MLTI-12. TIMING OF SYSTEMIC THERAPY ADMINISTRATION RELATIVE TO STEREOTACTIC RADIOSURGERY AND DEVELOPMENT OF RADIATION NECROSIS IN PATIENTS WITH BRAIN METASTASES

PURPOSE: The mainstay of oncologic therapy for patients with brain metastases involves brain-directed radiation, increasingly given via stereotactic radiosurgery (SRS), and systemic therapy for extracranial disease control. We sought to investigate the association between the timing of systemic therapy and SRS administration on development of radiation necrosis among patients with brain metastases. METHODS: We retrospectively identified 429 patients treated at Brigham and Women’s Hospital/Dana-Farber Cancer Institute with SRS for newly-diagnosed brain metastases between 2001–2015. Systemic therapy was tiered into 4 categories: chemotherapy, immunotherapy, hormonal therapy, and targeted therapy. All images were manually reviewed by two radiation oncologists specializing in brain tumors to assess the presence versus absence of radiographic necrosis. Patients with radiographic necrosis who harbored associated neurologic symptoms or were managed with steroids/bevacizumab/resection were considered to have symptomatic radiation necrosis. Data were analyzed using univariable Cox regression in SAS v9.4. The median follow-up in surviving patients was 1.79 years. RESULTS: In total, 252/429 and 361/429 patients received systemic therapy pre and/or post SRS, respectively. Patients receiving systemic therapy ≤5 days before SRS displayed higher rates of radiographic (HR 2.48, 95% CI 1.06–5.81, p=.04) and symptomatic (HR 3.74, 95% CI 1.08–12.98, p=.04) necrosis; a similar association was seen in patients receiving systemic therapy ≤5 days after SRS (HR 1.72, 95% CI 0.84–3.53, p=.14 and HR 4.42, 95% CI 1.75–11.14, p=.002, respectively). Trends towards increased necrosis risk were noted when comparing systemic therapy administration 1–5 days versus 6–10 days before/after SRS. The above 4 associations were significant when restricting the cohort to patients receiving targeted systemic therapy (HR-range 3.57–21.49, p-range 0.01–0.04). CONCLUSIONS: Our results suggest that a reasonable delay between SRS and systemic therapy administration may reduce rates of radiation necrosis, even among patients receiving targeted therapies. Validation in an independent data set would lend further support to this concept.