Monte Carlo modeling of Halcyon and Ethos radiotherapy beam using CAD geometry: validation and IAEA-compliant phase space

Abstract

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.

Technical note: TERMA scaling as an effective heterogeneity correction model for convolution‐based external‐beam photon dose calculations

“…36 The MC model of the treatment head with dual-layer multileaf collimator (MLC) implementation from computer aided design (CAD) drawings was used. 31 A detailed design specification was provided by the vendor and different components of the treatment head such as the target, the primary collimator, the ion chamber, and the secondary collimator were accurately modeled according to engineering drawings. The model was Geant4 based and the physics list "QGSP BIC" with "EM Opt4" was used in this work.…”

“…For FTDC, AAA, and AXB we use a multiple‐source model containing a set of physical parameters 36 . The MC model of the treatment head with dual‐layer multileaf collimator (MLC) implementation from computer aided design (CAD) drawings was used 31 . A detailed design specification was provided by the vendor and different components of the treatment head such as the target, the primary collimator, the ion chamber, and the secondary collimator were accurately modeled according to engineering drawings.…”

“…In this work, we propose a heterogeneity correction model within the single kernel convolution approach, which can significantly improve the accuracy while introducing marginal computational overhead. The improved accuracy of the resulting dose calculation method was demonstrated by comparing the obtained results with Acuros external beam (AXB), 28 AAA, 29,30 and MC, 31 for several heterogeneous geometries.…”

The delivery assessment for small targets on Halcyon radiotherapy system – Measured and calculated dose comparison

“…The system has proven to provide standardized and fast SRT treatments with curative intent and has demonstrated improved clinical workflow. 6,[8][9][10] The aim is to collect high-resolution measured dose distribution data with well-defined VMAT-SRT plans and compare the results to high-accuracy Monte Carlo (MC) model 11 and Acuros external beam (AXB) calculations. 12,13 AXB calculation was also included as it is the current clinical solution for the final dose calculation.…”

“…The aim is to collect high‐resolution measured dose distribution data with well‐defined VMAT‐SRT plans and compare the results to high‐accuracy Monte Carlo (MC) model 11 and Acuros external beam (AXB) calculations. 12 , 13 AXB calculation was also included as it is the current clinical solution for the final dose calculation.…”