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Authors: Allison N Palmiero, MS, Denise Fabian, MD, William St Clair, MD, PhD, Marcus Randall, MD, Damodar Pokhrel, PhD
Medical Physics Graduate Program, Department of Radiation Medicine, University of Kentucky, Lexington KY 40536 USA
Single-isocenter volumetric modulated arc therapy (VMAT) stereotactic radiosurgery (SRS) techniques to treat multiple brain metastases simultaneously can signiﬁcantly improve treatment delivery eﬃciency, patient compliance, and clinic workﬂow. However, due to large number of brain metastases sharing the same MLC pair causing island blocking, there is higher low- and intermediate-dose spillage to the normal brain and higher dose to organs-at-risk (OAR). To minimize this problem and improve plan quality, this study proposes a dual-isocenter planning strategy that groups lesions based on hemisphere location (left vs right-sided) in the brain parenchyma, providing less island blocking reducing the MLC travel distance. This technique offers simpliﬁed planning while also increasing patient comfort and compliance by allowing for large number of brain metastases to be treated in 2 groups. Seven complex patients with 5 to 16 metastases (64 total) were planned with a single-isocenter VMAT-SRS technique using a 10MV-FFF beam with a prescription of 20 Gy to each lesion. The isocenter was placed at the approximate geometric center of the targets. Each patient was replanned using the dual-isocenter approach, generating 2 plans and placing each isocenter at the approximate geometric center of the combined targets of each side with corresponding non-coplanar partial arcs. Compared to single-isocenter VMAT, dual-isocenter VMAT plans provided similar target coverage and dose conformity with less spread of intermediate dose to normal brain with reduction of dose to OAR. Reduction in total monitor units and beam-on time was observed, but due to the second isocenter setup and veriﬁcation, overall treatment time was increased. Dual-isocenter VMAT-SRS planning for multiple brain metastases is a simpliﬁed approach that provides superior treatment options for patient compliance who may not tolerate longer traditional treatment times as with individual isocenters to each target. This planning technique signiﬁcantly reduces the amount of low- and intermediate-dose spillage, further sparing OAR and normal brain, potentially im- proving target accuracy though localization of left vs right-sided tumors for each isocenter set up.