still be used in laminar regime at low particle concentrations in applications where the pumping power is not of great concern.

Conclusion

Convective flows of nanofluids in the transitional region have received little attention in the previous works. Motivated by this fact, experiments were conducted to determine the heat transfer coefficient and the pressure drop in transitional flow through a circular tube with constant heat flux boundary. The accuracy of the present set-up has been successfully verified through a series of experiments with deionized water. Indeed, the theoretical results are included in the range of the experimental error bars for both ∆P and hi. As a result, five concentrations of an Al2O3-water nanofluid have been studied. It has been shown that the presence of the nanoparticle does not affect the onset of the laminar-turbulent transition for this nanofluid up to 5w%, as the critical Reynolds number is around 2300 ± 50 for all tested fluids. Therefore, it suggests that this nanofluid behaves like a single-phase fluid in this range.

This assumption is strengthened by the good agreement between the experimental results for the friction factor and the theoretical relationship in laminar flow. However, the increase in pressure drop compared to water exceeds the enhancement of the convective heat transfer coefficient so that the thermal performance is lowered when these nanoparticles are added to water whatever the flow regime.

To overcome this problem, one could use nanoparticles with better thermal properties than Al2O3 like pure metals. Another possible solution would be to reduce the viscosity of the nanofluids using larger nanoparticles.

Figure 1 :

 1 Figure 1: Dynamic viscosity versus temperature for different concentrations of nanofluid.

Figure 1

 1 Figure1presents the dynamic viscosity (obtained by multiplying the kinematic viscosity with the density determined by Eq. (1)) as a function the temperature. The measured viscosity of water is in very good accordance with the literature[START_REF] Huber | New International Formulation for the Viscosity of H2O[END_REF] as the maximum relative deviation is 0.84%. The viscosity of the nanofluid clearly increases with mass concentration and its evolution as a function of the temperature follows the same trend as water. Actually, the relative viscosity B = 67 / >7 is

Figure 2 :

 2 Figure 2: Relative viscosity versus volume fraction.

Figure 2

 2 Figure2compares the experimental results with Eq. (5) and two models from the literature[START_REF] Einstein | Eine neue bestimmung der molekuldimensionen[END_REF][START_REF] Khanafer | A critical synthesis of thermophysical characteristics of nanofluids[END_REF]. It can be seen that the theoretical model of Einstein[START_REF] Einstein | Eine neue bestimmung der molekuldimensionen[END_REF] is not appropriated for this nanofluid while the correlation of Khanafer and Vafai[START_REF] Khanafer | A critical synthesis of thermophysical characteristics of nanofluids[END_REF] obtained from curve-fitting the measurements of Pak and Cho[START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles[END_REF] (Al2O3-water nanofluid, dNP = 13 nm) overestimates the present results. This discrepancy can be due to the difference of pH allowing a better dispersion of the nanoparticles in[START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles[END_REF], decreasing their effective diameter.

Figure 1 :

 1 Figure 1: Schematic diagram of the experimental set-up.

Figure 4 :

 4 Figure 4: Thermograms of the copper tube during heating (4a) and at rest (4b) and temperature profile (4c). The measurement area represents 6 × 2.4 mm 2

Figure 5 :

 5 Figure 5: Pressure drop in the test section versus Reynolds number for water.

Figure 6 :

 6 Figure 6: Convective heat transfer coefficient versus Reynolds number for water.

Figure 7 :

 7 Figure 7: Convective heat transfer coefficient versus Reynolds number for water and different concentrations of Al2O3-water nanofluid (a) with a zoom on the laminar region (b).

4. 2 .Figure 8 :

 28 Figure 8: Pressure drop in the test section versus Reynolds number for water and different concentrations of Al2O3-water nanofluid.

Figure 9

 9 Figure8shows the pressure drop in the test section as a function of the Reynolds number. One can note that the pressure drop significantly increases with the mass concentration whatever the flow regime so that at 5w%, it is more than doubled compared to that of water. Even at a very low mass concentration of 0.75%, the observed increase in pressure drop reaches 7%, which is much higher than the intensification of the convective heat transfer coefficient. The increase in pressure drop is due to the higher viscosity of the nanofluid compared to that of water creating more friction losses at the pipe wall. As the volumetric flow rate and the pressure drop of this nanofluid are higher than these of water, a greater pumping power is required to reach the same Reynolds number. This means that it can be difficult to obtain a fully turbulent flow with high concentrations of nanofluids. Figure9depicts the evolution of the friction factor as a function of the Reynolds number in laminar flow. The data points for the different concentrations of nanofluid are close to those of water. Moreover, they are in relatively good agreement with the theoretical law (eq. 15) up to a Reynolds number of 1500, confirming that the behaviour of this nanofluid is close to that of single-phase fluids. The error bars are bigger at low Reynolds numbers because the uncertainty of the pressure drop is a percentage of the full scale.

Figure 9 :

 9 Figure 9: Friction factor versus Reynolds number for water and different concentrations of Al2O3-water nanofluid in laminar regime.

Figure 10 :

 10 Figure 10: Convective heat transfer coefficient versus pumping power for water and different concentrations of Al2O3-water nanofluid (a) with a zoom on the laminar region (b).

Table 1 :

 1 Relative uncertainties

Acknowledgements

The authors are grateful to Didier Caron for his technical support. They also thank Maryline Parmentier for sharing with them the viscometer.

Appendix A. Measurement uncertainties

When a quantity X is recorded, (U, I, ∆P, Qv, Te, and T NF, in ), the best estimate is the arithmetic mean of the n measurements [START_REF] Taylor | An introduction to error analysis : the study of uncertainties in physical measure-ments[END_REF]:

The associated type A uncertainty δXA caused by random fluctuations of the measurements with a 95%confidence interval is determined by [START_REF] Taylor | An introduction to error analysis : the study of uncertainties in physical measure-ments[END_REF]:

where t 95%,n-1 is the value of the Student t-distribution with n -1 degrees of freedom at 95% confidence and σ(X) is the standard deviation defined by :

When a quantity q is derived from N possibly correlated input quantities Xi, i.e. q = f (X1, X2, ..., XN ), the combined type A uncertainty of q is :

with ri,j the correlation coefficient : .5) where σ(Xi, Xj) is the covariance between Xi and Xj.

On the other hand, type B uncertainty calculations due to systematic errors are based on manufacturers' specifications when available. Finally, type A and type B uncertainties are combined to obtain the overall uncertainty of q [START_REF] Taylor | An introduction to error analysis : the study of uncertainties in physical measure-ments[END_REF]: .6)