Stopping power ratio databases for proton therapy dose calculation

In modern radiation therapy, ion beams are increasingly used for cancer treatment. Protons are light ions that have Bragg peak characteristic in the depth dose distribution that is optimal for minimizing dose to surrounding normal tissues. In proton therapy treatment planning, the stopping power ratio (SPR) of a given medium to that of water is used for calculating the water-equivalent pathlength (WEPL) of tissues in the patient. Since SPR is related to the computed tomography (CT) number, the conversion from CT numbers to SPRs can be used to find WEPLs of tissue voxels, which are sequentially applied in selecting the initial proton energy. In this work, we investigated the effect of using different CT number to SPR conversion models on proton pencil beam dose calculation. In the SPR calculation, either elemental mean excitation energy (I-value) from the ICRU report or elemental mass stopping power from SRIM software were used. For each approach, four energy dependent scenarios were investigated. The ICRU model and the SRIM model showed a monotonic correlation of SPRs for most tissue types. The 2D dose distribution was calculated by a MATLAB-based proton therapy treatment planning system, PSPLAN. The results suggest that elemental I-values and elemental mass stopping power can be used interchangeably for SPR calculation. Moreover, CT number to SPR conversion models obtained for specific proton energy and average proton energy did not cause difference in pencil beam dose calculation unless the proton energy was relatively low.