Department of Radiation Medicine, Roswell Park Cancer Institute, Buffalo, NY 14263, USA.
J Appl Clin Med Phys. 2010 Jan 28;11(1):3150. doi: 10.1120/jacmp.v11i1.3150.
The GammaPlan treatment planning system does not account for the leakage and scatter dose during APS repositioning. In this study, the dose delivered to the target site and its periphery from the defocus stage and intershot couch transit (couch motion from the focus to defocus position and back) associated with APS repositioning are measured for the Gamma Knife model 4C. A stereotactic head-frame was attached to a Leksell 16 cm diameter spherical phantom with a calibrated ion chamber at its center. Using a fiducial box, CT images of the phantom were acquired and registered in the GammaPlan treatment planning system to determine the coordinates of the target (center of the phantom). An absorbed dose of 10 Gy to the 50% isodose line was prescribed to the target site for all measurements. Plans were generated for the 8, 14 and 18 mm collimator helmets to determine the relationship of measured dose to the number of repositions of the APS system and to the helmet size. The target coordinate was identical throughout entire study and there was no movement of the APS between various shots. This allowed for measurement of intershot transit dose at the target site and its periphery. The couch was paused in the defocus position, allowing defocus dose measurements at the intracranial target and periphery. Measured dose increases with frequency of repositioning and with helmet collimator size. During couch transit, the target receives more dose than peripheral regions; however, in the defocus position, the greatest dose is superior to the target site. The automatic positioning system for the Leksell Gamma Knife model 4C results in an additional dose of up to 3.87 +/- 0.07%, 4.97 +/- 0.04%, and 5.71 +/- 0.07% to the target site; its periphery receives additional dose that varies depending on its position relative to the target. There is also dose contribution to the patient in the defocus position, where the APS repositions the patient from one treatment coordinate to another. This may be important for treatment areas around critical structures within the brain. Further characterization of the defocus and transit exposures and development of a dose calculation algorithm to account for these doses would improve the accuracy of the delivered plan.
GammaPlan 治疗计划系统在 APS 重新定位期间不考虑漏射线和散射剂量。在这项研究中，测量了 Gamma Knife 模型 4C 在 APS 重新定位期间从散焦阶段和两次射击之间的治疗床转运（治疗床从焦点到散焦位置再返回）输送到靶区及其周围的剂量。将立体定向头架固定在装有中心校准电离室的 Leksell 16 厘米直径球形体模上。使用基准盒，对体模进行 CT 扫描并在 GammaPlan 治疗计划系统中进行注册，以确定靶区（体模中心）的坐标。对所有测量均规定靶区 50%等剂量线吸收剂量为 10 Gy。为 8、14 和 18 毫米准直器头盔生成计划，以确定测量剂量与 APS 系统重新定位次数以及头盔尺寸的关系。整个研究过程中靶区坐标保持不变，并且 APS 在各次射击之间没有移动。这允许在靶区及其周围测量两次射击之间的转运剂量。治疗床在散焦位置暂停，允许在颅内靶区及其周围测量散焦剂量。测量剂量随重新定位频率和头盔准直器尺寸的增加而增加。在治疗床转运期间，目标区域比周围区域接收到更多的剂量；然而，在散焦位置，最大剂量位于靶区上方。Leksell Gamma Knife 模型 4C 的自动定位系统会导致靶区增加多达 3.87 +/- 0.07%、4.97 +/- 0.04%和 5.71 +/- 0.07%的额外剂量；其周围区域会接收到与靶区位置相关的额外剂量。当 APS 将患者从一个治疗坐标重新定位到另一个坐标时，患者在散焦位置也会受到剂量照射。这对于大脑内关键结构周围的治疗区域可能很重要。进一步描述散焦和转运照射情况，并开发剂量计算算法以考虑这些剂量，将提高计划实施的准确性。
