ESTRO 2023 - Abstract Book

S1109

Digital Posters

ESTRO 2023

Materials and Methods From January 2018 to January 2021, patient with borderline/locally advanced (BR/LA) pancreatic adenocarcinoma were included. Cases without clearly identified LRR after a total neoadjuvant strategy were excluded. The neoadjuvant treatment was composed of induction chemotherapy (modified FOLFIRINOX) followed by iHD-SBRT (35Gy in 5 fractions with a simultaneous integrated boost up to 53Gy) and surgical exploration (if operable and no progression) ¹ . No elective nodal irradiation was used for iHD-SBRT and a tumour-vessels interface (TVI) structure was created by including the whole circumference of major abdominal vessels in direct contact with the GTV. During the follow-up, LRR were identified on CT or MRI. The same imaging modalities of LRR and RT-planning were imported in MIM (v7.1.5, MIMvista Inc, Cleveland, OH, USA), delineating the following structures : major abdominal great vessels, pancreas, primary tumour and LRR (only on the latter image set). A tailored DIR procedure was performed and validated (using qualitative and visual assessment) prior the back propagation of the LRR to the initial imaging. Finally, the 35Gy isodose line of the initial plan was used to classify the back-propagated LRRs as: in-field (IF), marginal and out-of-field (OF). (Figure1) Results Among 41 patients treated by iHD-SBRT for localized pancreatic cancer, a LRR was clearly identified in 17 patients (ten on CT and seven on MRI). The tailored DIR workflow confirmed plausible correspondence between the two image sets. The majority of the LRR were classified as OF (n=9), while four cases were identified as IF and the remaining four as marginal recurrences close to major abdominal vessels (table 1).

Conclusion Imaging based evaluation of loco-regional recurrence is an important part of the assessment of a neoadjuvant treatment strategy including iHD-SBRT for localised pancreatic cancer. Based on the tailored deformable image registration, four marginal LRR occurred, close to the vessels around the primary tumour. This highlight the need to include the whole circumference of the major abdominal vessels in direct contact with the GTV in the TVI structure, with at least 5mm margin on both sides of the GTV, in order to further minimize the risk of marginal LRR after iHD-SBRT.