Development of a new Hybrid Virtual Source Model to simulate Elekta Synergy MLCi2 linac

Abstract

Monte Carlo (MC) simulations of medical linear accelerators applying the phase space approach provide excellent results and substantially reduce execution time. This approach becomes impractical because a new complete MC simulation is needed for any new generation of linac heads. To overcome this disadvantage, the investigation exhibits an innovative hybrid Virtual Source Model (VSM) based on a theoretical reconstruction model utilizing the unchangeable parameters of linacs, taking as input the distribution of dose profiles and energy spectra weight. This VSM involves a couple of principal components, virtual source of photons and scattered particle sources from the Virtual Flattening Filter (VFF) and the asymmetric jaws. Extensive calculations and comparisons between the experimental dose distributions and the hybrid VSM are performed to evaluate VSM accuracy. The TOpas and Gate MC codes are utilized to achieve dose computational speedup and memory management optimization. For validation, several regular squared fields of varying sizes ranging from 3 × 3 to 20×20cm2 are considered, as well as two complex IMRT plans based on head&neck and pelvic treatments. Examining the VSM accuracy in the penumbra and the high dose gradient regions, applying gamma index (2%/2mm) criterion confirmed satisfactory results. A two and three dimensional γ index examination demonstrated an excellent agreement concerning the (3%/3mm) criteria for IMRT treatments. Furthermore, the speedup factor reached is 290 when applying this hybrid VSM compared to the entire simulation. Consequently, and according to the results achieved in this task, the hybrid VSM proposed is considered suitable for treatment planning systems (TPS) calculation purposes.