Objective: A Monte Carlo (MC) model of a Halcyon and Ethos (Varian Medical Systems, a Siemens Healthineers Company) radiotherapy beam was validated and field-independent phase space (PHSP) files were recorded above the dual-layer multileaf collimators (MLC). Approach: The treatment head geometry was modeled according to engineering drawings and the dual-layer MLC was imported from CAD (computer-aided design) files. The information for the incident electron beam was achieved from an iterative electromagnetic solver. The validation of the model was performed by comparing the dose delivered by the square MLC fields as well as complex field measurements. Main results: An electron phase space was generated from linac simulations and achieved improved MC results. The output factors for square fields were within 1% and the largest differences of 5% were found in the build-up region of PDDs and the penumbra region of profiles. With the more complicated MLC-shaped field (Fishbone), the largest differences of up to 8% were found in the MLC leaf tip region due to the uncertainty of the MLC positioning and the mechanical leaf gap (MLG) value. The impact of the collimator rotation on the PHSP solution has been assessed with both small and large fields, confirming negligible effects on in-field and out-of-field dose distributions. Significance: A computational model of the Halcyon and Ethos radiotherapy beam with a high accuracy implementation of the MLC was shown to be able to reproduce the radiation beam characteristics with square fields and more complex MLC-shaped fields. The field-independent PHSP files that were produced can be used as an accurate treatment head model above the MLC, and reduce the time to simulate particle transport through treatment head components.

Background: In highly heterogeneous medium, such as one with lung tissue or air cavities, the dose in the low-density region or after it, as calculated by the conventional methods based on convolution with an energy-spreading kernel, is usually overestimated in comparison with measurements or more accurate predictions. Purpose: To correct the overestimation, we propose a method of scaling the total energy released per mass (TERMA). Methods: The scaling depends on both the density distribution and the effective beam size in the lateral direction. Results: The corrected convolution method achieved a significantly improved accuracy in both the lung-like tissue and the water-like region after air, compared to the uncorrected method. The TERMA correction only adds about 10% to the overall computational cost. Conclusions: Due to the improvement in accuracy and the preservation of computational efficiency, the proposed dose calculation method will be valuable for inverse treatment planning.