SU‐E‐T‐31: A Fast Finite Size Pencil Beam (FSPB) Convolution Algorithm for a New Co‐60 Arc Therapy Machine

Abstract

Purpose:Present a fast Finite Size Pencil Beam (FSPB) convolution algorithm for a new Co‐60 arc therapy machine. The FSPB algorithm accounts for (i) strong angular divergence (short SAD), (ii) heterogeneity effect for primary attenuation, and (iii) source energy spectrum.Methods:The FSPB algorithm is based on a 0.5×0.5‐cm2 dose kernel calculated using the GEPTS (Gamma Electron and Positron Transport System) Monte Carlo code. The dose kernel is tabulated using a thin XYZ mesh (0.1 mm steps in lateral directions) for radius less than 1 cm and using an RZ mesh (with varying steps) for larger radial distance. To account for SSD effect, 11 dose kernels with SSDs varying between 30 cm to 80 cm are calculated. Maynord factor and “lateral stretching” are applied to account for differences between closest and actual SSD. Appropriate rotations and second order interpolation are used to calculate the dose from a given beamlet to a point.Results:Accuracy: Dose distributions in water with 80 cm SSD are calculated using the new FSPB convolution algorithm and full Monte Carlo simulation (gold standard). Figs 1–4 show excellent agreements between FSPB and Monte Carlo calculations for different field sizes and at different depths. The dose distribution for a prostate case is calculated using FSPB (Fig.5). Sixty conformal beams with rectum blocking are assumed. Figs 6–8 show the comparison with Monte Carlo simulation based on the same beam apertures. The excellent agreement demonstrates the accuracy of the new algorithm in handling SSD variation, oblique incidence, and scatter contribution.Speed: The FSPB convolution algorithm calculates 28 million dose points per second using a single 2.2‐GHz CPU. The present algorithm is seven times faster than a similar algorithm from Gu et al. (Phys. Med. Biol. 54, 2009, 6287–6297).Conclusion:A fast and accurate FSPB convolution algorithm was developed and benchmarked.