On the conversion from dose-to-medium to dose-to-water in heterogeneous phantoms with Acuros XB and Monte Carlo calculations

Abstract

The method implemented in Monte Carlo (MC) algorithm to convert dose-to-medium (Dm) to dose-to-water (Dw) is usually based on the Bragg-Gray cavity theory. Acuros XB (AXB) reports also Dm and Dw but the method to calculate Dw is based on the energy deposition cross sections for water in place of those for the local media. For both algorithms, the calculation of Dw in non-water media is similar to the dose received in a small volume of water, small enough not to disturb the fluence of charged particles. Recently, two new methods revised the Bragg-Gray cavity theory, one proposed by Andreo and the other by Reynaert et al. In this context, comparisons between AXB and MC were carried out in terms of dose-to-medium () and dose-to-water (), respectively. Multilayer slab heterogeneous phantoms made of lung, bone and polytetrafluoroethylene (PTFE) were investigated and measurements were carried out using radiochromic films. These latter were then compared to and to Dw which would be obtained according to the conversion methods proposed by Andreo and Reynaert et al agreed with for all cases (±1%). In lung, all Dw calculations and film measurements were in agreement. By contrast, and differed notably in bone (4.5%) and PTFE (3.5%), and both algorithms overestimated film measurements. These findings demonstrate that the conversion method is different between AXB and MC. Furthermore, films were not able to give dose in a small volume of water according to the definition of and . Applying either the fluence correction factor suggested by Andreo or the mass energy absorption ratios proposed by Reynaert et al, resulted in a good agreement (<1%) with film measurements. According to the method used for the conversion, different Dw could be obtained which might lead to several issues in clinical context.