Longitudinal Grouping of Target Volumes for Volumetric-Modulated Arc Therapy of Multiple Brain Metastases

Purpose Treatment of multiple brain metastases with single-isocenter volumetric modulated arc therapy causes unnecessary exposure to normal brain tissue. In this study, a longitudinal grouping method was developed to reduce such unnecessary exposure. Materials and Methods This method has two main aspects: grouping brain lesions longitudinally according to their longitudinal projection positions in beam's eye view, and rotating the collimator to 90 degrees to make the multiple leaf collimator leaves conform to the targets longitudinally group by group. For 11 patients with multiple (5-30) brain metastases, two single-isocenter volumetric modulated arc therapy plans were generated using a longitudinal grouping strategy (LGS) and the conventional strategy (CVS). The prescription dose was 52 Gy for 13 fractions. Dose normalization to 100% of the prescription dose in 95% of the planning target volume was adopted. For plan quality comparison, Paddick conformity and the gradient index of the planning target volume, and the mean dose, the V-100%, V-50%, V-25%, and V-10% volumes of normal brain tissue were calculated. Results There were no significant differences between the LGS and CVS plans in Paddick conformity (p = 0.374) and the gradient index (p = 0.182) of the combined planning target volumes or for V-100% (p = 0.266) and V-50% (p = 0.155) of the normal brain. However, the V-25% and V-10% of the normal brain which represented the low-dose region were significantly reduced in the LGS plans (p = 0.004 and p = 0.003, respectively). Consistently, the mean dose of the entire normal brain was 12.04 and 11.17 Gy in the CVS and LGS plans, respectively, a significant reduction in the LGS plans (p = 0.003). Conclusions The longitudinal grouping method can decrease unnecessary exposure and reduces the low-dose range in normal brain tissue.