Using a fixed-jaw technique to achieve superior delivery accuracy and plan quality in single-isocenter multiple-target stereotactic radiosurgery for brain metastases

To quantify the influence of the minimum dynamic leaf gap (DLG) on delivery accuracy and plan quality and determine whether the fixed-jaw technique can be used to reduce the adverse effects of DLG in single-isocenter volumetric modulated arc therapy (VMAT) stereotactic radiosurgery (SRS) for multiple brain metastases (BMs). Twelve patients, each with two BMs, were enrolled in this study. For each patient, five simulated cases were generated by adjusting the intertarget distance (ITD; range, 1-5 cm). For each case, two single-isocenter VMAT SRS plans with and without the fixed-jaw technique were designed. For the plan without the fixed-jaw technique, the jaw positions were automatically set to cover two targets (AJ-VMAT). For the other plan, the jaw was fixed to cover one lesion when another lesion was irradiated (FJ-VMAT). Dosimetric parameters and delivery parameters of two plans were compared. FJ-VMAT plans achieved similar target conformity, lower dose spillages (V10%, V25%), and lower normal brain tissue (NBT) mean dose, and better gradient index than AJ-VMAT plans regardless of ITD but with more monitor units and longer beam-on time (P < 0.05). For ITD =2 cm, FJ-VMAT plans also yielded lower V50% and 12-Gy isodose volume for NBT compared to AJ-VMAT, and the reduction became more distinct as the ITD increased. In addition, FJ-VMAT generated a significantly higher gamma passing rate for each criterion (P < 0.05), especially for larger ITDs (=2 cm) and tighter criteria (2%/1 mm) with a 4% improvement. Compared to conventional AJ-VMAT, FJ-VMAT could reduce the adverse effects from minimum DLG with superior delivery accuracy and plan quality in single-isocenter VMAT SRS for multiple BMs