Commissioning and evaluation of electron Monte Carlo dose calculation algorithm in the Monaco treatment planning system

Abstract

This study aimed to validate and commission the electron Monte Carlo (eMC) algorithm integrated into the Monaco Treatment Planning System (TPS) (version 6.1.2) in accordance with the American Association of Physicists in Medicine (AAPM) Medical Physics Practice Guideline (MPPG) 5a of 2015. The initial task involved validating the beam data used for beam modelling, followed by three verification tests using a homogeneous water-equivalent and a locally designed inhomogeneous phantom. The computed dose distributions from the eMC algorithm were evaluated against the measured data. In the homogeneous water-equivalent phantom, profiles, output factors and percentage depth dose (PDD) curves were evaluated for multiple electron energies at standard and extended source-to-surface distances (SSDs) using different applicators, including an irregularly shaped trapezoid cutout. PDDs and profiles were also computed at an oblique gantry angle of 20°. For the heterogeneity tests, low-density polystyrene was used to simulate lung tissue and solid Plaster of Paris (POP) was utilized to simulate bone. Cross-plane profiles distal to the inhomogeneities were computed by the eMC algorithm and compared to those measured with a planar detector. The study also investigated the performance of the algorithm by varying grid spacing and the number of histories. A one dimensional gamma analysis with 3 %/3 mm criterion was utilized to compare measured and computed PDDs. The eMC algorithm demonstrated agreement within 3 %/3 mm for PDDs and profiles across all applicators and beam energies. However, discrepancies were observed for the irregular trapezoid cutout, where the agreement fell below 90 percent, indicating that limitations exist in the dosimetry of field sizes below 3 cm radius at beam energies greater than 12 MeV. Additionally, deviations in output factors were noted, with a maximum error of 6.5 percent. Overall, the eMC algorithm computed the dose distributions with sufficient accuracy in both homogeneous and inhomogeneous phantoms, and the optimal input parameters were identified as a 0.2 cm grid spacing and 500,000 histories for all energies.