On the risks of a constant RBE for proton treatment planning

Abstract

Although it is well known that protons do not have a constant RBE of 1.1 this value is used in the daily clinical routine. To assess the impact of a variable RBE we have optimized proton treatment plans using a constant RBE and afterwards recalculated the RBE applying a method that is based on the linear-quadratic (LQ) model. We performed the study by creating treatment plans for four patients with nasopharyngeal carcinoma. A linear energy transfer (LET) dependence of the LQ-model was introduced by a linear dependence of the parameter alpha on the LET. For each volume of interest an individual set of model parameters alpha and beta derived from literature values has been used. The LET dependence was assumed to be the same for all tissues. The results have been compared as RBE-weighted absorbed dose distributions. It could be shown that the differences between variable and constant RBE distributions depend mainly on the choice of parameters but locally a severe increase in RBE-weighted absorbed dose could be observed due to regions of high LET. © 2009 Springer-Verlag.