Objective: Accurate surface dosimetry is important particularly in post-operative breast cancer radiotherapy where the target volume lies immediately beneath the skin and chest wall. This study aims to investigate the influence of different beam setups on surface dose calculations using Monte Carlo (MC) simulations.

Methods: A 6 MV x-ray beam for open and wedge fields from a Varian 21EX linear accelerator was modelled using the EGSnrc/BEAMnrc MC code. The modelled beam was verified by comparing the measured and calculated beam distributions. Surface doses were calculated in DOSRZnrc and normalized to a Dmax of 1.5 cm in a Solid Water (SW) phantom for different beam setups. Effects of different boundary crossing algorithms (PRESTA-I and EXACT) were compared as well as different materials and thicknesses of dose scoring voxels.

Results: We found no significant difference in homogeneous phantom calculations, with all surface doses agreeing to within 1.5%. Relatively large differences (up to 6%), however, were found in the calculated surface doses for an inhomogeneous phantom. PRESTA-I was found to give larger surface doses than EXACT.

Conclusions: We found that the inhomogeneous phantom surface dose calculations are more sensitive to changes in MC beam parameters than the homogeneous phantom case. This suggests that care must be taken when using MC for surface dose calculations in complex geometries.