Zero Setup Margin Mask versus Frame Immobilization during Gamma Knife

We compared the clinical outcomes of BM treated with mask immobilization with zero-SM (i.e., zero-PTV) to standard zero-SM frame immobilization SRS. Consecutive patients with BM, 0.5-2.0 cm in maximal diameter, treated with single-fraction SRS (22-24 Gy) during March 2019-February 2021 were included. Univariable and multivariable analysis were performed using the Kaplan-Meier method and Cox proportional hazards regression. A total of 150 patients with 453 BM met inclusion criteria. A total of 129 (28.5%) lesions were treated with a zero-SM mask immobilization and 324 (71.5%) with zero-SM frame immobilization. Frame immobilization treatments were associated with a higher proportion of gastrointestinal and fewer breast-cancer metastases (