35 Monte Carlo dose calculation of VMAT clinical treatment plans for Novalis TrueBeam STx linac using PRIMO

Introduction The aim of patient specific QA is to check if calculated dose by TPS corresponds to delivered dose to patient by the linac. With complex techniques like VMAT this is both more important and more challenging. QA both focus on dose calculation and dose delivery. Regarding dose calculation, Monte Carlo methods are considered as golden standard. Methods We use PRIMO to calculate VMAT clinical plans, for a TrueBeam. PRIMO combines the PENELOPE Monte Carlo code, the geometry of some linacs including their MLC, and a graphical user interface. PRIMO is adapted to perform Monte Carlo simulation in a clinical environment. The main reason is the geometry definition. Version 0.1.5 officially doesn’t support VMAT plans calculation. With in-house additional scripts and huge efforts, we succeeded to calculate VMAT plans, on CT, with TrueBeam. Version 0.3.1 now allows VMAT plans calculation in a very simple way. This latest version discontinues the support of Elekta linacs. We compared dose from Eclipse (Acuros 13.7, dose to medium) with PRIMO. As the beam production geometry is not known for TrueBeam, we use phase space made available by Varian. PRIMO 0.3.1 includes an additional Monte Carlo code: DPM. It is optimized for radiotherapy but with a reduced accuracy in air. Results With PRIMO 0.3.1, we compared dose for 11 clinical plans. 3D gamma index (2%/2 mm) is always above 95%. For 2 lung cases, this required use of PENELOPE rather than DPM. Calculation time with DPM vary from 20 to 60 min, mainly depending on PTV size. Calculation time is 6 times longer with PENELOPE (24 cores workstation). Uncertainty (2 sigma) is about 2.5%. Median dose difference is lower than 1%. We also successfully calculated SRS plan from iPlan. Conclusions PRIMO is a valid tool to calculate VMAT clinical plan, for a TrueBeam. It is adapted for use in a clinical environment.