Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
J Appl Clin Med Phys. 2022 Oct;23(10):e13774. doi: 10.1002/acm2.13774. Epub 2022 Sep 15.
Iodination of rectal hydrogel spacer increases the computed tomography (CT) visibility. The effect of iodinated hydrogel spacer material on the accuracy of proton dosimetry has not been fully studied yet. We presented a systematic study to determine the effect of iodination on proton dosimetry accuracy during proton therapy (PT).
PT plans were designed for 20 prostate cancer patients with rectal hydrogel spacer. Three variations of hydrogel density were considered. First, as the ground truth, the true elemental composition of hydrogel true material (TM), verified by our measurement of spacer stopping power ratio, was used for plan optimization and Monte Carlo dose calculation. The dose distribution was recalculated with (1) no material (NM) override based on the CT intensity of the iodinated spacer, and (2) the water material (WM) override, where spacer material was replaced by water. The plans were compared with the ground truth using the metrics of gamma index (GI) and dosimetric indices.
The iodination of hydrogel spacer affected the proton dose distribution with the NM scenario showing the most deviation from the ground truth. The iodination of spacer resulted in a notable increase in CT intensity and led to the treatment planning systems mistreating the iodinated spacer as a high-density material. Among the structures adjacent to the target, neurovascular bundles showed the largest dose difference, up to 350 cGy or about 5% of the prescribed dose with NM. Compared to the WM scenario, dose distribution similarity and GI passing ratios were lower in the NM scenario.
The inaccurate CT intensity-based material for iodinated spacer resulted in errors in PT dose calculation. We found that the error was negligible if the iodinated spacer was replaced with water. Water density can be used as a clinically accessible and convenient alternative material override to true spacer material.
直肠水凝胶间隔物的碘化可提高计算机断层扫描（CT）的可见度。碘化水凝胶间隔物材料对质子剂量准确性的影响尚未得到充分研究。我们进行了一项系统研究，以确定在质子治疗（PT）期间碘化对质子剂量准确性的影响。
为 20 名患有直肠水凝胶间隔物的前列腺癌患者设计了 PT 计划。考虑了三种水凝胶密度变化。首先，作为基准，通过测量间隔物阻止比来验证水凝胶真实材料（TM）的真实元素组成，用于计划优化和蒙特卡罗剂量计算。使用（1）基于碘化间隔物的 CT 强度的无材料（NM）覆盖和（2）水材料（WM）覆盖，重新计算剂量分布，其中用水替代间隔物材料。使用伽马指数（GI）和剂量学指标将计划与基准进行比较。
水凝胶间隔物的碘化影响了质子剂量分布，NM 方案显示与基准的偏差最大。间隔物的碘化导致 CT 强度显著增加，导致治疗计划系统将碘化间隔物误作为高密度材料进行处理。在与靶区相邻的结构中，神经血管束的剂量差异最大，NM 时最大剂量差异可达 350 cGy 或约 5%的处方剂量。与 WM 方案相比，NM 方案的剂量分布相似性和 GI 通过率较低。
基于碘化间隔物不准确的 CT 强度的材料导致 PT 剂量计算错误。我们发现，如果用WM 替换碘化间隔物，误差可以忽略不计。水密度可以用作替代真实间隔物材料的临床可及且方便的替代材料覆盖。
