Hippocampal sparing in patients receiving radiosurgery for ≥25 brain metastases

•89 patients treated with stereotactic radiosurgery for ≥25 brain metastases.•More than half had tumors within the hippocampal avoidance region.•Patients with tumors involving the hippocampus had the highest median Dmin.•Even with tumors in the hippocampal avoidance regions, stereotactic affords hippocampal sparing. To report our dosimetric analysis of the hippocampi (HC) and the incidence of perihippocampal tumor location in patients with ≥25 brain metastases who received stereotactic radiosurgery (SRS) in single or multiple sessions. Analysis of our prospective registry identified 89 patients treated with SRS for ≥25 brain metastases. HC avoidance regions (HA-region) were created on treatment planning MRIs by 5 mm expansion of HC. Doses from each session were summed to calculate HC dose. The distribution of metastases relative to the HA-region and the HC was analyzed. Median number of tumors irradiated per patient was 33 (range 25–116) in a median of 3 (range1–12) sessions. Median bilateral HC Dmin (D100), D40, D50, Dmax, and Dmean (Gy) was 1.88, 3.94, 3.62, 16.6, and 3.97 for all patients, and 1.43, 2.99, 2.88, 5.64, and 3.07 for patients with tumors outside the HA-region. Multivariate linear regression showed that the median HC D40, D50, and Dmin were significantly correlated with the tumor number and tumor volume (p