Nicolas Courbin
email: nicolas.courbin@cefe.cnrs.fr

Tendai Chinho

Lorien Pichegru

Ambalika Verma-Grémillet

Clara Péron

Peter G Ryan

David Grémillet

The dance of the Cape gannet may contain social information on foraging behaviour

Keywords: Bayesian model averaging, ceremonial dance, foraging movements, GPS Duration

Social information percolates through a variety of channels to influence animal decisionmaking, with a notable incidence on reproductive and feeding success. Colonial central place foragers can reduce time to locate ephemeral food patches and/or increase foraging rate by following their informed peers, parasitizing direction of returning successful foragers, or being intentionally informed on distant food locations at the colony (e.g., the waggle dance of the honeybee). Ceremonial behaviours may also deliver social foraging information between mates, which can spread inadvertently to neighbours. Here we tested for information display in Cape gannets (Morus capensis), a socially monogamous species, during the elaborate dance ceremony performed each time a partner returns to the nest during the breeding season. We tracked fine-scale foraging behaviour of gannets using bird-borne GPS recorders, and videorecorded their subsequent dance ceremony, which involved up to 14 different displays. As we hypothesized, dance characteristics were associated with foraging trip features. Notably, overall dance duration was negatively linked to foraging trip duration, which was highly positively correlated with foraging range, foraging path length and time spent foraging during the trip. Overall dance duration was also negatively linked with distance to the main foraging grounds. Additionally, the duration of the preening behaviour was related to the bearing of the main feeding spot. The latter relationship was supported by a Bayesian model averaging analysis, allowing inferences robust to multiple comparisons. Overall, ceremonial behaviour may provide social foraging information on feeding locations, while evidence for further information transfer to the mate or neighbours was not tested here. Frequent updating on prey spatial distribution, inadvertently communicated or not, should be particularly valuable for predators tracking ephemeral prey patches, providing an additional advantage to colonial living. Our results may have strong implications for cultural evolution in animal societies.

tracking, social information, Morus capensis, South Africa.

4

Animals may use the behaviour of their surrounding conspecifics or heterospecifics to acquire and update information about the value of alternative options, and are thereby provided with intentionally communicated or inadvertent social information [START_REF] Danchin | Public information: from nosy neighbors to cultural evolution[END_REF][START_REF] Rieucau | Exploring the costs and benefits of social information use: an appraisal of current experimental evidence[END_REF]. Inadvertent social information has initially been observed in foraging situations (e.g. [START_REF] Ward | The importance of certain assemblages of birds as 'information-centres' for food-finding[END_REF][START_REF] Greene | Individuals in an osprey colony discriminate between high and low quality information[END_REF][START_REF] Valone | Group foraging, public information, and patch estimation[END_REF], later applied in the context of breeding habitat selection [START_REF] Boulinier | The use of conspecific reproductive success for breeding patch selection in terrestrial migratory species[END_REF][START_REF] Doligez | Public information and breeding habitat selection in a wild bird population[END_REF], and finally expanded to information about the quality of any feature [START_REF] Dall | Information and its use by animals in evolutionary ecology[END_REF]. The use of social information is widespread across animals, including humans, and is predicted to be an essential promoter of cultural evolution [START_REF] Danchin | Public information: from nosy neighbors to cultural evolution[END_REF][START_REF] Rieucau | Exploring the costs and benefits of social information use: an appraisal of current experimental evidence[END_REF]. Social information use is particularly marked in colonial species, where it serves predator defence, defence against brood parasites, behavioural coordination in breeding and migration, breeding habitat selection, sexual and commodity selection, and optimal foraging [START_REF] Evans | Information use in colonial living[END_REF]. In the latter case a series of working hypotheses foresee evolutionary benefits to colonial breeding and communal roosting (reviewed in [START_REF] Evans | Information use in colonial living[END_REF]. Notably, these hypotheses propose that individuals provide and share information on the location of food sources (Information Centre Hypothesis: [START_REF] Ward | The importance of certain assemblages of birds as 'information-centres' for food-finding[END_REF][START_REF] Greene | Individuals in an osprey colony discriminate between high and low quality information[END_REF]. The working hypotheses also posit that individuals benefit from feeding in groups, and hence use colonies to recruit conspecifics (Recruitment Centre Hypothesis: [START_REF] Richner | Communal life: honest signaling and the recruitment center hypothesis[END_REF]Danchin & Richner, 2001), or suggest that subordinate individuals benefit from information on feeding locations, whereas successful foragers reduce predation risk by occupying safer central positions within the colony (Two-strategies Hypothesis: [START_REF] Weatherhead | Two principal strategies in avian communal roosts[END_REF]). Although there is still considerable debate regarding these hypotheses (see the fervent exchanges between Danchin & Richner, 2001;[START_REF] Marzluff | Raven roosts are still information centres[END_REF]Richner & Danchin, 2001), uninformed individuals may thereby increase their foraging success, by physically following informed conspecifics towards prey patches (e.g., evening bat Nycticeius humeralis; [START_REF] Wilkinson | Information transfer at evening bat colonies[END_REF]; Eurasian griffon vulture Gyps fulvus; [START_REF] Harel | Social foraging and individual consistency in following behaviour: testing the information centre hypothesis in free-ranging vultures[END_REF]. Information may also be delivered through visual cues, at the colony or its close proximity, based on flight directions of returning conspecifics [START_REF] Weimerskirch | Use of social information in seabirds: compass rafts indicate the heading of food patches[END_REF][START_REF] Racine | Evidence of social information on food location in a ring-billed gull colony, but the birds do not use it[END_REF], or afar the colony when individuals attract each other in the vicinity of food patches (Local Enhancement: [START_REF] Krebs | Colonial nesting and social feeding as strategies for exploiting food resources in great blue heron (Ardea herodias)[END_REF][START_REF] Buckley | Food finding and the influence of information, local enhancement, and communal roosting on foraging success of North American vultures[END_REF]. This later mechanism has been identified repeatedly in colonial seabirds (e.g., [START_REF] Weimerskirch | Use of social information in seabirds: compass rafts indicate the heading of food patches[END_REF][START_REF] Machovsky-Capuska | The contribution of private and public information in foraging by Australasian gannets[END_REF]Thiebault, Mullers, Pistorius, & Tremblay, 2014a;[START_REF] Boyd | Effectiveness of social information used by seabirds searching for unpredictable and ephemeral prey[END_REF].

Individuals can also benefit from social information without physically following their conspecifics. Notably, some species have evolved signalling for social information transfer. This is famously the case in honeybees (Apis mellifera) that inform each other on the location of feeding spots in a complex waggle dance at the hive [START_REF] Von Frisch | The dance language and orientation of bees[END_REF][START_REF] Riley | The flight paths of honeybees recruited by the waggle dance[END_REF][START_REF] Grüter | Informational conflicts created by the waggle dance[END_REF] or in ants (Lasius niger) and naked mole-rats (Heterocephalus glaber) that use scent trails to find food [START_REF] Judd | Naked-mole rats recruit colony mates to food sources[END_REF]Evinson, Petchey, Beckerman, & Ratnieks, 2008). However, it has been far more challenging to establish whether colonial vertebrate animals also provide information about distant foraging grounds through their behaviour at the breeding site in absence of signalling, as recently proposed for seabirds [START_REF] Weimerskirch | Seabirds-individuals in colonies[END_REF]. Some seabirds, such as albatrosses and gannets, are well-known for their elaborate courtship behaviour [START_REF] Townsend | Courtship in birds[END_REF][START_REF] Pickering | Courtship behavior of the wandering albatross Diomedea exulans at Bird Island, South Georgia[END_REF], not only while mating, but also each time one of the partners returns to the nest after a foraging trip at sea (Fig. 1, see Movie A1). The gannet dance ceremony involves a series of displays, eloquently described by [START_REF] Townsend | Courtship in birds[END_REF] and later Nelson (1978a). Gannets can vocally identify each other [START_REF] White | Acoustic basis for individual recognition by voice in the gannet[END_REF][START_REF] Krull | Analyses of sex and individual differences in vocalizations of Australasian gannets using a dynamic time warping algorithm[END_REF] and recent studies indicate that they are visually sensitive to coloured signalling of conspecific plumage features, suggesting the importance of observing partners and breeding neighbours in e.g. sexual selection [START_REF] Ismar | Sexual plumage dichromatism in a size monomorphic seabird[END_REF]. Their high visual capabilities may allow them to recognize each other and to potentially detect the information provided. However, until very recently, gannet dance had only been studied in the context of ecological and evolutionary processes occurring within the breeding colony, such as competition for breeding space and sexual selection [START_REF] Nelson | The Sulidae: gannets and boobies[END_REF], and sexual and individual recognition [START_REF] White | Acoustic basis for individual recognition by voice in the gannet[END_REF][START_REF] Krull | Analyses of sex and individual differences in vocalizations of Australasian gannets using a dynamic time warping algorithm[END_REF]. This was probably due to the lack of precise information about the foraging movements of individuals at sea. Thanks to improvement of fine scale-tracking technologies, this gap has now been filled. Notably, [START_REF] Machovsky-Capuska | The contribution of private and public information in foraging by Australasian gannets[END_REF] used Global Positioning System (GPS)-tracking of breeding Australasian gannets (Morus serrator) during their trips at sea, to test firstly for linkages between foraging trip characteristics and those of the ceremonial dance displayed upon the birds' return to the nest site. They then tested whether partners leaving the nest used information transferred during the dance. The authors did not find significant relationships, but their study suffered from a small sample size (n = 6 breeding pairs), leaving room for potential type II errors (i.e., falsely infer the absence of true relationships), and from the fact that they reduced the elaborate gannet dance (see Movie A1) to only one display, i.e., the number of bill touches, in addition to dance duration.

We therefore repeated these observations with a larger sample size of Cape gannets (Morus capensis), also by using high-resolution GPS tracking of bird foraging movements in combination with video recordings of the ceremonial dance displayed upon arrival of GPStracked birds at the nest. Here, we focused on the first part of the process of social information transfer (information display). Thereby, considering the whole ceremonial dance complexity, we hypothesized that gannet dance may contain social information related to feeding grounds. Such information should decrease uncertainties in food localization from an evolutionary perspective. We thus expected significant links between foraging trip characteristics and features of the gannet dance ceremony.

METHODS

The study was conducted in October-November 2013-2016 on Malgas Island (33.05° S, 17.93° E) in the Western Cape, South Africa, which hosts ca. 19 000 breeding pairs of Cape gannets [START_REF] Crawford | Winners and losers-responses to recent environmental change by South African seabirds that compete with purse-seine fisheries for food[END_REF].

Ethical Note

All experiments were performed under permit from South African National Parks with respect to animal ethics (N° RYAP/AGR/001-2002/V1).

Breeding individuals raising chicks 2-6 weeks' old were caught at the nest and equipped with GPS trackers (CatTrack1, Catnip Technologies, Hong-Kong, PRC, 70×32×15 mm, 30 g, 1.2% of a mean bird body mass of 2.5 kg) attached to the lower back with waterproof Tesa® tape only. Indeed, unlike glue which does have an impact on the plumage, tape can be completely removed when recovering the GPS from the bird. Cape gannets are relatively tame at the nest and can be easily caught using a neck-hook mounted on a 3m pole.

Once the bird is caught with the pole, it is gently walked away from its nest, to the edge of the colony, where it is caught by hand. The procedure is used twice for each bird (deployment and recovery of the GPS data loggers). Handled birds return straight to their nest when released. Capture may cause momentarily disturbance of the neighbours, which may walk a few steps away from their nests, but also come back onto them within seconds/minutes. We recaptured all birds after 1-3 days, and removed the loggers as well as all tape. We minimized handling stress as much as possible, by handling birds in <10 min, by keeping them whenever 8 possible in the shade to avoid over-heating, and by covering their heads. The fieldwork procedure closely followed [START_REF] Grémillet | Offshore diplomacy, or how seabirds mitigate intra-specific competition: a case study based on GPS tracking of Cape gannets from neighbouring colonies[END_REF], who measured no effect of devices on foraging trip duration. GPS-tagged birds were filmed upon their return to the nest. Unlike most northern gannets (Morus bassanus), Cape gannets can be approached to a few meters without resulting in any observable effect. Filming and observations, which were performed in the open and away from the edge of the colony, at an approximate distance of 10m, therefore did not disturb any bird of the colony.

Foraging Trip Characteristics

We followed the foraging movements of 79 Cape gannets (one individual per breeding pair) using GPS loggers between 2013 and 2016. GPSs recorded position and speed at 30-s to one minute intervals. All GPS tracks were rediscretized to regular one minute intervals with the adehabitatLT package [START_REF] Calenge | The package adehabitat for the R software: a tool for the analysis of space and habitat use by animals[END_REF] for the R software (R Core Team, 2016). We only retained foraging trips for which we also had complete video recordings of gannet dance upon their subsequent return to the nest (see Gannet dance ceremony section), resulting in a subset of one foraging trip per individual for 28 birds, with 6 birds in 2013, 2 in 2014, 9 in 2015 and 11 in 2016 (Fig. 2). Yet, information gathered through GPS tracking, for birds which could not be videoed upon their return, was not lost in any manner since it was used in a long-term monitoring program of Cape gannets as sentinels of environmental change in the Benguela upwelling (e.g. [START_REF] Cohen | Changes in prey availability impact the foraging behaviour and fitness of Cape gannets over a decade[END_REF].

We defined a foraging trip as a round trip from the nest of an individual, consisting of all its at-sea GPS locations. We first characterized each foraging trip by its average range (km, i.e., the average distance to the nest across at-sea locations), its maximum range (km, to the farthest at-sea location), its total travelled distance (km, i.e., the cumulative distance between consecutive at-sea locations) and its duration (h). We also calculated time spent in foraging during the trip (min), and the distance (km) and direction (degree) to the main foraging spot, i.e., the average distance and direction from the colony across locations within a single foraging event and constituting the foraging spot where gannet spent most of their time consecutively during the trip. Given the ephemeral nature of food patches at sea, information on the last foraging spot may seem more suitable in a context of information exchange. In our study, the last and main foraging spots were highly correlated both in their direction (circular correlation r = 0.97, N = 28) and distance (Pearson's correlation r = 0.64, N = 28) to the colony, and were identical or spatially close (median = 7.5 km, IQR = 0 -26.3 km, N = 28).

Therefore, the main foraging spot was a source of both recent and qualitative information (i.e., most valuable patch during the trip inferred from the amount of time spent feeding there). We distinguished the foraging phases from other movement phases (resting and commuting) along the trips with a segmentation-clustering method, which outperforms methods previously and commonly used [START_REF] Patin | Identifying stationary phases in multivariate time series for highlighting behavioural modes and home range settlements[END_REF]; see full details in Appendix and Figs A1, A2). To avoid correlation issues, we used only trip duration, distance and direction to the main foraging spot in the subsequent statistical analyses (Appendix Table A1).

Gannet Dance Ceremony

We filmed adult Cape gannets that had been equipped with GPS trackers as they returned to the colony, to document the full dance ceremony upon their arrival at the nest site (see Movie A1). As noted by [START_REF] Machovsky-Capuska | The contribution of private and public information in foraging by Australasian gannets[END_REF] this is extremely challenging, as it requires one to start filming tagged birds before they arrive at the nest, their exact time of arrival at the colony being unpredictable. To improve the detection of equipped bird returning to the nest, they were marked with a small blue/red spot (~10cm of diameter) on the head and chest using a biocompatible marker which faded after a few days. Studied birds were selected at random within the colony, and after a sit-and-wait period for the observer, the dance was filmed using a Nikon D90, or a Sony HD Camcorder set on a tripod. We collected a total of 79 behavioural sequences for 79 GPS-tagged birds between 2013 and 2016. However, because we missed filming the start of the dance, and/or because the foraging trip was not entirely recorded (rare cases), or both reasons, we retained only 28 complete sequences for which we also had complete GPS-tracking information on the whole preceding at-sea voyage.

We considered the dance ceremony in a broad sense (i.e., not only in an inter-sexual context) including all behaviours displayed during changes of guard at the nest between the mates. Our approach allowed us to avoid making a strong a priori hypothesis on the role of the different dance features (i.e., we did not exclude some dancing behaviours potentially implied in an inter-sexual context) and thus avoided potential pitfalls omitting key relationships between dance parameters and foraging features. More specifically, the start of the dance was defined as the time of arrival of the returning gannet at the nest (bird within ≤2m of its partner), either by spot-landing at the nest site, or by landing at a distance and walking towards the nest. The GPS-equipped gannet then displayed some, or all of 14 behaviours. "Nape-biting": one partner grabs the other by the neck and shakes its head sideways. "Scissors/Bowing": both partners face each other and cross their beaks and heads, moving them repeatedly right and left (i.e., the bill fencing, Fig. 1), regularly interrupting this display to bow towards the ground as in a reverence. "Sky pointing": the bird stretches its neck, head and beak upwards. "Head-shake": heads-shake without upward beak, hence different from "Scissors". "Mutual-preening": partners preen each other with their beaks. "Preening": the GPS-equipped bird preens itself. "Shaking": the GPS-equipped bird ruffles its feathers. "Flapping": wing-flapping in the GPS-equipped bird. "Chick grooming": the GPSequipped bird preens the chick with its beak. "Chick-feeding": the GPS-equipped bird feeds the chick. "Neighbour fight": aggressive behaviour of the GPS-equipped bird towards breeding neighbours, in the form of grabbing attempts. "Yawning": the GPS-equipped bird yawns. "Pause": none of the other listed behaviours are recorded, and the bird is standing/sitting still. "Changeover": at some stage during the dance ceremony, the returning (GPS-equipped bird) climbs onto the nest, and takes over chick-guarding duties. The end of the dance ceremony was set when the off-duty partner left the nest (>2m away from the nest), leaving the GPS-equipped bird in charge of chick-guarding. All video analyses were performed by the same person (AVG) with Avidemux 2.6, which allows frame-by-frame visualization, and the durations and the start and end times of each behavioural pattern were recorded to the nearest 1/100s. AVG was naïve to the hypothesis tested and blinded to the foraging/travel data.

Linking Foraging Movements and Dancing Behaviour

We tested for the links between foraging movements and dance features based on the 28 birds for which we had both the complete dance ceremony and the entire preceding foraging trip (N = 28). To avoid that correlations among dance ceremony features blurred our understanding of their various relationships with trip features, we proceeded in a two-step approach. First, our aim was to model how dance ceremony duration varied with duration of the previous foraging trip, distance and direction to the main foraging spot. Due to high correlation between foraging covariates (Appendix Table A1), we fitted three independent Cox proportional hazards models [START_REF] Cox | Regression models and life tables (with discussion)[END_REF]. Cox regressions were estimated with the survival package (coxph function; [START_REF] Therneau | Local enhancement in a seabird: reaction distances and foraging consequence of predator aggregations[END_REF] in R software (R Core Team, 2016) and had the form:

ℎ = ℎ equation 1
where ℎ is the hazard function, i.e., the risk that the dance ceremony ended at time t, is a vector, is the estimated coefficient for the continuous foraging trip covariate , and ℎ is an unspecified baseline hazard function. The proportional hazard assumption was satisfied for each regression. We accounted for multiple comparisons using Benjamini-Yekutieli corrections [START_REF] Benjamini | The control of the false discovery rate in multiple testing under dependency[END_REF]. For more convenience in interpretation, we present in the results the predicted total duration of the dance ceremony (not the risk of terminating the dance) given the foraging covariates, i.e., the duration-based quantities estimated from Cox regression with the coxed package (coxed function; [START_REF] Kropko | coxed: duration-based quantities of interest for the Cox proportional hazards model[END_REF].

Second, we used Bayesian Model Averaging (BMA) analyses to evaluate relationships between the patterns of the foraging trips and those of the dance ceremony.

| = ∆ ⁄ ∑ ∆ ⁄ , equation 2
where is the Bayesian information criterion (BIC; [START_REF] Schwarz | Estimating the dimension of a model[END_REF] for the model and comparisons in a BMA framework [START_REF] Neath | Bayesian multiple comparisons and model selection[END_REF]. The number of candidate models being rapidly high (2^n umber of predictors ) in a model averaging process, BMA uses the leaps and bounds algorithm [START_REF] Furnival | Regression by leaps and bounds[END_REF] and an Occam's window to retain a set of 'best' candidate models [START_REF] Volinsky | Bayesian model averaging in proportional hazard models: assessing the risk of a stroke[END_REF][START_REF] Hoeting | Bayesian model averaging: a tutorial[END_REF].

We conducted three distinct BMA analyses, using either BMA for Cox proportional hazards models for the Trip duration response variable, or using BMA for linear models for

Distance to the main foraging spot and Direction to the main foraging spot response variables. Distance to the main foraging spot was log transformed to satisfy normality of residuals. Considering the high correlation between some foraging trip features (r ≥ 0.7, N = 28; Appendix Table A1), we limited our analyses to the effect of dance ceremony patterns on these three key parameters of foraging trips, commonly used. We considered direction to the main foraging spot as a linear (not circular) dependent variable because colony was settled along an approximate straight coastline (Fig. 2). Consequently, no high direction values occurred (max = 209°) and values had a range close to 180° (range was of 195°). We characterized gannet dance patterns by the proportions of time spent in eight different behaviours (i.e., chick grooming, change over, head shake, scissor bowing, nape bite, mutual preening, individual preening and sky pointing) and used them as predictors (Appendix Tables A2 andA3). We excluded behaviours that represented <1%, on average, of dance duration, and the proportion of time spent to rest (pause behaviour) from BMA analyses (Appendix Tables A3 andA4). All predictors were mean-centred and scaled. Candidate models selected by BMA did not contain strongly correlated predictors (r < 0.7, N = 28;

Appendix Tables A2 andA5). All predictors had an equal weight of 0.5 a priori. Finally, we calculated a sharp confidence interval of 99% for Bayesian parameter estimates, to account for the fact that three BMA analyses were fitted from the same set of predictors. BMA was performed with the BMA package [START_REF] Raftery | BMA: Bayesian Model Averaging[END_REF] and BAS package [START_REF] Clyde | Bayesian variable selection and model averaging using Bayesian adaptive sampling[END_REF] for the R software (R Core Team, 2016).

RESULTS

Foraging Trip and Dance Ceremony Characteristics

Average values showed that GPS-tracked Cape gannets foraged within 108 km of the breeding colony, mainly to the West and the South, and travelling on average 346 km during trips lasting 22 h (Fig. 2, full details on foraging parameters are provided in Appendix Table A4). Birds met their main foraging spot on average at 77 km from the colony, and displayed active foraging for 6.45 h across their trip (Fig. 2). Trip duration was highly positively correlated with foraging range, foraging path length and time spent in foraging during the trip (Appendix Table A1). Classification of the different behavioural modes were consistent with biological expectations, and we found that gannet moved with high tortuosity and average mean smoothed speed of 16.81 km.h -1 during the feeding phases (see full details in Appendix and Figs A1, A2).

On average, upon their return to the nest, birds displayed a dance ceremony which lasted 5.7 min, with, by order of importance in terms of time, 60.1 s of individual preening (13% of total dance duration on average), 45.2 s of mutual preening (9%), 44.0 s of scissors/bowings (23%), 10.8 s of head shaking (4%), 7.9 s of chick grooming (2%), 6.7 s of sky pointing (2%), 2.3 s of chick feeding (<1%), 1.3 s of nape biting (<1%), 1.0 s of wing flapping (<1%), 0.8 s of shaking (<1%), 0.4 s of neighbour fighting (<1%), 0.3 s of yawning (<1%), and 156 s in pause (43%; Appendix Table A4).

Evidence for Related Foraging Movements and Dancing Behaviour

Dance ceremony duration was negatively linked to the duration of the previous foraging trip, with a decrease of ~11 s (range 15 to <1 s) in dance duration for each additional hour spent at sea (Fig. 3a, Appendix Table A6). Dance duration also decreased with an increase of the distance to the main foraging spot, with a decrease of ~2 s (range 3 to 1 s) in dance duration for each supplementary kilometre between the nest and the main feeding area (Fig. 3b, Appendix Table A6).

Each BMA analysis indicated moderate model uncertainty (highest posterior model probability was 0.134 for trip duration model, 0.260 for the distance to the main foraging spot model, and 0.331 for the direction to the main foraging spot model, Appendix Table A5), emphasizing the need to consider it within a BMA framework for robust inferences. Trip duration not only impacted dance duration, but also the structural properties of the dance ceremony. Thereby, the average model for trip duration indicated a posterior effect probability of 0.978 for the scissors/bowings predictor (Appendix Table A3). Gannets spent higher proportion of their dance ceremony displaying scissors/bowings movement heads when the foraging trip duration increased (i.e., when the risk of returning to the nest was low; Fig. 4a, Appendix Table A3). Considering that an increase in foraging trip duration was related to a decrease in dance duration and an increase in the proportion of scissors/bowings, gannet seemed to spend an incompressible amount of time displaying scissors/bowings during the dance ceremony. Consistently, dance duration was uncorrelated with the total time spent in scissors/bowings (r = 0.06, N = 28, P = 0.75), a variable with a relatively low dispersion (Appendix Table A4), but was negatively correlated with the proportion of time in scissors/bowings (r = -0.58, N = 28, P = 0.001). Trip duration was not related to any other features of dance ceremony.

Distance to the main foraging spot had no significant impact on the proportions of the different behaviours displayed during the dance ceremony (Appendix Table A3). In contrast, individual preening behaviour might be an indicator of the direction to the main foraging spot.

Indeed, gannets spent a lower proportion of their time preening during the dance ceremony when they had mainly foraged to the South than to the West of the colony (Fig. 4b, Appendix Table A3). This result was supported by a posterior effect probability of 1.000 for the individual preening predictor (Appendix Table A3). Importantly, the direction to the main foraging spot was not related to dance duration (r = -0.32, N = 28, P = 0.09). Using a principal component analysis confirmed the results of BMAs (Appendix Fig. A3).

DISCUSSION

We identified significant linkages between at-sea Cape gannet foraging movements and their subsequent dance ceremony when returning to the nest site, using a two-step analytical framework robust to multiple comparisons. Notably, we found that dance ceremony is an indicator of foraging effort and direction and distance to the recent and most used prey patch visited during the last trip. It is, to the best of our knowledge, the first evidence that ceremonial behaviour might be a channel of social foraging information on distant feeding locations at a vertebrate breeding aggregation. Our correlative approach did not allow testing whether dance features constitute social information per se. Indeed, this information is probably supplied inadvertently through the dance; yet, we do not know if the mate and neighbours receive and use the foraging information displayed during the dance.

Gannet Dance Could Socially Inform Conspecifics on Prey Patch Location

Overall dance duration decreased with increasing duration of the preceding foraging trip and distance to the main foraging spot. Gannet foraging trip duration is tightly linked to structural (foraging range and foraging path length) and functional (time spent in foraging during the trip; Appendix Table A1) trip features, and hence a reliable proxy for foraging effort [START_REF] Lewis | Evidence of intra-specific competition for food in a pelagic seabird[END_REF]. Shorter dancing duration could constitute a social cue for conspecifics, of how hard gannets foraged at sea and at which distance they mainly fed.

The negative correlation between the dance duration and the duration of the preceding foraging trip also suggests that the partner which had been staying at the nest of the GPSequipped bird to guard their chick was in a hurry to start its own foraging sequence and/or that the incoming bird was exhausted, and each second seemed to count in that respect. This starvation hypothesis of the waiting partner (or exhaustion hypothesis of the incoming bird) may be reinforced by the fact that the variance in dance duration was much greater for short and medium foraging trip durations (Fig. 3a). Overall, even if the hungry state of the waiting partner is the main mechanism driving dance duration, the outcome information of the dance remains the same for the potentially eavesdropping neighbours: short dance duration corresponds to long at-sea foraging trip. Cape gannets competing with fisheries are experiencing nutritional stress leading to lower body condition [START_REF] Grémillet | Starving seabirds: unprofitable foraging and its fitness consequences in Cape gannets competing with fisheries in the Benguela upwelling ecosystem[END_REF]. If birds are in a hurry to feed this may lead to a reduction in time available for information displays at the colony, which in turn may trigger a negative feedback loop on their population-level foraging performance. This negative feedback could be enhanced by the fact that an incompressible part of the ceremonial dance seems to be dedicated to scissors/bowing behaviour. This incompressibility of scissors/bowing behaviour reveals a key parameter of the gannet dance, perhaps more related to courtship display [START_REF] Townsend | Courtship in birds[END_REF], pair bonding [START_REF] Nelson | The Sulidae: gannets and boobies[END_REF], mate recognition [START_REF] Meseth | The dance of the Laysan albatross (Diomedea immutabilis)[END_REF] or individual's condition assessment rather than foraging information display, echoing previous work by Machovsky-Capuska et al.

(2014).

Moreover, gannets preened more during the dance ceremony when they had travelled to the West than to the South. By preening, gannet may convey inadvertent cues on food patches direction to others. Such differences in preening behaviour might be explained by the fact that Cape gannets from the study colony preferentially target sardines and avoid purseseine fishing vessels when foraging in the South of their at-sea home-range, and feed on fishery wastes behind hake trawlers when foraging to the West [START_REF] Tew Kai | Are Cape gannets dependent upon fishery waste? A multi-scale analysis using seabird GPS-tracking, hydro-acoustic surveys of pelagic fish and vessel monitoring systems[END_REF]. Indeed, birds attending trawlers are more likely to get into contact with fish oil and other elements affecting their plumage, and we speculate that this might then lead Cape gannets to preen more often. Nevertheless, for the partner or neighbours observing the displaying individual, deducting the direction to the main foraging spot from preening behaviour requires strong cognitive abilities (e.g., associative learning, memory), which remain to be measured in seabirds.

One important condition for the emergence of social information is the temporal match between the duration of a prey patch occurrence in the environment and the time required for an informed conspecific to revisit the patch. If prey patches disappear before conspecifics can revisit the patches, information on patch location is useless. The duration of a prey patch is highly variable in our study area, and the number of prey patches is probably more limiting than the size of individual patches, thereby determining encounter rates of Cape gannets with their prey (Discussed in [START_REF] Thiebault | How to capture fish in a school? Effect of successive predator attacks on seabird feeding success[END_REF]. We found that the main foraging patch is also often the last foraging patch, or at least close to it, and hence gannet dance provides recently updated information (median time elapsed since departure from the main foraging spot was 2.4h). When prey patches occur for sufficiently long time, allowing the same individual to revisit them, sharing information is still valuable if predators forage more efficiently in groups (Danchin & Richner, 2001), as it is the case for northern and Cape gannets [START_REF] Wakefield | Space partitioning without territoriality in gannets[END_REF][START_REF] Thiebault | How to capture fish in a school? Effect of successive predator attacks on seabird feeding success[END_REF].

Conspecifics may also acquire some information from the dance ceremony by observing for how long their neighbours are away at sea, yet this requires keeping track of time at the scale of hours, even days, something which will not always be possible since those neighbours may also have to leave the colony to forage in the meantime. Contrastingly, the dance ceremony provides inadvertent information on a wide range of foraging parameters within just a few minutes. Considering the large number of available neighbours at our Cape gannet study colony, the accumulation of easily-observable dance ceremonies could then provide permanently-updated robust knowledge on foraging grounds within the colonyspecific home-range, depending on oceanographic conditions.

On the Relevance of Information Displays

Yet, signals on foraging behaviour provided during the dance of the Cape gannet are far less subtle and detailed than those generated by honeybees and other insects, which may actively provide the accurate direction and distance to the resource, but also information on its chemical properties [START_REF] Riley | The flight paths of honeybees recruited by the waggle dance[END_REF][START_REF] Leadbeater | Social learning in insects -from miniature brains to consensus building[END_REF][START_REF] Grüter | Informational conflicts created by the waggle dance[END_REF].

Further, despite all our efforts to gain an adequate sample size, the statistical power of some of our analyses may remain limited. Distinguishing between inadvertent (as suggested here)

and intentional information display through the gannet dance is therefore particularly challenging. Moreover, foraging information contained in gannet dance might be only relevant if it is actually used by birds (mate or neighbours), and a previous study on ringbilled gull (Larus delawarensis) showed that information provided by leaving and arrival flight direction of conspecifics was irrelevant at the colony [START_REF] Racine | Evidence of social information on food location in a ring-billed gull colony, but the birds do not use it[END_REF]. These aspects should be tested through further studies, in vertebrate species performing behavioural ceremonies following, or preceding, foraging activity. To go beyond our correlative approach these studies should, ideally, be experimental [START_REF] Reader | Experimental identification of social learning in wild animals[END_REF], testing for example if dancing behaviour facilitates localization of foraging grounds, by following each individual in the pairs with GPS [START_REF] Machovsky-Capuska | The contribution of private and public information in foraging by Australasian gannets[END_REF], or testing if the starvation state of the waiting partner mainly drives dance duration, with a food supplementation experiment. Also, the dance ceremony could provide feedback on the nutritional state (e.g., body condition, coloration) of the foraging partner [START_REF] Machovsky-Capuska | Sex-specific macronutrient foraging strategies in a highly successful marine predator: the Australasian gannet[END_REF] or be tightly linked to the sex-specific foraging behaviour of gannets (not in trip duration but possibly in feeding areas; [START_REF] Lewis | Sex-specific foraging behaviour in a monomorphic seabird[END_REF].

Social Foraging Information Percolates from Multiple Pathways in Seabirds

Our results echo recent work demonstrating that Cape gannets use social information en route to their feeding sites and gannets anticipate the spatio-temporal occurrence of their prey [START_REF] Pettex | Northern gannets anticipate the spatio-temporal occurrence of their prey[END_REF]. For instance, investigations combining GPS-tracking with bird-borne video recordings demonstrated that gannets which reacted to the presence of conspecifics during their initial flight from the colony to a food patch reached this patch twice as fast as those which did not make use of social information [START_REF] Thiebault | From colony to first patch: processes of prey searching and social information in Cape gannets[END_REF]. This following behaviour has also been clearly established in vultures [START_REF] Harel | Social foraging and individual consistency in following behaviour: testing the information centre hypothesis in free-ranging vultures[END_REF] and is known to reduce searching time in uninformed ant individuals [START_REF] Franks | Teaching in tandem-running ants[END_REF].

Further, our findings are highly coherent with a growing body of work explaining foraging spatial asymmetries in seabirds through the use of social information, available at the breeding colony and/or at sea. Specifically, [START_REF] Grémillet | Offshore diplomacy, or how seabirds mitigate intra-specific competition: a case study based on GPS tracking of Cape gannets from neighbouring colonies[END_REF] noticed that GPS-tracked Cape gannets from neighbouring colonies had mutually exclusive at-sea home ranges during the breeding season, even when neutral diffusion models predicted that those areas should overlap widely. The authors termed this system 'Offshore diplomacy', and their observations have meanwhile been confirmed in a large number of seabird species. Notably, [START_REF] Wakefield | Space partitioning without territoriality in gannets[END_REF] found mutually-exclusive at-sea home ranges across 12 breeding colonies of northern gannets around the British Isles, pointing to space partitioning without territoriality.

The authors developed individual-based models of gannet foraging distributions, which strongly suggested the importance of bird memory and social information for the emergence of foraging asymmetries leading to offshore diplomacy. They speculated that information must be made public, probably inadvertently, by birds at sea and at the breeding site, but did not propose specific behavioural pathways. Our findings indicate that the dance of the gannet may convey social information to the partners and neighbours of gannets returning from the sea. This information channel seems particularly likely and pertinent, since Cape gannets have one of the highest breeding densities of any seabird, with on average 3 nests per square meter [START_REF] Randall | Increasing population of Cape gannets on Bird Island, Algoa Bay, and observations on breeding success[END_REF]. The use of multiple social information pathways could offer a selective advantage to colonial species, as it could minimize uncertainty in locating ephemeral prey patches [START_REF] Harel | Social foraging and individual consistency in following behaviour: testing the information centre hypothesis in free-ranging vultures[END_REF][START_REF] Courbin | Short-term prey field lability constrains individual specialisation in resource selection and foraging site fidelity in a marine predator[END_REF]. Overall, insights gained through our study open exciting perspectives with respect to the emergence of coloniality [START_REF] Danchin | Public information: from nosy neighbors to cultural evolution[END_REF][START_REF] Rieucau | Exploring the costs and benefits of social information use: an appraisal of current experimental evidence[END_REF][START_REF] Evans | Information use in colonial living[END_REF].

performed with the segclust2d package [START_REF] Patin | Segclust2d: bivariate segmentation / clustering methods and tools[END_REF] for R software (R Core Team, 2016), with Lmin set to 5 locations.

Consistently with biological expectations, we found that Cape gannet moved with high tortuosity and moderate speed during the feeding phases (average SD of turning angle of feeding clusters was of 1.28 rad and average mean smoothed speed of feeding clusters was of 16.81 km.h -1 ), with lower tortuosity and lower speed during the resting movement bouts (average SD of turning angle was of 0.27 rad and average mean speed was of 1.63 km.h -1 ), and transited using movements with lower tortuosity and higher speed (average SD of turning angle was of 0.23 rad and average mean speed was of 47.32 km.h -1 ; Appendix Fig. A2). foraging trip features and the 15 dance ceremony features used in our analyses. We showed the correlation circle between the 1 st and 2 nd axes, the 1 st and 3 rd axes, and the 1 st and 4 th axes.

The length of the arrows corresponds to the cos2 value, a meaning of the qualitative representation of the variables on the correlation circle. We used a colour coding to distinguish between the dance duration (red), the dance displays (blue) and the foraging trip features (green).

  . Bayesian model parameter estimates and Bayesian interval estimates for a mean µi for predictor i are based on the mixture distribution: & ' ( | , is the posterior distribution on µi given data under model . Since the relative weight | on each model may decrease as the set of candidate models grow larger, the use of BIC weights as posterior probabilities allows valid multiple

Figure 1 .

 1 Figure 1. Cape gannet (Morus capensis) pair displaying 'scissors' to neighbours during its

Figure 2 .

 2 Figure 2. Distribution of the 28 foraging trips of Cape gannets breeding in the Malgas Island

Figure 3 .

 3 Figure 3. Predicted total time of dance ceremony (in s, solid line) with its 95% confidence

Figure 4 .

 4 Figure 4. Predicted relationship (solid line) between the features of dance ceremony and those

Figure A1 .

 A1 Figure A1. Example of the result of the segmentation-clustering model for the foraging track

Figure A2 .

 A2 Figure A2. Distributions of means (mu) and their standard deviations (SD) of the speeds and

Figure A3 .

 A3 Figure A3. Correlation circles based on a principal component analysis including the three

Table A2 .

 A2 Pearson's correlation coefficients among the features of the Cape gannet dance.

	Duration	Chick	Chick	Change	Flapping	Head	Mutual	Nape	Neighbour	Individual	Pause	Scissors	Shaking	Sky	Yawning
		feeding	grooming	over		shaking	preening	bite	fight	preening		bowing		pointing

Table A5 .

 A5 List of the candidate models selected for the Bayesian model averaging after the 'leaps and bounds' and Occam's window exclusion

	733	1 Figure captions.			•			•	•		0.260	-59.06
	734	processes. 2			•				•		0.129	-57.66
		Model number 3	Chick	Change	Head •	Scissor / •	Nape	Mutual	Individual •	Sky	Posterior model 0.085	BIC -56.82
		4	grooming	over	shake •	Bowing	bite	preening •	preening	pointing	probabilities, 0.064	-56.26
		5			•	•					0.058	|	-56.06
		Trip duration, 27 models were selected in total, cumulative posterior probability for the 5 best candidate models was 0.484 Posterior effect 0.039 0.024 0.815 0.272 0.027 0.514 0.689 0.083
		1 probabilities, P(β ≠ 0 | )				•					0.134	-8.78
		2				•	•				0.103	-8.26
		3 Direction to the main foraging spot, 19 models were selected in total, cumulative posterior probability for the 5 best candidate models was 0.803 • • 0.093 -8.05
		4 1	•		• •	•		•	•		0.091 0.331	-8.02 -57.59
		5 2			• •	•	•		•		0.063 0.223	-7.27 -56.81
		Posterior effect 3	0.149	0.105	0.351 0.978 •	0.349 0.213	0.084 •	0.127 •	0.089	-54.97
		probabilities, P(β ≠ 0 | ) 4			•	•			•		0.087	-54.93
		5	•		•				•		0.073	-54.57
		Distance to the main foraging spot, 19 models were selected in total, cumulative posterior probability for the 5 best candidate models was 0.596 Posterior effect 0.073 0.047 0.907 0.087 0.058 0.363 1.000 0.107

ACKNOWLEDGMENTS

This work was supported by the Centre National de la Recherche Scientifique (CNRS) and the University of Cape Town. We warmly thank Pierre Nel and South African National Parks for accommodation and logistical support. We are also grateful to Itai Mukutyu, Plaxedes Rukuni, Muku Muyahamba, Paige Potter and David Green for their extensive help in the field. We address special thanks to Simon Benhamou for its input on the segmentation/clustering analysis and Rémi Patin and Henri Couffin for sharing the R code.

Thanks to Thierry Boulinier, Damien Farine, Andrea Thiebault and three anonymous reviewers for their insights on the manuscript.

Data Availability

Data are available online at https://mycore.core-cloud.net/index.php/s/44d1PNWFE1d7wiW.

Author Contributions

DG designed the study; NC designed and conducted the statistical analyses; DG, CP, TC, and LP collected field data; PGR supervised TC; AVG analyzed the videos; NC and DG drafted the manuscript; All authors contributed to the writing. All authors gave final approval for publication.

Competing Interests

No competing interests declared.

Supplementary Material

Movie A1 is available online at https://mycore.corecloud.net/index.php/s/IuxM0jHCjdHVNuo

Appendix

Identification of behavioural modes with segmentation-clustering method

We assumed that a foraging trip is composed by the succession of homogeneous bouts in terms of speed and sinuosity (stationary phases along the signal). We looked for phases that are stationary in terms of these metrics (i.e. characterized by a roughly constant mean and variance, different from those of the previous and next phases ;[START_REF] Benhamou | Of scales and stationarity in animal movements[END_REF]. In the present study, we considered that these phases corresponded to three types of behavioural mode: transiting, feeding and resting. They were identified using a segmentation-clustering method [START_REF] Patin | Identifying stationary phases in multivariate time series for highlighting behavioural modes and home range settlements[END_REF] applied to two metrics simultaneously: the smoothed speed (i.e., speed smoothed over two steps before and after the focus location) and the turning angle measured at constant step length (i.e., angle between the focus location, the first location entering a circle of radius equal to the median step length, and the last location inside the circle), which were showed to provide the best contrast between the behavioural modes considered. This method both segments the bivariate signal provided by the two metrics into stationary phases and assigns the phases to one out of the three behavioural modes considered, as a Hidden Markov model-based approach would do, but in way that is both simpler and less sensitive to random noise [START_REF] Patin | Identifying stationary phases in multivariate time series for highlighting behavioural modes and home range settlements[END_REF]. Considering that gannets rest at sea surface and that current velocities change in space and time, the segmentation-clustering ability of the method decreased with the length of the time-series (i.e., the duration of the foraging trip). Indeed, we observed a higher likelihood to classify separately resting phases with different speed and merge the transiting and feeding phases (i.e., identification of two resting phases and one global movement mode). To compensate this artefact, we therefore allowed four modes in the segmentation-clustering analysis, and grouped them later into the three expected behaviours (Appendix Fig. A1). Behavioural mode identification was


Vanina Isnardi

Sébastien Thureau
email: sebastien.thureau@chb.unicancer.fr

Pierre Vera

Impact of positon emission tomography with computed tomography for image-guided radiotherapy Impact de la tomographie par émission de positons combinée à la scanographie pour la radiothérapie

Keywords: PET, radiotherapy, adaptative radiation TEP, radiothérapie, radiothérapie adaptative

Therapeutic effectiveness in radiotherapy is partly related to correct staging of the disease and then precise therapeutic targeting. Positon emission tomography (PET) allows the stage of many cancers to be determined and therefore is essential before deciding on radiation treatment. The definition of the therapeutic target is essential to obtain correct tumour control and limit side effects. The part of

Introduction

Latest technical and technological improvements in the field of radiotherapy allow to deliver higher doses to the tumour, and to improve protection of healthy tissues. Thus, an accurate delineation of the target volume is crucial, in order to improve local control, with limited toxicity.

To achieve this purpose, target volume delineation relies on multimodality imaging. CT, even with contrast media is often limited but remains mandatory to establish treatment planification. Magnetic resonance imaging (MRI) and positon emission tomography with computed tomography (PET/CT), with ( 18 F)-fluorodeoxyglucose (FDG) or other tracers, give further information about tumour boundaries and intrinsic characteristics, and has become essential to the radiotherapist's toolbox.

Moreover, both modalities could lead to a more personalized treatment with the concepts of dose escalation or de-escalation, dose painting, and adaptative radiotherapy.

In 2009, the International Atomic Energy Agency (IAEA) expert report already encouraged the routine use of FDG PET/CT in radiotherapy planning for non-small cell lung cancer and the potential interest for head and neck cancer, lymphoma and oesophageal cancer [START_REF] Macmanus | Use of PET and PET/CT for radiation therapy planning: IAEA expert report 2006-2007[END_REF]. However, it also alerted to the need of rigorous and consistently applied protocols, with a chain of quality control. The segmentation method of PET/CT images also needs to be chosen carefully, as to this date, no reference segmentation method exists [START_REF] Im | Current methods to define metabolic tumor volume in positron emission tomography: which one is better?[END_REF]. These items have already been discussed in a previous issue [START_REF] Thureau | Definition of target volume: When et how can the radiation oncologist use PET?[END_REF].

In this article we will present data supporting the use of PET/CT for radiotherapy planning in daily practice, in lung cancer, head and neck cancer, oesophagus cancer and lymphoma. Then we will discuss about other tumour location in which PET/CT could improve tumour delineation and the potential role of PET/CT for dose escalation, dose painting and adaptative radiotherapy, with the help of FDG or other tracers.

Data supporting the use of PET/CT for radiotherapy planning in daily practice

Non-small cell lung cancer

FDG PET/CT is currently recommended in the initial staging of non-small cell lung cancer for non-or oligometastatic patient in order to confirm that the patient is candidate to a local treatment [START_REF] Salaün | Good clinical practice recommendations for the use of PET/CT in oncology[END_REF]. It enables detection of unknown metastases in about 10 to 20 % patients with apparent stage III cancer [START_REF] Bradley | A phase II comparative study of gross tumor volume definition with or without PET/CT fusion in dosimetric planning for non-smallcell lung cancer (NSCLC): primary analysis of Radiation Therapy Oncology Group (RTOG) 0515[END_REF]. Several studies have shown an impact of FDG PET/CT on radiotherapy planning in more than 30% of patients, leading to a cancellation in case of distant metastases or to a modification of radiotherapy target volume [START_REF] Bradley | A phase II comparative study of gross tumor volume definition with or without PET/CT fusion in dosimetric planning for non-smallcell lung cancer (NSCLC): primary analysis of Radiation Therapy Oncology Group (RTOG) 0515[END_REF](6)[START_REF] Pommier | Impact of ( 18 F)-FDG PET on treatment strategy and 3D radiotherapy planning in non-small cell lung cancer: A prospective multicenter study[END_REF].

Concerning the primary tumour, FDG PET/CT allows to exclude atelectasis and thus to reduce target volume, and to improve inter-observer variability in tumour delineation [START_REF] Nestle | 18 F)-deoxyglucose positron emission tomography (FDG-PET) for the planning of radiotherapy in lung cancer: high impact in patients with atelectasis[END_REF][START_REF] Steenbakkers | Reduction of observer variation using matched CT-PET for lung cancer delineation: A three-dimensional analysis[END_REF].

Concerning the nodal target volume, in a phase II comparative study, Bradley et al. have shown that disagreements between CT and FDG PET/CT occurred in about 50% of patients, confined mainly to one or two nodal stations [START_REF] Bradley | A phase II comparative study of gross tumor volume definition with or without PET/CT fusion in dosimetric planning for non-smallcell lung cancer (NSCLC): primary analysis of Radiation Therapy Oncology Group (RTOG) 0515[END_REF]. FDG PET/CT shows good performances for nodal staging in non-small cell lung cancer, better than CT, with a median sensitivity and a specificity of 85% and 90% respectively (61 % and 79% for CT), and a negative predictive value superior to 90% in case of T1-T2 tumour [START_REF] Gould | Test performance of positron emission tomography and computed tomography for mediastinal staging in patients with non-small-cell lung cancer: a meta-analysis[END_REF][START_REF] Wang | Negative predictive value of positron emission tomography and computed tomography for stage T1-2N0 non-small-cell lung cancer: a meta-analysis[END_REF]. Two prospective studies have demonstrated low rate of isolated nodal failure, less than 5 % with selective nodal irradiation principle, based on FDG PET/CT for non-small cell lung cancer [START_REF] Bradley | A phase II comparative study of gross tumor volume definition with or without PET/CT fusion in dosimetric planning for non-smallcell lung cancer (NSCLC): primary analysis of Radiation Therapy Oncology Group (RTOG) 0515[END_REF][START_REF] De Ruysscher | Selective mediastinal node irradiation based on FDG-PET scan data in patients with non-small-cell lung cancer: a prospective clinical study[END_REF]. In summary, as specified in the guidelines from the European Organization for Research and Treatment of Cancer (EORTC) published in 2010 (13):

•

it is reasonable to delineate the tumour on the basis of FDG PET/CT to reduce the irradiated volume in case of atelectasis;

• it is recommended to include FDG positive nodes in the gross tumour volume, even if they are not enlarged in CT, and if histologic examination is not feasible to exclude false positive;

• but it requires strict standardization of PET/CT acquisition, and to keep in mind that a rapid decrease of FDG uptake could occur soon after the onset of chemotherapy.

Squamous cell carcinoma of the head and neck

FDG PET/CT is currently recommended for initial staging of locally advanced squamous cell carcinoma of the head and neck (stage III/IV) in order to detect distant metastasis, but could also be proposed in any stage if synchronous localisations could impact later treatment (4).

FDG PET/CT helps primary tumour detection in squamous cell carcinoma of the head and neck and undifferentiated nasopharyngeal tumour with a reported overall sensitivity of 93 to 100%, specificity of 90 to 100% and an accuracy of 94 to 98% [START_REF] Troost | Clinical evidence on PET-CT for radiation therapy planning in head and neck tumours[END_REF]. And in case of cervical lymph node metastasis of an unknown primary tumour, FDG PET/CT is able to detect about 25% of occult primary tumours [START_REF] Daisne | Tumor volume in pharyngolaryngeal squamous cell carcinoma: comparison at CT, MR imaging, and FDG PET and validation with surgical specimen[END_REF]. Furthermore, it has been shown that the introduction of FDG PET/CT led to significantly smaller gross tumour volumes for pharyngolaryngeal squamous cell carcinoma than those delineated on CT or MRI [START_REF] Daisne | Tumor volume in pharyngolaryngeal squamous cell carcinoma: comparison at CT, MR imaging, and FDG PET and validation with surgical specimen[END_REF]. A comparison with the laryngeal specimens showed that FDG PET gross tumour volumes were the closest to the histologic ones.

Concerning nodal metastases detection, FDG PET/CT shows diagnostic performances comparable to CT and anatomical MRI, and seemed to be outperformed by diffusion-weighted MRI [START_REF] Grégoire | Molecular imaging-guided radiotherapy for the treatment of head-and-neck squamous cell carcinoma: does it fulfill the promises?[END_REF]. In addition, its sensitivity dropped off in case of no clinical evidence of metastatic nodes [START_REF] Kyzas | 18 F)-fluorodeoxyglucose positron emission tomography to evaluate cervical node metastases in patients with head and neck squamous cell carcinoma: a meta-analysis[END_REF]. Thus it is not routinely recommended for the nodal staging. International guidelines published in 2018 recommend the inclusion of FDG PET/CT data for the delineation of the gross tumour volume of the primary cancer, for locally advanced tumour, besides clinical examination, endoscopic, CT and MRI data [START_REF] Lee | International guideline for the delineation of the clinical target volumes (CTV) for nasopharyngeal carcinoma[END_REF].

Although there is currently no standard segmentation defined, they recommend the use of userindependent segmentation algorithms.

Oesophageal cancer

FDG PET/CT is currently recommended in the initial staging of oesophageal cancer as it allows for improved detection of distant metastasis, with a sensitivity and specificity of 71% and 93%, better than CT (52% and 91 % respectively), especially coeliac and supraclavicular metastatic nodes [START_REF] Salaün | Good clinical practice recommendations for the use of PET/CT in oncology[END_REF][START_REF] Van Vliet | Staging investigations for oesophageal cancer: a meta-analysis[END_REF].

Some studies reported a significant positive correlation between FDG PET/CT-based tumour lengths and pathological findings, often with a visual segmentation method or with the use of a predefined threshold [START_REF] Van Vliet | Staging investigations for oesophageal cancer: a meta-analysis[END_REF][START_REF] Foley | Impact of positron emission tomography and endoscopic ultrasound length of disease difference on treatment planning in patients with oesophageal cancer[END_REF][START_REF] Moureau-Zabotto | Impact of computed tomography (CT) and 18F-deoxyglucose positron emission tomography (FDG-PET) image fusion for conformal radiotherapy in esophageal carcinoma[END_REF]. However a more recent study has shown significant differences in tumour lengths estimated by PET or endoscopic ultrasound [START_REF] Foley | Impact of positron emission tomography and endoscopic ultrasound length of disease difference on treatment planning in patients with oesophageal cancer[END_REF]. The endoscopic ultrasound length was more than 2 cm longer than the PET length in 38% of cases, and the PET length was greater than 2cm longer than the endoscopic ultrasound length in 5% of cases, suggesting the complementarity of both modalities to define the gross tumour volume of the primary cancer.

Thus, the potential impact of FDG PET/CT on the planning target volume relies on its ability to detect distant node metastasis, and to help to better define craniocaudal limits, as confirmed in few studies [START_REF] Moureau-Zabotto | Impact of computed tomography (CT) and 18F-deoxyglucose positron emission tomography (FDG-PET) image fusion for conformal radiotherapy in esophageal carcinoma[END_REF][START_REF] Schreurs | Impact of 18-fluorodeoxyglucose positron emission tomography on computed tomography defined target volumes in radiation treatment planning of esophageal cancer: reduction in geographic misses with equal inter-observer variability: PET/CT improves esophageal target definition[END_REF][START_REF] Grange | Defining the target in cancer of the oesophagus: direct radiotherapy planning with fluorodeoxyglucose positron emission tomography-computed tomography[END_REF][START_REF] Muijs | Clinical validation of FDG-PET/CT in the radiation treatment planning for patients with oesophageal cancer[END_REF]. More recently, Metzger et al. have reported that the use of FDG PET/CT for staging and planning of primary or neoadjuvant chemoradiotherapy for oesophageal cancer has a favourable impact on patient survival (25).

High grade lymphoma

FDG PET/CT is well known as the most accurate imaging method for determining disease extent, nodal and extranodal, in high grade lymphoma, Hodgkin and non-Hodgkin diseases [START_REF] Gallamini | Role of PET in lymphoma[END_REF]. Moreover, FDG PET/CT performed after treatment is an effective tool for discriminating residual active disease from fibrotic masses, with a higher sensitivity for Hodgkin lymphoma and higher specificity for non-Hodgkin lymphoma [START_REF] Zijlstra | 18 F)fluorodeoxyglucose positron emission tomography for post-treatment evaluation of malignant lymphoma: a systematic review[END_REF][START_REF] Terasawa | 18 F)-FDG PET for posttherapy assessment of Hodgkin's disease and aggressive Non-Hodgkin's lymphoma: a systematic review[END_REF].

Simultaneously, new radiotherapeutic strategies for lymphoma has been developed, more targeted, with more accurate and reduced field, in order to limit long-term adverse events. In particular, the involved site and the involved node radiotherapy principles have emerged for definitive radiotherapy of early stage Hodgkin lymphoma.

Thus, FDG PET/CT is mandatory first for selecting patients candidate to definitive radiotherapy, by confirming an early stage for Hodgkin lymphoma, and characterizing a residual mass at the end of treatment of non-Hodgkin lymphoma, and second for helping to define treatment field. In particular, pretreatment FDG PET/CT is required for the application of the involved site and involved node radiotherapy principles in both early stage of Hodgkin and non Hodgkin lymphomas [START_REF] Specht | Modern radiation therapy for Hodgkin lymphoma: field and dose guidelines from the international lymphoma radiation oncology group (ILROG)[END_REF][START_REF] Zelenetz | Non-Hodgkin's Lymphomas, version 3.2012[END_REF][START_REF] Girinsky | Is FDG-PET scan in patients with early stage Hodgkin lymphoma of any value in the implementation of the involved-node radiotherapy concept and dose painting?[END_REF][START_REF] Girinsky | The conundrum of Hodgkin lymphoma nodes: to be or not to be included in the involved node radiation fields. The EORTC-GELA lymphoma group guidelines[END_REF]. This pretreatment FDG PET/CT needs to be performed in treatment position in case of involved node radiotherapy [START_REF] Édeline | Multimodality imaging using PET/CT ( 18 F)-fluorodeoxyglucose for radiotherapy field delineation of localized Hodgkin lymphoma[END_REF]. But as it is often logistically difficult to obtain this first PET/CT in treatment position, involved site radiotherapy allows for larger margins around the initially involved nodes.

Furthermore, close collaboration between radiation oncologists and nuclear medicine physicians is highly encouraged, and the latter should use a common atlas of nodal station in their reports [START_REF] Édeline | Multimodality imaging using PET/CT ( 18 F)-fluorodeoxyglucose for radiotherapy field delineation of localized Hodgkin lymphoma[END_REF][START_REF] Terezakis | A prospective study of 18 FDG-PET with CT coregistration for radiation treatment planning of lymphomas and other hematologic malignancies[END_REF][START_REF] Girinsky | Role of FDG-PET in the implementation of involved-node radiation therapy for Hodgkin lymphoma patients[END_REF].

Other tumour locations

There is growing interest for the use of PET/CT in other tumour locations with FDG or other PET tracers.

Prostate-specific PET tracers are of interest for patients with a biochemical recurrence of prostate cancer, especially when salvage radiotherapy could be performed. Salvage radiotherapy should be performed as early as possible, optimally when serum prostate-specific antigen levels are below 0.5 ng/ml. Prostate-specific membrane antigen (PSMA) PET tracers show better performances than choline tracers and fuciclovine, with compared median detection rate of 51.5% , 19.5% and 38% respectively for PSA levels below 1 ng/ml [START_REF] Evans | Prostate cancer-specific PET radiotracers: A review on the clinical utility in recurrent disease[END_REF]. In a recent meta-analysis, Perera et al. reported PSMA PET detection rates of 33 % for PSA levels below 0.2 ng/ml and 46% for PSA levels 0.2 to less than 0.5 ng/ml, in patients with biochemical recurrence after radical prostatectomy [START_REF] Perera | Sensitivity, specificity, and predictors of positive ( 68 Ga)-prostate-specific membrane antigen positron emission tomography in advanced prostate cancer: a systematic review and meta-analysis[END_REF]. In addition, another meta-analysis reported that PSMA PET increased the proportion of patients assigned to radiotherapy from 56% to 62%, and that 24 % of patients received increased dose to the sites of suspected recurrence and/or an enlarged target volume including the sites of recurrence, while 20% received stereotactic body radiotherapy to single metastases or oligometastasis [START_REF] Han | Impact of ( 68 Ga)-PSMA PET on the management of patients with prostate cancer: a systematic review and meta-analysis[END_REF].

For anal cancer, FDG PET/CT could provide additional information that could lead to major changes in treatment strategy in almost one in five patients, but induces changes in irradiated field in only 4% FDG PET/CT could also be useful in other tumour location, once the lesions show a significant uptake, as for example in liver metastasis and cholangiocarinomas, in primary cardiac tumour, or in case of reirradiation, particularly when conventionnal imaging is of limited performance, when iodine contrast media is contra-indicated or when the patient is wearing a pacemaker or any metallic devices preventing MRI [START_REF] Bundschuh | Respiratory gated ( 18 F)FDG PET/CT for target volume delineation in stereotactic radiation treatment of liver metastases[END_REF][START_REF] Onal | Comparison of computed tomography-and positron emission tomography-based radiotherapy planning in cholangiocarcinoma[END_REF][START_REF] Elsayad | Advances in image-guided radiation therapy for primary cardiac angiosarcoma: the role of PET-CT and MRI[END_REF]. But one should keep in mind the intrinsic PET limitations such as a rather low spatial resolution and a high level of noise [START_REF] Grégoire | Molecular imaging-guided radiotherapy for the treatment of head-and-neck squamous cell carcinoma: does it fulfill the promises?[END_REF].

Potential role of PET/CT for dose escalation, dose painting and adaptative radiotherapy

Current radiotherapy techniques allow us to consider delivering heterogeneous doses within the target volume. With the routine use of intensity-modulated radiation for the treatment of many cancer (head and neck, brain tumour, lung cancer, etc.) allowing to deliver a very precise dose to the gross tumour volume, but there is still a risk of local recurrences confirming the presence of radioresistant areas.

Functional imaging allows considering the selection of these areas (biological target volume) in order to deliver a variable dose depending on the risk of recurrence [START_REF] Roberts | Painting Dose: The ART of radiation[END_REF]. This imaging can be used before the start of radiotherapy as a complement to the planning scanner or during treatment to adapt to the early response. Several tracers can be used to target areas and the choice of tracer will depend on the location treated; the most commonly used tracers are those used to target hypoxia, proliferation or tumour metabolism. In this context, the method of definition and segmentation of target volumes is important and it has been demonstrated that the delineation method has an impact on the defined volume. Many methods exist and have been reworked in a previous article (3).

Non small cell lung cancer

As mentioned in the previous chapter, PET/CT is recommended in the staging and the planning for non small cell lung cancer treated by radiation. While the level of evidence is lower and currently does not allow consideration of dose painting radiotherapy from PET, many studies confirm its value in defining patients at risk of recurrence and radioresistant areas. For example, several studies have shown the prognostic implications of changes in standardized uptake value and suggested that FDG PET/CT could predict the response to chemoradiation or induction chemotherapy [START_REF] Abramyuk | Is pretherapeutical FDG-PET/CT capable to detect high risk tumor subvolumes responsible for local failure in non-small cell lung cancer?[END_REF][START_REF] Ganem | Prognostic value of postinduction chemotherapy ( 18 F)-FDG PET-CT in stage II/III non-small cell lung cancer before (chemo-) radiation[END_REF]. FDG is the tracer that has been most used to propose a dose adapted to pretherapeutic PET. Three studies have shown that the most hypermetabolic area before radiotherapy was an area at risk of local recurrence [START_REF] Aerts | Identification of residual metabolic-active areas within individual NSCLC tumours using a pre-radiotherapy ( 18 F)fluorodeoxyglucose-PET-CT scan[END_REF][START_REF] Calais | Areas of high ( 18 F)-FDG uptake on preradiotherapy PET/CT identify preferential sites of local relapse after chemoradiotherapy for non-small cell lung cancer[END_REF][START_REF] Jouglar | Patterns of locoregional failure in locally advanced non-small cell lung cancer treated with definitive conformal radiotherapy: Results from the Gating 2006 trial[END_REF]. The same results were found for stereotactic radiotherapy for lung cancer [START_REF] Park | Maximum standardized uptake value at pretreatment PET in estimating lung cancer progression after stereotactic body radiotherapy[END_REF]. A randomized phase II study (NCT01024829) has now been completed and has focused on increasing the dose with an integrated boost to the area of the primary tumor with 50% of the maximum standardized uptake value (72Gy) according to the normal tissue tolerance. Initial toxicity results demonstrate the feasibility of this dose increase [START_REF] Van Diessen | The acute and late toxicity results of a randomized phase II dose-escalation trial in non-small cell lung cancer (PETboost trial)[END_REF]. The first alternative to this adapted radiotherapy to FDG PET is the use of other tracers in pretherapeutic examinations. For the proliferation with 3'-( 18 F)-fluoro-3'deoxythymidine (FLT), there is only preclinical data or studies showing the feasibility of using this tracer for lung cancer [START_REF] Li | Spatial concordance of tumor proliferation and accelerated repopulation from pathologic images to 3'-( 18 F)-fluoro-3'-deoxythymidine PET images: a basic study guided for PET-based radiotherapy dose painting[END_REF][START_REF] Vera | Simultaneous positron emission tomography (PET) assessment of metabolism with (¹⁸F)-fluoro-2-deoxy-d-glucose (FDG), proliferation with (¹⁸F)-fluoro-thymidine (FLT), and hypoxia with (¹⁸F)-fluoro-misonidazole (F-miso) before and during radiotherapy in patients with non-small-cell lung cancer (NSCLC): a pilot study[END_REF][START_REF] Lelandais | Fusion of multi-tracer PET images for dose painting[END_REF]. The tracer for which we have the most data is the ( 18 F)-labelled misonidazole that allows to define hypoxic zones with PET/CT. In particular, there is a French phase II trial that confirms the possibility of boosting the hypoxic zone without increasing toxicity. In addition, a boost to the hypoxic volumes appears to improve the survival of hypoxic patients compared to those receiving a standard dose (55, 56) (figure 1). This strategy based on hypoxia is different from a boost on hypermetabolic volume with a specific therapeutic impact [START_REF] Thureau | FDG and FMISO PET-guided dose escalation with intensity-modulated radiotherapy in lung cancer[END_REF][START_REF] Thureau | Comparison of hypermetabolic and hypoxic volumes delineated on ( 18 F)-FDG and ( 18 F)-fluoromisonidazole PET/CT in non-small-cell lung cancer patients[END_REF]. However, Di Perri et al.

consider that there is a good correlation between hypoxic volumes and FDG hotspots, but not from our group [START_REF] Thureau | Comparison of hypermetabolic and hypoxic volumes delineated on ( 18 F)-FDG and ( 18 F)-fluoromisonidazole PET/CT in non-small-cell lung cancer patients[END_REF][START_REF] Perri | Correlation analysis of ( 18 F)fluorodeoxyglucose and ( 18 F)-fluoroazomycin arabinoside uptake distributions in lung tumours during radiation therapy[END_REF].

The third alternative is to adapt the dose to changes in PET during radiotherapy. In a previous study, we had shown that it was possible to define a pertreatment segmentation method for both primary tumour and lymph nodes [START_REF] Ganem | Delineation of lung cancer with FDG PET/CT during radiation therapy[END_REF]. In this context, a French phase II/III study is ongoing to demonstrate the interest of increasing the dose at 74Gy on the hypermetabolic volume during radiotherapy (at 42Gy) (RTEP7 -NCT02473133). A similar phase II trial has just completed including 138 patients to study the use of FDG PET/CT between the 18th and 19th fraction with a dose determined by contraints at organs at risk (RTOG 1106 -NCT01507428).

Squamous cell carcinoma of the head and neck

With lung cancers, head and neck cancers are the ones for which there are the most published and ongoing studies. It's for this location that the Brussels teams proposed a dose adapted to PET fixation using two methods. The first strategy is a boost at a subvolume. It is the simplest technique and close to the usual techniques in case of several dose levels, the name generally used for this strategy is "dose painting by volume". The second strategy "dose painting by numbers" is very interesting because it proposes to assign a dose to each voxel according to its level of PET fixation [START_REF] Grégoire | Radiotherapy for head and neck tumours in 2012 and beyond: conformal, tailored, and adaptive?[END_REF].

Many studies confirm the feasibility of a boost based on FDG fixation without increasing the dose to the organs at risk [START_REF] Thorwarth | Dose painting with IMPT, helical tomotherapy and IMXT: a dosimetric comparison[END_REF][START_REF] Differding | Methodology for adaptive and robust FDG-PET escalated dose painting by numbers in head and neck tumors[END_REF]. Two studies are in progress on ClinicalTrial.gov focusing on dose painting using FDG PET with adaptive dose escalation compared to standard treatment. The dose usually proposed is about 80Gy over the radioresistance area; it has been shown that a higher dose can be harmful with a risk of late mucosal ulcers [START_REF] Madani | Maximum tolerated dose in a phase I trial on adaptive dose painting by numbers for head and neck cancer[END_REF].

As confirmed very recently, hypoxia makes it possible to select patients with a good or poor prognosis, with a hazard ratio at 9 for local control in a study involving 37 patients [START_REF] Thorwarth | Prospective evaluation of a tumor control probability model based on dynamic ( 18 F)-FMISO PET for head-andneck cancer radiotherapy[END_REF]. As FDG, it has been shown that it is possible to increase the dose to more than 80Gy without increasing the dose to at-risk organs. For head and neck, several tracers were tested, such as ( 18 F)-labelled misonidazole azomycin arabinoside, flortanidazole (HX4), or erythronitroimidazole, or ( 64 Cu)-labelled diacetylbis(N4-methylthiosemicarbazone (ATSM). Studies based on an equivalent probability of tumor control between normoxic and hypoxic tumors show that an increase of 15 to 20% in hypoxic volumes is possible without increasing the dose to organs at risk [START_REF] De Figueiredo | Hypoxia imaging with ( 18 F)-FMISO-PET for guided dose escalation with intensitymodulated radiotherapy in head-and-neck cancers[END_REF].

A new strategy could be considered with a dose reduction for Human papilloma virus positive cancers with hypoxia. If this hypoxia is resolved within 1 week of treatment, a selectively de-escalate dose to the nodes (60Gy) is possible with a good local control [START_REF] Lee | Strategy of using intratreatment hypoxia imaging to selectively and safely guide radiation dose de-escalation concurrent with chemotherapy for locoregionally advanced human papillomavirus-related oropharyngeal carcinoma[END_REF].

The problem remains linked to the anatomical modification of lesions during radiotherapy and therefore to the difficulty of targeting radioresistant voxels (68).

Oesophageal cancer

For oesophageal cancer, there are far fewer studies for adaptive radiotherapy. However, it is a cancer for which the optimal dose of radiotherapy remains to be defined and for which the level of toxicity remains high.

As with lung cancers, it appears that the most hypermetabolic area before treatment is an area at risk of recurrence [START_REF] Calais | High FDG uptake areas on pre-radiotherapy PET/CT identify preferential sites of local relapse after chemoradiotherapy for locally advanced oesophageal cancer[END_REF]. Another French study proposes to define the target volume based on an automatic segmentation of the FDG PET to allow a decrease in dose to organs at risk or an increase in dose up to 66Gy [START_REF] Encaoua | Radiotherapy volume delineation based on ( 18 F)-fluorodeoxyglucose positron emission tomography for locally advanced or inoperable oesophageal cancer[END_REF].

FLT PET, which represents proliferation, appears to be an interesting tracer both in initial planning and in the early response. [START_REF] Yue | Measuring tumor cell proliferation with 18F-FLT PET during radiotherapy of esophageal squamous cell carcinoma: a pilot clinical study[END_REF][START_REF] Zhang | Gradient-based delineation of the primary GTV on FLT PET in squamous cell cancer of the thoracic esophagus and impact on radiotherapy planning[END_REF][START_REF] Gerbaudo | Pilot study of serial FLT and FDG-PET/CT imaging to monitor response to neoadjuvant chemoradiotherapy of esophageal adenocarcinoma: correlation with histopathologic response[END_REF]. Data on hypoxia in oesophageal cancers are very limited. While it is confirmed that there is significant binding of hypoxia tracers in esophageal cancers, there is no adaptive radiotherapy study based on these tracers (74).

Other tumour locations

Gynaecological cancer

In most other cancers, pretreatment PET is used to consider targeted boosts. This is case for pelvic cancers, particularly cervical and prostate cancer. PET is often associated with MRI. For cervical cancer, it has been shown that it is possible to consider boosting pathological pelvic lymph nodes to about 60Gy [START_REF] Arnesen | Short-course PET based simultaneous integrated boost for locally advanced cervical cancer[END_REF][START_REF] Wujanto | Does external beam radiation boost to pelvic lymph nodes improve outcomes in patients with locally advanced cervical cancer?[END_REF]. However, this strategy remains controversial and if it does not increase toxicity, it is difficult to demonstrate the value in terms of tumour control. Indeed, the most important study on the interest of a boost on pathological lymph nodes was retrospective and did not use PET (77).

Prostatic cancer

As we have seen previously, there are specific tracers for prostatic cancer with choline or PSMA. FDG PET/CT has no place in radiotherapy planning. During definitive primary radiotherapy, dose escalation to the prostate should improve the local control, but the local toxicity remains high. More accurate determination of the dominant intraprostatic lesion, which is the more aggressive and less responding lesion, should lead to diminish local toxicity while preserving local tumour control [START_REF] Monninkhof | Standard whole prostate gland radiotherapy with and without lesion boost in prostate cancer: Toxicity in the FLAME randomized controlled trial[END_REF][START_REF] Pinkawa | Dose-escalation using intensity-modulated radiotherapy for prostate cancer -evaluation of quality of life with and without ( 18 F)-choline PET-CT detected simultaneous integrated boost[END_REF].

There is growing of interest for PSMA PET/CT to this purpose. As PSMA uptake is known to be correlated to the aggressiveness of the lesions, a miss of low grade intraprostatic lesions seems to be acceptable for radiotherapy purposes (80), and a PSMA PET gross tumour volume contouring finally appears to be feasible in 95 % of patients [START_REF] Zamboglou | 68 Ga)-HBED-CC-PSMA PET/CT versus histopathology in primary localized prostate cancer: a voxel-wise comparison[END_REF]. Some studies suggested that ( 68 Ga)-labelled PSMA PET gives complementary data to the multiparametric MRI for the delineation of the dominant intraprostatic lesion [START_REF] Zschaeck | PSMA-PET based radiotherapy: a review of initial experiences, survey on current practice and future perspectives[END_REF]. It has been demonstrated that it is possible to increase the dose up to 95Gy from combining multiparametric MRI and PET data using tumour control probability calculations (83).

Conclusion

PET/CT has become essential in the planning of many cancers including lung, head and neck, Hodgkin's disease and prostate cancer (table 1). However, it sometimes has to be combined with MRI imaging and segmentation methods have yet to be defined. The interest of PET/MRI must be demonstrated in prospective studies. The development of targeted treatments for recurrent areas requires imaging with very good sensitivity and specificity, as is the case for prostate cancers. Current techniques allow us to consider treatments by dose painting or even adaptive dose painting, and in this context, it seems that functional imaging is essential. However, it is now impossible to say that the best functional imaging is still that of metabolism, hypoxia or proliferation.

  39). Nevertheless, Bassi et al. observed a change in gross tumour volume, with a mean increase of 25 % (40).

Figure 1 .

 1 Figure 1. Impact of positon emission tomography with computed tomography for image-guided radiotherapy. (a,b) PET imaging before treatment; (a) FDG PET; (b) Fmiso PET; (c) FDG PET during radiation with good response but an oesophagitis. FDG: fluorodeoxyglucose; Fmiso: fluoromisonidazole; PET: positon emission tomography.

Table 1 .

 1 Synthesis of current recommendations for the use of positon emission tomography for radiotherapy planification.

	Primary tumour	Staging	Targeting
	Lung	Recommended for non or	Exclusion of atelectasis
		oligometastatic non small cell	Inclusion of PET positive nodes (if
		lung cancer	histologic examination is not feasible to
			exclude false positive)
	Head and neck	Recommended for locally	Help in delineation of the primary gross
		advanced head and neck	tumour volume for locally advanced cancer
		squamous cell carcinoma	User-independent segmentation algorithms
	Oesophagus	Recommended before	Detection of distant nodes
		chemoradiotherapy	Help in defining craniocaudal limits
	Lymphoma	Recommended for high grade	Detection of involved nodes or involved
		lymphoma (Hodgkin and non	sites, especially in case of involved node or
		Hodgkin diseases)	involved site radiotherapy

Disclosure

The authors declare that they have no conflict of interests.