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 (D m) to dose-to-water (D w) is usually based on the Bragg-Gray cavity theory. Acuros XB (AXB) reports also D m and D w but the method to calculate D w is based on the energy deposition cross sections for water in place of those for the local media. For both algorithms, the calculation of D w 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 ([Formula see text]) and dose-to-water ([Formula see text]), 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 [Formula see text] and to D w which would be obtained according to the conversion methods proposed by Andreo and Reynaert et al [Formula see text] agreed with [Formula see text] for all cases (±1%). In lung, all D w calculations and film measurements were in agreement. By contrast, [Formula see text] and [Formula see text] 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 [Formula see text] and [Formula see text]. 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 D w could be obtained which might lead to several issues in clinical context.