Performance of a Semi-Analytical Dose Engine Designed for Spot Scanning Proton Therapy with Small Spot Sizes

Abstract

To study the clinical performance of a semi-analytical dose engine developed for spot scanning proton delivery systems that feature small spot sizes. The dose engine used a ray-tracing pencil beam algorithm with three lateral Gaussian components. It was commissioned using Monte Carlo (MC) data generated by an experimentally well-benchmarked Geant4 code and then fine-tuned using point dose measurements with various combinations of energy layers, field sizes, depths, and SOBPs. The dose engine used water-equivalent distance in both the beam direction and the lateral direction to account for inhomogeneity. Ten patients representing different disease sites were randomly selected for validation. Comparisons were done in water with data calculated from a fast MC code and with ionization chamber array measurements for patient-specific QA (including point doses, dose profiles, 2D-3D, and 3D-3D gamma analysis) and in patient geometries with MC data (including dose volume histogram indices and 3D-3D gamma analysis). In-water gamma passing rates using 3%/3mm criteria for these ten patients were greater than 97% when compared to MC and greater than 95% when compared to dose plane measurements. Excellent agreement was observed for dose profiles and point dose measurements in water. Good agreement between the semi-analytical engine and the MC code was also observed in patient geometries. This dose engine has been successfully used at our institution as a fast, independent second check for over two years. A ray-tracing dose engine with three lateral Gaussian components can accurately calculate dose distributions for our proton delivery system.Abstract 1167; TableExample treatment plan DVH indicesMonte CarloDose engineCTV statisticsMean dose [cGy(RBE)]5228.65229.1D95% [cGy(RBE)]4943.45006.6Head and NeckBrain D1% [cGy(RBE)]4336.34330.6OARsBrain stem D1% [cGy(RBE)]3906.94062.2Spinal cord D1% [cGy(RBE)]2684.32728.5Eye mean dose (L/R) [cGy(RBE)]1362.7 / 1947.21389.7 / 1890.1 Open table in a new tab