ESTRO 36 Abstract Book

S473 ESTRO 36 2017 _______________________________________________________________________________________________

Results Patients show no significant variations in OARs doses during the treatment (Table 1).

Poster: Physics track: Adaptive radiotherapy for inter- fraction motion management

PO-0875 Dosimetric effects of anatomical changes in proton therapy of head and neck (H&N) cancer G. Miori 1,2 , L. WIdesott 1 , F. Fracchiolla 1 , S. Lorentini 1 , P. Farace 1 , R. Righetto 1 , C. Algranati 1 , M. Schwarz 1,3 1 Trento Hospital, Protontherapy, Trento, Italy 2 University of Rome Tor Vergata, Postgraduate School of Purpose or Objective Anatomical changes in H&N patients can affect dose distributions especially in proton therapy. A retrospective analysis of H&N patients undergoing repeat CTs and treated at our Proton Therapy Center was done to evaluate dose changes and to identify a dosimetric index for the need of replanning. Furthermore, TCP analysis was performed to evaluate the magnitude of changes with radiobiological parameters. Finally, non-adapted and adapted plans were compared. Material and Methods All H&N patients treated in our center between October 2014 and September 2016 with at least one repeat CT (eCT) were considered. 21 patients were identified: 18 patients had at least one eCT (1 to 6 eCTs), but did not need replanning, and 3 patients needed replanning at some stage of the treatment. The original plan was recalculated on each eCT. Differences were calculated for each treatment fraction, considering a stepwedge interpolation on fractions where the eCT was missing. D1 variations (ΔD1) for cord, brainstem, optic chiasm and optic nerves, and Dmax differences (ΔDmax) for lenses were considered. Target coverage analysis was based on differences in CTV V95 (ΔV95). ΔV95 values were included in Non-replanned ( controls ) if they came from non- replanned patients or from replanned patient calculations on CT preceding the replanning CT (rCT). On the contrary, ΔV95 were included in Replanned ( cases ) if they came from replanned patients on the rCT and the following CTs. The choice was made to consider the trend in target coverage after the point identified for replanning. A cut- off ΔV95 for the need of replanning was identified by the maximum Youden’s index on the ROC analysis between control and cases. Next, TCP differences with respect to the planning TCP (ΔTCP) were calculated. ΔTCP values were divided in Non-replanned and Replanned as for DV95 analysis. Finally, a comparison between adapted and non- adapted plans for the 3 replanned patients was done. All statistics were made by t-Student tests. Medical Physics, Rome, Italy 3 INFN, TIFPA, Trento, Italy

Target coverage analysis shows large differences between Replanned and Non-replanned (p<0.001). The maximum Youden’s index identifies CTV ΔV95= -5% as an optimized threshold level for replanning (sensitivity=87.5%; specificity=100%). TCP analysis shows large variations between Replanned and Non-replanned (p<0.001). ΔV95 and ΔTCP results are summarized in Table 1. ΔV95 comparison between non-adapted and adapted plans shows significant CTV coverage improvements (Figure 1). Conclusion OARs doses were not affected by anatomical changes in all H&N patients studied. On the contrary, there was a significant difference in the effect of anatomical changes for replanned and non-replanned patients, confirmed by radiobiological changes. Therefore, ART reveal great benefits in target coverage for patients that need replanning which can be identified by a threshold dosimetric index. PO-0876 Treatment adaptation is mandatory for intensity modulated proton therapy of advanced lung cancer L. Hoffmann 1 , M. Alber 2 , M. Jensen 3 , M. Holt 3 , D. Møller 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 2 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany 3 Aarhus University Hospital, Department of Radiation Oncology, Aarhus, Denmark Purpose or Objective Large anatomical changes during radiotherapy are seen for a large proportion of lung cancer patients. Precise delivery of proton therapy is highly sensitive to these changes