Dosimetric study of split-arc technique based on personalized collimator optimization in VMAT for multiple brain metastases.

Purpose

To evaluate the multiple split-arc technique in mitigating the "island blocking effect (IBE)" during volumetric modulated arc therapy (VMAT) for multiple brain metastases (MBM) and identify the optimal clinical protocol.

Methods

A retrospective analysis of 20 MBM patients (2-5 targets) was performed. Four VMAT plans were generated per patient: a control group (2A, conventional dual arcs) and three experimental groups (4sA, 6sA, 8sA) where arcs were split with personalized collimator optimization based on beam's eye view (BEV). Dosimetric parameters for planning target volumes (PTV), normal brain tissue (NBT), organs at risk (OARs), and delivery metrics (MU, complexity metric [CM], control point mean jaw area [AJaw, mean], control point mean jaw-X/Y width [WJaw-X/Y, mean]) were compared.

Results

All strategies met prescription requirements with no significant differences in CI or HI (P > 0.05). Experimental groups showed significant improvements in GI, NBT mean dose (Dmean), V12Gy, MU, CM, AJaw, mean and WJaw-X/Y, mean (P < 0.05), with improvements correlating with the number of sub-arcs.Specifically, 6sA and 8sA groups significantly reduced NBT's Dmean and V12Gy (P < 0.01), and OAR exposure (P < 0.05). The 8sA group achieved the best performance, with GI, MU, CM, and WJaw-X, mean reduced by 12.05%, 14.43%, 10.80%, and 27.52%, respectively, while WJaw-Y, mean increased by 24.04% compared to the 2A group (P < 0.001). However, no statistically significant differences were found between the 6sA and 8sA groups regarding MU, CM, or most OAR sparing (P > 0.05).

Conclusion

The split-arc technique with collimator optimization effectively mitigates IBE and reduces NBT dose. Balancing dosimetric quality and efficiency, the 6-split arc (6sA) strategy is recommended as the preferred protocol for MBM.