SU‐E‐T‐692: Evaluation of Surface Dose Calculation of Superposition‐Convolution Algorithms Using Monte Carlo Simulation

Purpose: This study evaluated the surfacedosimetry predicted by the anisotropic analytical algorithm (AAA) and collapsed cone convolution (CCC) algorithm using the tangential‐like photon beam and phantom geometry. Monte Carlo(MC) simulation (EGSnrc code) was used as a benchmark for comparison. Methods: The 6 and 15 MV photon beams with field sizes of 4×4 (or 7×7), 10×10 and 20×20 cm2, produced by a Varian 21EX linear accelerator were used. Horizontal dose profiles at different depths, phantom skin profiles (i.e. vertical dose profiles at a distance of 2 mm from the phantom lateral surface), gamma dose distributions, and dose‐volume histograms (DVHs) of skin slab were determined. Results: For dose profiles at different depths, the CCC agreed better with doses in the air‐phantom region, while both the AAA and CCC agreed well with doses in the penumbra region, when compared to the MC. Gamma evaluations between the AAA/CCC and MC revealed that deviations of 2D dose distribution occurred in both beam edges in the phantom and air‐phantom interface. DVHs of skin slab showed that both the AAA and CCC underestimated the width of the slope for both the 6 and 15 MV photon beams. The mean dose differences along the phantom skin profiles for the AAA and CCC were respectively: 7.6±2.6% and 2.1±1.3% for a 10×10 cm2 field, 6 MV; 16.3±2.1% and 6.7±2.1% for a 20×20 cm2 field, 6 MV; 5.5±1.2% and 1.7±1.4% for a 10×10 cm2, 15 MV; 18.0±1.3% and 8.3±1.8% for a 20×20 cm2, 15 MV. Conclusions: As surfacedose with tangential‐like photon beam geometry is important in some radiation treatment sites such as breast, chest wall and sarcoma, the dosimetry data and beam and phantom geometry in this study are worthwhile to be considered, when carrying out quality assurance and commissioning for treatment planning systems.