Rethinking the limits: SRS for ≥20 brain metastases with favorable brain and hippocampal doses

Whole-brain radiotherapy (WBRT) is commonly used for patients with a high burden of brain metastases (BM), but its well-documented toxicity profile raises concerns. Initial results from a pivotal phase III randomized trial comparing stereotactic radiosurgery (SRS) and WBRT for up to 20 lesions demonstrated that SRS had less negative patient impact without compromising oncologic or survival outcomes. However, for patients with very high BM burden (e.g., >20 lesions), outcomes following single-course SRS remain underexplored, particularly regarding cumulative brain dose.

We conducted a retrospective cohort study of patients treated with single-course SRS for ≥ 20 intracranial lesions. Dosimetric parameters, including mean dose to the brain and hippocampi, were analyzed.

Twenty patients underwent 21 courses of single-course SRS targeting ≥ 20 lesions. The median number of BM treated per course was 25, with a median aggregate target volume of 3.36 cc. The median mean dose to the brain was 2.49 Gy, and to the hippocampi was 2.28 Gy. Two patients developed probable radiation necrosis.

Single-course SRS for ≥ 20 BM appears safe and dosimetrically favorable compared with WBRT, where whole-brain doses typically range from 3-4 Gy. Hippocampal doses were notably lower than those seen with hippocampal-avoidant WBRT (HA-WBRT). These findings support the consideration of SRS in carefully selected patients with extensive intracranial disease, even when lesion counts exceed 20.