Hippocampal sparing in patients receiving radiosurgery for ≥25 brain metastases.

Purpose/objectives: 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.

Materials/methods: 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.

Results: Median number of tumors irradiated per patient was 33 (range 25-116) in a median of 3 (range1-12) sessions. Median bilateral HC D (D), D, D, D, and D (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 D, D, and D were significantly correlated with the tumor number and tumor volume (p < 0.001). Of the total 3059 treated tumors, 83 (2.7%) were located in the HA-region in 57% evaluable patients; 38 tumors (1.2%) abutted or involved the HC itself.

Conclusions: Hippocampal dose is higher in patients with tumors in the HA-region; however, even for patients with a high burden of intracranial disease and tumors located in the HA-regions, SRS affords hippocampal sparing. This is particularly relevant in light of our finding of eventual perihippocampal metastases in more than half of our patients.