Single Fraction Frameless Stereotactic Radiosurgery on the Gamma Knife Icon for Patients With Brain Metastases: Time to Abandon the Frame?

PURPOSE: The latest version of the Gamma Knife, the Icon, allows for immobilization with a mask in lieu of the traditional frame during stereotactic radiosurgery. There have been some concerns regarding extent of immobilization during single fraction frameless treatment and potential effect on outcomes. As such, we reviewed outcomes in patients with brain metastases treated in a single fraction using either a frame or mask on the Gamma Knife Icon at our institution. METHODS AND MATERIALS: We reviewed the records of 95 patients with a total of 374 metastases treated between May 2019 and January 2021. Thirty-nine patients (41%) were treated using the Leksell frame with the remainder being immobilized with a mask. The median number of metastatic lesions was 2 (1–20). The median prescription dose was 20 Gy (11.5–24 Gy). Odds ratios were generated to identify predictors of mask use. Kaplan-Meier analysis was used to calculate survival, local failure, and distant failure rates. Cox regression was used to identify predictors of survival. Propensity matching was used to account for indication bias. RESULTS: Of the 95 patients treated, 88 (93%) had follow-up with a median duration of 5 months (1–18). Frame utilization was more likely with 6 to 10 brain metastases. Median overall survival was not reached and was 70% and 60% at 6 and 12 months for the entire cohort, respectively. There was no significant difference in survival by immobilization method (P = .12). Six patients had local failure in 10 total lesions (3 patients in each group). After propensity matching the 12 month tumor local control was 96% and 85% for framed and frameless cases, respectively (P = .07). CONCLUSIONS: Frameless mask-based stereotactic radiosurgery using the Gamma Knife Icon is feasible and maintains the excellent local control seen with the use of the headframe.