P36. Dosimetric impact of treatment couch modelling and contention in pulmonary stereotactic treatment

Introduction During a treatment of lung lesion in stereotactic condition, for greater reproducibility, patients are immobilized with a complex and customized contention. The objective of this study is to determine the dosimetric impact of contention used in CGFL with the treatment couch. Materials and methods Pulmonary stereotactic contentions used at CGFL are customizable vacuum cushions (BlueBAG™ BodyFIX). Ten treatment plans was performed without contention and couch. Each uses an arrangement of non modulated beams of 6 MV. Dose distributions of the resulting plans were re-calculated with the obtained Monitor Units, but with including of BlueBAG and treatment couch (“Exact IGRT” of Varian). Hounsfield Units used for internal structure couch are −945 and −300 for external structure. All dose distributions were calculated with Acuros XB V.11 (Varian Medical System). Results New dose calculation with treatment couch and contention indicate a deterioration of around 1.9% of the D 98% . Note that in 9 of the 10 case of the study, this D 98% remains above the prescribe dose. The volume PTV receiving a dose is reduced by 1.6%. The most significant degradation was observed on the D 50% and is 2.5% on average. Conclusions Dosimetric impact of treatment couch modelling and contention in pulmonary stereotactic treatment is moderate. Although a significant deterioration of the dosimetric index was observed, coverage of prescribed dose remains acceptable.