Department of Medical Physics and Engineering, St. James's Institute of Oncology, St. James's University Hospital, Leeds, West Yorkshire LS9 7TF, United Kingdom.
Med Phys. 2011 Feb;38(2):884-90. doi: 10.1118/1.3532824.
It is recommended to have a method for independently verifying planned doses in stereotactic radiosurgery. The problem is one of how to model the geometry of a skull sampled by a limited number of points and how to subsequently calculate numerous attenuation pathlengths through the modeled skull. While methods of verification have been previously published for model B and C Gamma Knife units, the aims of the current work were to apply the principles of these previously published techniques for the verification of plans for Gamma Knife PERFEXIONTM, to present a new method of verification, and to compare all methods in terms of their agreement with GammaPlan.
Four algorithms were implemented: the previously published spherical approximation method (SAM) and bubble helmet skull (BHS), plus a modified BHS named interpolated BHS (IBHS) and a newly developed variable radius SAM (VRSAM). Reference point doses calculated by the four algorithms were compared to those reported by GammaPlan for 54 simple test plans and for 35 targets in 20 recent patient plans.
For test plans, the mean (standard deviation) discrepancies against GammaPlan-reported doses were 0.3 (1.3)%, 0.3 (1.3)%, -1.6 (3.4)%, and -0.4 (1.0)% for SAM, VRSAM, BHS, and IBHS, respectively. For patient plans both the VRSAM and IBHS showed insignificant (p=0.22 and p = 0.50) discrepancies against GammaPlan of 0.38 (1.86)% and -0.11 (1.86)%, respectively. More significant discrepancies against GammaPlan (p < 0.0001) of 2.64 (2.98)% and -4.43 (3.39)% were observed for the SAM and BHS.
The SAM can lead to large discrepancies against GammaPlan when a sphere is a poor approximation of the true skull surface, and in peripheral locations can lead to nonreal solutions to the attenuation pathlength calculations. While the BHS does not suffer the same geometric assumptions of the SAM, it can underestimate dose for peripherally located shots. The IBHS exhibits better agreement with GammaPlan than does the BHS, but requires two-dimensional interpolation that was found to be impractical to implement in the Excel-based software used in the current work. Combining aspects of both the previously published SAM and BHS algorithms, the newly presented VRSAM exhibits comparable results to the IBHS but without the need for interpolation and is therefore considered the preferred technique of the four implemented.
建议有一种方法可以独立验证立体定向放射外科中的计划剂量。问题之一是如何对通过有限数量的点采样的颅骨几何形状进行建模，以及如何随后计算通过建模颅骨的大量衰减路径长度。虽然之前已经为模型 B 和 C Gamma Knife 单位发布了验证方法，但当前工作的目的是将之前发表的这些技术的原理应用于 Gamma Knife PERFEXIONTM 计划的验证，提出一种新的验证方法，并根据与 GammaPlan 的一致性比较所有方法。
实施了四种算法：先前发表的球形逼近方法 (SAM) 和泡沫头盔颅骨 (BHS)，以及一种名为插值 BHS (IBHS) 的改良 BHS 和一种新开发的可变半径 SAM (VRSAM)。由四个算法计算的参考点剂量与 GammaPlan 为 54 个简单测试计划和 20 个最近患者计划中的 35 个靶区报告的剂量进行了比较。
对于测试计划，SAM、VRSAM、BHS 和 IBHS 分别与 GammaPlan 报告剂量的平均（标准差）偏差为 0.3（1.3）%、0.3（1.3）%、-1.6（3.4）%和-0.4（1.0）%。对于患者计划，VRSAM 和 IBHS 与 GammaPlan 的偏差分别为 0.38（1.86）%和-0.11（1.86）%，均无统计学意义（p=0.22 和 p=0.50）。SAM 和 BHS 与 GammaPlan 的差异更为显著（p<0.0001），分别为 2.64（2.98）%和-4.43（3.39）%。
当球体不是真实颅骨表面的良好近似值时，SAM 可能导致与 GammaPlan 之间的较大差异，并且在周边位置可能导致衰减路径长度计算的非真实解。虽然 BHS 没有 SAM 那样的几何假设，但它可能会低估外围位置的剂量。IBHS 与 GammaPlan 的一致性优于 BHS，但需要二维插值，在当前工作中使用的基于 Excel 的软件中发现该插值不切实际。新提出的 VRSAM 结合了先前发表的 SAM 和 BHS 算法的各个方面，与 IBHS 相比具有相当的结果，但无需插值，因此被认为是四种实现方法中的首选技术。
