LAUSR

Simple Summary The Gamma Knife® Icon™ allows for mask immobilization for stereotactic radiosurgery (SRS) as an alternative to frame immobilization. However, standardized recommendations for setup margins (SM) to create the planning target volume (PTV) with the mask immobilization do not exist and, therefore, practice patterns vary. Adding a SM might be the correct approach, if the possibility of significant intrafraction motion exists; on the other hand, it may be unnecessary as it increases the risk of radiation necrosis. This study, comprising 150 patients with 453 brain metastases (BM) treated for a median of 15 months of follow up, demonstrates that zero-SM mask immobilization had comparable clinical outcomes compared to a control group of similar patients undergoing frame immobilization SRS. There was no difference in freedom from local failure (FFLF) between the mask and frame immobilization groups on univariable or multivariable analysis. The initial findings support omitting a SM when using mask immobilization with this treatment approach on a GK Icon™. Abstract 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 (p = 0.024), and a higher number of treated lesions per SRS course (median 7 vs. 3; p < 0.001). With a median follow up of 15 months, there was no difference in FFLF between the mask and frame immobilization groups on univariable (p = 0.29) or multivariable analysis (p = 0.518). Actuarial FFLF at 1 year was 90.5% for mask and 92% for frame immobilization (p = 0.272). Radiation necrosis rates at 1 year were 12.5% for mask and 4.1% for frame immobilization (p = 0.502). For BM 0.5–2.0 cm in maximal diameter treated with single-fraction SRS using 22–24 Gy, mask immobilization with zero SM produces comparable clinical outcomes to frame immobilization. The initial findings support omitting a SM when using mask immobilization with this treatment approach on a Gamma Knife® Icon™.