ESTRO 2022 - Abstract Book

S566

Abstract book

ESTRO 2022

Conclusion The currently available models appear to be of suboptimal quality and therefore caution should be considered for using these models in clinical practice. Future model development studies should adhere to methodological guidelines because unreliable models could misguide clinical decision making. The identified predictors in this study can be used as the potential predictors in future models.

Mini-Oral: 15: Treatment plan optimisation & adaptation

MO-0635 The need for treatment adaptation in carbon ion radiotherapy of pancreatic cancer

S. molinelli 1 , A. Vai 1 , S. Russo 1 , P. Loap 2 , G. Meschini 3 , G. Magro 1 , C. Paganelli 3 , A. Barcellini 4 , V. Vitolo 4 , M. Ciocca 1 , E. Orlandi 4 1 Fondazione CNAO, Medical Physics, pavia, Italy; 2 Institut Curie, Radiation Oncology, Paris, France; 3 Politecnico di Milano, Dipartimento di Elettronica, Informazione e Bioingegneria, milan, Italy; 4 Fondazione CNAO, Clinical Department, pavia, Italy Purpose or Objective To quantify potential benefits of 4D robust optimization on multiple 4DCT acquisitions combined with off-line treatment adaptation (ART) for pre-operative carbon ion therapy (CIRT) of borderline resectable pancreatic adenocarcinomas (phase II clinical trial - NCT03822936). Materials and Methods For 10 previously treated patients, a 4DCT was acquired at -15 (CT Plan ), -5 (RV 1 ), 0 (RV 2 ) and +6 (RV 3 ) days from RT start. Treatment plans were optimized to a dose prescription of 38.4 Gy(RBE), in 8 fractions, with a constraint of 38 Gy(RBE) to 1% of the Gastrointestinal organs at risk (GI-OARs) volume (D 1% ) and a plan objective of D 5cc <36 Gy(RBE). The employed motion mitigation strategy involves immobilization with a solid thermoplastic mask, gated dose delivery, centered on maximum expiration (0Ex), combined with 5 rescans. Three adaptive strategies were tested: (A) robust optimization on CTPlan-0Ex accounting for 3 mm set-up and 3% range uncertainty, including the 30%-inspiration phase; (B) robust optimization with the addition of RV 1 -0Ex as uncertainty scenario; (C) plan recalculation at each RV i and re-optimization (RP i ) according to pre-defined thresholds on dose deviation from clinical goals. The cumulative variation of target coverage and GI-OARs doses was evaluated, for each pipeline, assuming RV 2 and RV 3 representative of the subsequent 4 fractions. The duodenum contour of all available 4DCT for each patient was registered on CT Plan . The capacity of pre-RT acquisitions to predict duodenum position, along the RT course, was investigated by computing the intersection of the contours at CT plan , RV 1 , or the union of both, with respect to post-RT 4DCT and the CTV, coupled with increasing margin expansions. Results No RC i ever exceeded the near-to-maximum (D 1% ) constraint to GI-OARs (A). The use of robust optimization alone (B) improved CTV D 98% on average of (3.0±2.2)%, but was still sub-optimal on a patient-specific basis. According to (C), half of the plans would be re-optimized to recover target coverage and/or minimize duodenum D 5cc , at least at one RV i (Figure 1). A significant difference was observed between duodenum contours on CT plan , RV 1 , or their union when intersecting subsequent contours, suggesting that the duodenum position could not be predicted by any combination of pre-RT contours, neither with a margin expansion, without substantially compromising target coverage (Figure 2).

Figure 1: % Variation of CTV D 98% (a) and duodenum D 5cc (b) for each patient, according to the three strategies (A blue – B red – C green), at planning and at each RV i with respect to optimization goals.