Effects of surface curvature on electron Monte Carlo (eMC) calculation results

Some clinical situations in radiotherapy require electron beams to be incident on curved patient surfaces. This study presents central-axis dose output (cGy/MU) and percent dose versus depth (PDD) data that show the effects of curvature on results computed by the Varian eMC v15.6 algorithm using 6, 9, 12, 16, and 20 MeV electron beams incident on virtual phantoms with curved surfaces. The phantoms were designed to simulate common treatment sites. The dose outputs at the depth of maximum dose (dmax) on the central axis were observed to decrease 0%-14%, and several features of the PDDs for the A10 applicator changed, including up to 12% increased entrance dose. These dosimetric changes have the greatest effect on treatment sites with a radius of curvature of 10 cm or less, such as the scalp, nose, neck, and extremities. The concept of applying a curvature correction factor based on relative output data is presented to help clinical users mitigate discrepancies between calculations performed by simple monitor unit verification systems and accurate treatment planning dose algorithms.