Li Tianfang, Ozhasoglu Cihat, Burton Steven, Flickinger John, Heron Dwight E, Huq M Saiful
University of Pittsburgh Cancer Institute.
J Appl Clin Med Phys. 2015 Nov 8;16(6):333-339. doi: 10.1120/jacmp.v16i6.5748.
For targets with substantial volume, collimators of relatively large size are usually selected to minimize the treatment time in robotic radiosurgery. Their large penumbrae may adversely affect the dose gradient around the target. In this study, we implement and evaluate an inner-shell planning method to increase the dose gradient and reduce dose to normal tissues. Ten patients previously treated with CyberKnife M6 system were randomly selected with the only criterion being that PTV be larger than 2 cm(3). A new plan was generated for each patient in which the PTV was split into two regions: a 5 mm inner shell and a core, and a 7.5 mm Iris collimator was exclusively applied to the shell, with other appropriate collimators applied to the core depending on its size. The optimization objective, functions, and constraints were the same as in the corresponding clinical plan. The results were analyzed for V12 Gy, V9 Gy, V5 Gy, and gradient index (GI). Volume reduction was found for the inner-shell method at all studied dose levels as compared to the clinical plans. The absolute dose-volume reduction ranged from 0.05 cm(3) to 18.5 cm(3) with a mean of 5.6 cm(3) for 12 Gy, from 0.2 cm(3) to 38.1 cm(3) with a mean of 9.8 cm(3) for 9 Gy, and from 1.5 cm(3) to 115.7 cm(3) with a mean of 24.8 cm(3) for 5 Gy, respectively. The GI reduction ranged from 3.2% to 23.6%, with a mean of 12.6%. Paired t-test for GI has a p-value of 0.0014. The range for treatment time increase is from -3 min to 20 min, with a mean of 7.0 min. We conclude that irradiating the PTV periphery exclusively with the 7.5 mm Iris collimator, rather than applying mixed collimators to the whole PTV, can substantially improve the dose gradient, while maintaining good coverage, conformity, and reasonable treatment time.
对于体积较大的靶区，在机器人放射外科手术中通常选择尺寸相对较大的准直器，以尽量缩短治疗时间。其较大的半值层可能会对靶区周围的剂量梯度产生不利影响。在本研究中，我们实施并评估了一种内壳计划方法，以增加剂量梯度并减少对正常组织的剂量。随机选择10例先前接受射波刀M6系统治疗的患者，唯一标准是计划靶体积（PTV）大于2 cm³。为每位患者生成一个新计划，其中PTV被分为两个区域：一个5 mm的内壳和一个核心区，仅将一个7.5 mm的虹膜准直器应用于内壳，根据核心区大小将其他合适的准直器应用于核心区。优化目标、函数和约束条件与相应的临床计划相同。分析了V12 Gy、V9 Gy、V5 Gy和梯度指数（GI）的结果。与临床计划相比，在内壳方法的所有研究剂量水平下均发现体积减小。12 Gy时，绝对剂量体积减小范围为0.05 cm³至18.5 cm³，平均为5.6 cm³；9 Gy时，范围为0.2 cm³至38.1 cm³，平均为9.8 cm³；5 Gy时，范围为1.5 cm³至115.7 cm³，平均为24.8 cm³。GI降低范围为3.2%至23.6%，平均为12.6%。GI的配对t检验p值为0.0014。治疗时间增加范围为 -3分钟至20分钟，平均为7.0分钟。我们得出结论，仅用7.5 mm虹膜准直器照射PTV周边，而不是对整个PTV应用混合准直器，可以在保持良好的覆盖、适形性和合理治疗时间的同时，显著改善剂量梯度。
