Dogan N, Siebers J V, Keall P J
Radiation Oncology Department, Virginia Commonwealth University Medical Center, 401 College Street, Richmond, 23298, USA.
Phys Med Biol. 2006 Oct 7;51(19):4967-80. doi: 10.1088/0031-9155/51/19/015. Epub 2006 Sep 18.
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 (D(98)), up to 5.8% for maximum (D(2)) 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 (D(98)) to the PTVs, up to 2.0% for the maximum (D(2)) 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 D(m) and D(w) will result in systematic dose errors ranging from 0% to 8%.
传统的光子放射治疗剂量计算算法通常计算并报告水的吸收剂量（D(w)）。然而，蒙特卡罗（MC）剂量计算算法一般计算并报告物质的吸收剂量（D(m)）。随着MC计算算法被引入常规临床应用，D(w)和D(m)之间是否存在临床显著差异的问题依然存在。本研究的目的是评估MC计算的调强放疗（IMRT）计划中D(m)和D(w)的剂量体积指数之间的差异。本研究选取了10例头颈部（H&N）癌患者和10例前列腺癌患者。使用基于EGS4的系统进行MC计算。将基于MC计算的D(m)转换为D(w)是在MC计算后完成的过程。使用基于剂量体积的指数评估靶区和关键结构的D(w)和D(m)结果。对于H&N的IMRT计划，用D(w)和D(m)计算的剂量体积指数之间的系统差异，对于计划靶体积（PTV）处方剂量（D(98)）高达2.9%，对于PTV的最大剂量（D(2)）高达5.8%，对于关键结构剂量指数高达2.7%。对于前列腺IMRT计划，基于D(w)和D(m)计算的指数之间的系统差异，对于PTV的处方剂量（D(98)）高达3.5%，对于PTV的最大剂量（D(2)）高达2.0%，对于股骨头由于其较高的水/骨质量阻止本领比差异高达8%。本研究表明，在MC计算的IMRT治疗计划中将D(m)转换为D(w)会在靶区和关键结构的剂量体积直方图（DVH）中引入系统误差。在某些情况下，当存在如股骨头等含硬骨结构时，对于H&N病例这种系统误差可能高达5.8%，对于前列腺病例高达8.0%。忽略D(m)和D(w)之间的差异将导致0%至8%的系统剂量误差。
