Department of Radiation Oncology, University of Washington, Seattle, Washington, USA.
Department of Radiation Oncology, University of Vermont Medical Center, Burlington, Vermont, USA.
J Appl Clin Med Phys. 2022 Apr;23(4):e13513. doi: 10.1002/acm2.13513. Epub 2022 Jan 5.
Total body irradiation (TBI) is an integral part of stem cell transplant. However, patients are at risk of treatment-related toxicities, including radiation pneumonitis. While lung dose is one of the most crucial aspects of TBI dosimetry, currently available data are based on point doses. As volumetric dose distribution could be substantially altered by lung block parameters, we used 3D dosimetry in our treatment planning system to estimate volumetric lung dose and measure the impact of various lung block designs.
We commissioned a TBI beam model in RayStation that matches the measured tissue-phantom ratio under our clinical TBI setup. Cerrobend blocks were automatically generated in RayStation on thoracic Computed Tomography (CT) scans from three anonymized patients using the lung, clavicle, spine, and diaphragmatic contours. The margin for block edge was varied to 0, 1, or 2 cm from the superior, lateral, and inferior thoracic borders, with a uniform margin 2.5 cm lateral to the vertebral bodies. The lung dose was calculated and compared with a prescription dose of 1200 cGy in six fractions (three with blocks and three without).
The point dose at midplane under the block and the average lung dose are at the range of 73%-76% and 80%-88% of prescription dose respectively regardless of the block margins. In contrast, the percent lung volume receiving 10 Gy increased by nearly two-fold, from 31% to 60% over the margins from 0 to 2 cm.
The TPS-derived 3D lung dose is substantially different from the nominal dose assumed with HVL lung blocks. Point doses under the block are insufficient to accurately gauge the relationship between dose and pneumonitis, and TBI dosimetry could be highly variable between patients and institutions as more descriptive parameters are not included in protocols. Much progress remains to be made to optimize and standardize technical aspects of TBI, and better dosimetry could provide more precise dosimetric predictors for pneumonitis risk.
全身照射（TBI）是干细胞移植的一个组成部分。然而，患者存在与治疗相关的毒性风险，包括放射性肺炎。虽然肺剂量是 TBI 剂量学中最重要的方面之一，但目前可用的数据基于点剂量。由于肺挡块参数会极大地改变容积剂量分布，因此我们在治疗计划系统中使用 3D 剂量学来估计容积肺剂量，并测量各种肺挡块设计的影响。
我们在 RayStation 中委托制作了一个 TBI 射束模型，该模型与我们临床 TBI 设置下的组织-体模比相匹配。在 RayStation 中，使用来自三名匿名患者的胸部 CT 扫描，自动生成 Cerrobend 挡块，挡块边缘与上、侧和下胸壁的距离分别为 0、1 或 2cm，椎体外侧的统一边缘为 2.5cm。计算并比较了肺剂量与 1200cGy 的处方剂量（带挡块和不带挡块各 3 次）。
挡块下中平面的点剂量和平均肺剂量分别在处方剂量的 73%-76%和 80%-88%范围内，无论挡块边缘如何。相比之下，肺体积接受 10Gy 的比例增加了近两倍，从 0 到 2cm 边缘时从 31%增加到 60%。
TPS 衍生的 3D 肺剂量与 HVL 肺挡块假设的名义剂量有很大差异。挡块下的点剂量不足以准确评估剂量与肺炎之间的关系，并且由于协议中未包含更多描述性参数，TBI 剂量学在患者和机构之间可能存在很大差异。为了优化和标准化 TBI 的技术方面，还有很多工作要做，更好的剂量学可以为肺炎风险提供更精确的剂量预测。
