Curvature correction factors for the independent verification of dose monitor units of electron treatment plans calculated in Eclipse.

Abstract

Abstract Electron Monte Carlo (MC) dose calculation algorithms are sensitive to the incidence angle and curvature of the body contour of the patient. Over 10% difference between Treatment Planning System (TPS) and Independent Monitor Unit (IMU) calculations have been reported in the literature. We also observed similar results while using ClearCalc IMU calculation for electrons treatments planned in the Eclipse TPS. These plans can have over 10% difference to ClearCalc calculations when there is significant curvature of the body contour on the incident beam. The aim of this project is to measure and apply curvature correction factors (CCFs) in the clinical setting. Spherical and cylindrical phantoms were 3D printed from water equivalent PLA of varying diameters. The custom phantom was placed on solid water and measurements were carried out using NACP chamber at dmax depth. The CCF was defined as the ratio of the chamber reading with the custom phantom to that with solid water at the same depth. CCFs were measured for three electron energies (6MeV, 9MeV and 12MeV) and compared to calculations made in the Eclipse TPS. CCFs are then applied to ClearCalc calculations for patient cases to compare against the Eclipse MU calculation. Varian’s Eclipse API scripting is used to assist in calculating the patient curvature for the IMU calculations. The mean difference between the measured and TPS calculated CCFs was -0.6% with a maximum difference of 7.8% for the smallest diameter phantom (2.8cm) with 6MeV energy. The measured CCF’s can be used for correcting surface curvature to improve independent MU calculations for electron plans.