Clinical comparison of head and neck and prostate IMRT plans using absorbed dose to medium and absorbed dose to water

Conventional photon radiation therapy dose-calculation algorithms typically compute and report the absorbed dose to water (D-w). Monte Carlo (MC) dose-calculation algorithms, however, generally compute and report the absorbed dose to the material (D-m). As MC-calculation algorithms are being introduced into routine clinical usage, the question as to whether there is a clinically significant difference between D-w and D-m remains. The goal of the current study is to assess the differences between dose-volume indices for D-m and D-w MC-calculated IMRT plans. Ten head-and-neck (H&N) and ten prostate cancer patients were selected for this study. MC calculations were performed using an EGS4-based system. Converting D-m to D-w for MC-based calculations was accomplished as a post-MC calculation process. D-w and D-m results for target and critical structures were evaluated using the dose-volume-based indices. For H&N IMRT plans, systematic differences between dose-volume indices computed with D-w and D-m were up to 2.9% for the PTV prescription dose (D98), up to 5.8% for maximum (D2) dose to the PTV and up to 2.7% for the critical structure dose indices. For prostate IMRT plans, the systematic differences between D-w- and D-m- based computed indices were up to 3.5% for the prescription dose ( D98) to the PTVs, up to 2.0% for the maximum ( D2) dose to the PTVs and up to 8% for the femoral heads due to their higher water/bone mass stopping power ratio. This study showed that converting D-m to D-w in MC-calculated IMRT treatment plans introduces a systematic error in target and critical structure DVHs. In some cases, this systematic error may reach up to 5.8% for H&N and 8.0% for prostate cases when the hard-bone-containing structures such as femoral heads are present. Ignoring differences between Dm and D-w will result in systematic dose errors ranging from 0% to 8%.