Utilization of CBCT to improve the delivery accuracy of Gamma Knife radiosurgery with G-frame

Purpose: To quantify the G-frame based stereotactic coordinate definition accuracy of Leksell coordinate G-frame-based Gamma Knife radiosurgery (GKRS) by the on-board cone-beam CT (CBCT) and establish remedial action rules to minimize the delivery errors. Methods: We analyzed the data of 108 patients (a total of 201 tumors) treated by GKRS with G-frame for head fixation. After co-registering the CBCT images and plan reference images, the Leksell GammaPlan (LGP) treatment planning system provided the amount of geometric translation and rotation required to minimize the position difference between the plan and treatment. The software also calculated maximum displacement, which characterizes the position shift more clearly. We studied how much these predicted dosimetric quantities changed if the treatment was delivered without correcting the patient's position. Results: The maximum displacement of the patient position obtained from the co-registration of CBCT and plan reference images was 0.81 ± 0.38 mm (0.24–2.03 mm). The target coverage decreased by 3.3 ± 7.0% on average (−48.5% to +35.7%). The decrease of the target coverage, however, became smaller as the target volume increased. In particular, if the volume was greater than 2 cm³, the %change in target coverage was always less than −5%. Conclusions: The position differences reported by the registration module of LGP were within the accuracy limit of image registration for most clinical cases, but the errors could be larger in some cases. Therefore, we propose the following decision process. We do not advise position adjustment for G-frame based GKRS if the maximum displacement is less than 1 mm. When this limit is exceeded, however, another criterion should be applied to the decision making by considering the tumor size (or the treatment volume) together with the acceptable change of the tumor coverage.