Determination and use of patient curvature correction factors by measurement in electron monitor unit calculations.

In our department, monitor units (MU) for electron treatments are calculated using a Monte Carlo algorithm in the RayStation treatment planning system and independently checked using RadCalc software. An audit found that 22 of 50 independent checks failed the ± 5% local tolerance level. One of the causes for the discrepancy was thought to be that the RadCalc calculation takes no account of the patient surface curvature. Three-dimensional printed moulds were used to create wax "domes" and "bowls" with curvatures ranging from +0.33 cm-1 to -0.13 cm-1. The dose per MU delivered at dmax, under the wax phantoms, to a calibrated NACP type chamber was determined by measurement. The fields were delivered using 6 to 18 MeV electrons from a Varian TrueBeam linac. The measured data shows that the dose varies linearly, for a given electron energy, with the curvature of the wax mould. This information was used to create a set of curvature correction factors to be applied to the 50 independent MU checks. When the appropriate correction factor was applied to the RadCalc MU only 3 of the 50 calculations were beyond the tolerance level, improving agreement and reducing the number of out of tolerance results.