ESTRO 2022 - Abstract Book

S1297

Abstract book

ESTRO 2022

Conclusion The TPS Monaco models successfully the reduction in the radiation transmission when the closed gap moves off axis, provided the MLC Agility gap measures physically 1 mm at the leaves plane. The existence of this minimun gap between adjacent leaves makes it necessary to close the gap off axis in order to reduce the transmission radiation through it. During the accelerator commissioning, the size of this gap should be checked to ensure minimum transmission. In the case of dynamic treatments in which the leaves are closed within the field, the correct modelling of this parameter is particularly important.

PO-1518 Reduction of second cancer risks with proton therapy vs photon VMAT in seminoma patients

W. Heemsbergen 1 , M. Dirkx 1 , D. De Regt 2 , M. Franckena 1 , Y. Klaver 3 , S. Habraken 3 , R. Nout 1

1 Erasmus MC Cancer Institute, Radiotherapy, Rotterdam, The Netherlands; 2 InHolland , University of Applied Science, Haarlem, The Netherlands; 3 Holland Proton Therapy Center, Radiotherapy, Delft, The Netherlands Purpose or Objective Elective para-aortic (PAO) radiotherapy (RT) has been the standard adjuvant therapy after local surgery in stage I seminoma for many years, using photons and an APPA field setup. In the past decade this policy is however shifting towards watchful waiting, since several cohort studies demonstrated that this young patient category with excellent overall survival is at considerable risk for late effects including second primary cancer (SPC) risks related to the radiation exposure of organs at risk (OAR), in particular the stomach, pancreas, and kidneys. Current developments in RT offer more advanced options with VMAT and proton therapy, with the potential to reduce OAR dose levels and therefore SPC risks. In the current study we compared dose to the OARs between VMAT, proton therapy and an APPA reference group. Mean dose was considered as a suitable proxy for SPC risks, since a linear dose-response is assumed in the observed OAR dose range of ≈ 0.1-15 Gy, causing cell damage but not cell kill, according to the theoretical models (linear, linear plateau, linear exponential). Materials and Methods In six recent clinical treatment plans we added delineations of pancreas and stomach and re-optimized the VMAT plan (13x2 Gy, single-ARC, 10 MV, clinical constraints) applying a uniform 7 mm PTV-CTV margin and created an additional proton plan. All received RT to the PAO lymph nodes and two had additional RT to para-iliacal (PAI) lymph nodes. Proton therapy plans were optimized using in-house developed software for the prioritized multi-criteria optimization of RT treatment