1323 Pathologic Incidence of Tumor Positivity Versus Necrosis After Prior Radiotherapy in Patients Undergoing Craniotomy and Resection for Presumed Recurrent Brain Metastasis: Insights from a Prospective Multi-Institutional Registry

INTRODUCTION: Determining true recurrence versus necrosis alone after previous radiotherapy (RT) for brain metastasis based on imaging alone is challenging. Accurate diagnosis is critical to patient management, as further RT is contraindicated in the setting of radiation necrosis without tumor (TUM-). METHODS: Rates of intraoperative frozen section pathology reporting tumor +/- necrosis (TUM+) or necrosis without tumor (TUM-) were examined in patients undergoing resection for presumed RBM after prior same-site RT. All cases were prospectively enrolled on a multi-institutional registry for patients undergoing resection and intraoperative cesium-131 collagen tile brachytherapy (NCT04427384)(GammaTile, GT Medical Technologies, Tempe AZ, USA). Preoperative evaluation varied by center, and patient demographics, primary site, lesion size, and prior therapies were also examined. RESULTS: From 10/2020 to 2/2024, 60 patients (64 lesions) underwent resection and intraoperative frozen section pathologic evaluation. Per patient, primary sites were 53% lung, 15% melanoma, 13% breast, 7% renal, and 10% other. Median age was 62, median preoperative maximum diameter was 2.9 cm, F:M ratio was 31:29, and median time from prior RT was 15.4 months. Across all histologies, TUM+ was seen in 88% (53/60) and TUM- in 12% (7/60). Rates of TUM- by primary type were highest for lung (16%), breast (13%), and melanoma (11%). The TUM- rate for lung metastasis was 16% vs 7% for non-lung origin. All TUM- patients received RT and prior chemotherapy, immunotherapy, or both. CONCLUSIONS: For all previously irradiated metastasis, pathologic evaluation at time of presumed radiographic recurrence demonstrated an actual 12% rate of TUM-. These findings underscore the importance of pathologic tumor confirmation before considering re-RT for presumed radiographic recurrence.