A comparison of electron beam dose calculation accuracy between treatment planning systems using either a pencil beam or a Monte Carlo algorithm

Purpose: To present a comparison of the accuracy of two commercial electron beam treatment planning systems: one uses a Monte Carlo algorithm and the other uses a pencil beam model for dose calculations. Methods and Materials: For the same inhomogeneous phantoms and incident beams, measured dose distributions are compared with those predicted by the commercial treatment planning systems at different source-to-surface distances (SSDs). The accuracy of the pencil beam system for monitor unit calculations is also tested at various SSDs. Beam energies of 6-20 MeV are used. Results: The pencil beam model shows some serious limitations in predicting hot and cold spots in inhomogeneous phantoms for small low- or high-density inhomogeneities, especially for low-energy electron beams, such as 9 MeV. Errors (> 10%) are seen in predicting high- and low-dose variations for three-dimensional inhomogeneous phantoms. The Monte Carlo calculated results generally agree much better with measurements. Conclusions: The accuracy of the pencil beam calculations is difficult to predict because it depends on both the inhomogeneity geometry and location. The pencil beam calculations using CADPLAN result in large errors in phantoms containing three-dimensional type inhomogeneities. The Monte Carlo method in Theraplan Plus dose calculation module is shown to be more robust in accurately predicting dose distributions and monitor units under the tested conditions. (c) 2005 Elsevier Inc.