Assessment of real-world surveillance strategies for patients undergoing systemic therapy for brain metastases.

Introduction: Radiotherapy has long been first-line treatment for brain metastases. However, CNS penetrating systemic therapies are increasingly used as first-line alternatives. There is little evidence guiding surveillance in this setting. Here we retrospectively assessed surveillance strategies at a single institution and associated outcomes. Methods: Records from 33 patients, evaluated from 2021-2024 and treated with systemic therapy alone for active brain metastases, were reviewed. Time from diagnosis to 1st, 2nd, and 3rd surveillance MRI, incidence of intracranial progression, survival, and neurological adverse events were assessed. Patients were stratified by whether treatment was supported by the 2021 ASCO-SNO-ASTRO guidelines. Results: Treatment met guidelines for 14/33 patients. Median (range) time to 1st, 2nd, and 3rd MRI was 45 (18-207), 95 (46-204), and 180 (74-329) days, respectively. One-year cumulative incidence of local progression in the guideline group was 0.43 (95% CI [0.18-0.66]) and in the non-guideline group was 0.32 (95% CI [0.13-0.52], p=0.5). One-year cumulative incidence of brain radiation in the guideline group was 0.29 (95% CI [0.09-0.53]) and in the non-guideline group was 0.44 (95% CI [0.22-0.64], p=0.87). Two patients experienced safety events on systemic therapy (seizures). Discussion: Use of CNS-penetrating systemic therapies to treat brain metastases outpaced national guidelines in this study. While surveillance imaging frequency was variable and recurrences were common, many patients were able to delay radiation for over one year, with infrequent safety events. As interest in and options for CNS-penetrating systemic therapy grow, evidence-based guidelines for surveillance will be warranted.