Validation of a Monte Carlo model for multi leaf collimator based electron delivery.

PURPOSE To develop and validate a Monte Carlo model of the Varian TrueBeam to study electron collimation using the existing photon multi-leaf collimators (pMLC), instead of conventional electron applicators and apertures. MATERIALS AND METHODS A complete Monte Carlo model of the Varian TrueBeam was developed using Tool for particle simulation (TOPAS) (version 3.1.p3). Vendor-supplied information was used to model the treatment head components and the source parameters. A phase space plane was setup above the collimating jaws and captured particles were reused until a statistical uncertainty of 1% was achieved in the central axis. Electron energies 6, 9, 12, 16, and 20 MeV with a jaw-defined field of 20 × 20 cm2 at iso-center, pMLC-defined fields of 6.8 × 6.8 cm2 and 11.4 × 11.4 cm2 at 80 cm source-to-surface distance (SSD) and an applicator-defined field of 10 × 10 cm2 at iso-center were evaluated. All the measurements except the applicator-defined fields were measured using an ionization chamber in a water tank using 80 cm SSD. The dose difference, distance-to-agreement and gamma index were used to evaluate the agreement between the Monte Carlo calculations and measurements. Contributions of electron scattering off pMLC leaves and inter-leaf leakage on dose profiles were evaluated and compared with Monte Carlo calculations. Electron transport through a heterogeneous phantom was simulated and the resulting dose distributions were compared with film measurements. The validated Monte Carlo model was used to simulate several clinically motivated cases to demonstrate the benefit of pMLC-based electron delivery compared to applicator-based electron delivery. RESULTS Calculated and measured percentage depth-dose (PDD) curves agree within 2% after normalization. The agreement between normalized percentage depth dose curves were evaluated using one-dimensional gamma analysis with a local tolerance of 2%/1 mm and the %points passing gamma criteria was 100% for all energies. For jaw-defined fields, calculated profiles agree with measurements with pass rates of >97% for 2%/2 mm gamma criteria. Calculated FWHM and penumbra width agree with measurements within 0.4 cm. For fields with tertiary collimation using an pMLC or applicator, the average gamma pass rate of compared profiles was 98% with 2%/2 mm gamma criteria. The profiles measured to evaluate the pMLC leaf scattering agreed with Monte Carlo calculations with an average gamma pass rate of 96.5% with 3%/2 mm gamma criteria. Measured dose profiles below the heterogenous phantom agreed well with calculated profiles and matched within 2.5% for most points. The calculated clinically applicable cases using TOPAS MC and Eclipse TPS for single enface electron beam, electron-photon mixed beam and a matched electron-electron beam exhibited a reasonable agreement in PDDs, profiles and dose volume histograms. CONCLUSION We present a validation of a Monte Carlo model of Varian TrueBeam for pMLC-based electron delivery. Monte Carlo calculations agreed with measurements satisfying gamma criterion of 1%/1 mm for depth dose curves and 2%/1 mm for dose profiles. The simulation of clinically applicable cases demonstrated the clinical utility of pMLC-based electrons and the use of MC simulations for development of advanced radiation therapy techniques.