OBJECT Radiation necrosis (RN), or its imaging equivalent, treatment-related imaging changes (TRIC), is an inflammatory reaction to high-dose radiation in the brain. The authors sought to investigate the hypothesis that immunotherapy increases the risk of developing RN/TRIC after stereotactic Gamma Knife (GK) radiosurgery for brain metastases. METHODS A total of 180 patients who underwent GK surgery for brain metastases between 2006 and 2012 were studied. The systemic therapy they received was classified as cytotoxic chemotherapy (CT), targeted therapy (TT), or immunotherapy (IT). The timing of systemic therapy in relation to GK treatment was also recorded. Logistic regression was used to calculate the odds of developing RN according to type of systemic therapy received. RESULTS The median follow-up time was 11.7 months. Of 180 patients, 39 (21.7%) developed RN/TRIC. RN/TRIC rates were 37.5% (12 of 32) in patients who received IT alone, 16.9% (14 of 83) in those who received CT only, and 25.0% (5 of 20) in those who received TT only. Median overall survival was significantly longer in patients who developed RN/TRIC (23.7 vs 9.9 months, respectively). The RN/TRIC rate was increased significantly in patients who received IT alone (OR 2.40 [95% CI 1.06–5.44]; p = 0.03), whereas receipt of any CT was associated with a lower risk of RN/TRIC (OR 0.38 [95% CI 0.18–0.78]; p = 0.01). The timing of development of RN/TRIC was not different between patients who received IT and those who received CT. CONCLUSIONS Patients who receive IT alone may have an increased rate of RN/TRIC compared with those who receive CT or TT alone after stereotactic radiosurgery, whereas receiving any CT may in fact be protective against RN/TRIC. As the use of immunotherapies increases, the rate of RN/TRIC may be expected to increase compared with rates in the chemotherapy era.