Department of Radiation Oncology, MD Anderson Cancer Center at Cooper, Camden, NJ, 08103, USA.
Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA, 19111, USA.
Med Phys. 2017 Dec;44(12):6159-6165. doi: 10.1002/mp.12630. Epub 2017 Nov 14.
There is no consensus on the optimal prescription isodose line (IDL) in CyberKnife (CK) SRS. We designed a strategy to search for optimal CK plans at different levels of IDLs and investigated the dosimetric impact on the quality of CK plans.
The retrospective study consisted of 13 CK patients with 16 brain tumors. The mean volume and size of the tumors was 9.7 ± 10.4 cc and 30.3 ± 10.9 mm, respectively. Four shells were created at distances of 2-3 mm to 60 mm from the target. The constraint dose of the innermost shell (D1) was the primary optimization parameter. For isolated brain tumors, D1 started from the prescription dose and gradually reduced after optimization started over. The optimal plans were reached when the coverage started to degrade and the desired IDL was achieved. For eight tumors abutting an OAR, both the D1 and constraint dose to the OAR were gradually pushed until an optimal plan was reached for the desired IDL.
For the isolated tumors, the V5 Gy, V10 Gy, V15 Gy, V20 Gy, and V25 Gy of low IDL (49.6 ± 2.1%) plans were on average 23.6%, 28.6%, 33.8%, 26.2%, and 10.6% lower, respectively, comparing to the high IDL (88.6 ± 1.3%) plans. The Conformality Index (CI) of the low IDL plans outperformed the high IDL plans (mean: 1.15 vs. 1.24), except for a lesion under 0.5 cc. The quality of the middle IDL plans (69.6 ± 1.5%) was close to the low IDL plans. Similar results were observed for tumors abutting an OAR.
Low IDL plans outperformed high IDL plans for all metrics in tumors > 0.5 cc. The lower dose exposure of normal brain tissue and better CI could potentially reduce radiation necrosis while the higher maximum dose could improve local control.
在 CyberKnife（CK）SRS 中，对于最佳的处方等剂量线（IDL）尚无共识。我们设计了一种策略，以寻找不同 IDL 水平的最佳 CK 计划，并研究了剂量学对 CK 计划质量的影响。
本回顾性研究纳入了 13 例患有 16 个脑肿瘤的 CK 患者。肿瘤的平均体积和大小分别为 9.7±10.4cc 和 30.3±10.9mm。从靶区向 2-3mm 到 60mm 距离处创建了 4 个壳层。最内层壳层（D1）的约束剂量是主要的优化参数。对于孤立性脑肿瘤，D1 从处方剂量开始，在优化开始后逐渐降低。当覆盖范围开始下降并达到所需 IDL 时，即可达到最佳方案。对于 8 个毗邻 OAR 的肿瘤，D1 和 OAR 的约束剂量逐渐增加，直到达到所需 IDL 的最佳方案。
对于孤立性肿瘤，低 IDL（49.6±2.1%）计划的 V5Gy、V10Gy、V15Gy、V20Gy 和 V25Gy 平均分别降低了 23.6%、28.6%、33.8%、26.2%和 10.6%，而高 IDL（88.6±1.3%）计划则提高了 23.6%、28.6%、33.8%、26.2%和 10.6%。低 IDL 计划的适形指数（CI）优于高 IDL 计划（平均值：1.15 对 1.24），但小于 0.5cc 的病变除外。中 IDL 计划（69.6±1.5%）的质量接近低 IDL 计划。毗邻 OAR 的肿瘤也观察到类似的结果。
对于所有大于 0.5cc 的肿瘤，低 IDL 计划在所有指标上均优于高 IDL 计划。较低的正常脑组织剂量暴露和更好的 CI 可能会降低放射性坏死的风险，而较高的最大剂量可能会提高局部控制率。
