RADI-09. DEFINING PROGRESSION IN PATIENTS TREATED WITH TEN OR MORE BRAIN METASTASES FOLLOWING STEREOTACTIC RADIOSURGERY

BACKGROUND: An increasing trend has been to elect for Stereotactic Radiosurgery (SRS) for the treatment of brain metastases. Progression following treatment is typically defined as a 20% increase in the initial lesion volume treated. Challenges in defining progression can arise as the reported incidence of pseudoprogression or radiation necrosis following treatment ranges from 5%-30%. The purpose of this study was to assess patterns of failure in patients treated with 10 or more brain metastases. METHODS: From March 2014 to April 2018, fifty-five patients with 10 or more total brain metastases were retrospectively reviewed following frame-based radiosurgery to a dose of 12–20 Gy. Post-treatment MRI scans were used to assess tumor response in 3 month intervals. Tumor control was defined as tumor volume ≤ 1.2 times the baseline tumor volume at each measured interval. RESULTS: Fifty-five patients received 75 total radiosurgery treatments to 692 tumors. Forty patients received synchronous treatment, while 15 received metachronous treatment. 20 patients (36%) and 72 tumors (10%) experienced progression following treatment. 46 tumors were larger after first MRI in 15 patients (28%). Of these 15 patients, eight had complete resolution in 15 tumors on subsequent scan. Of the eight patients who had resolution, six patients received immunotherapy during and after treatment and all but one patient saw an initial increase >100% of their initial tumor volume. Median overall survival was 11 months. Univariate analysis revealed an association between larger brain volumes irradiated with 12 Gy and decreased overall survival (p < 0.05). CONCLUSION: It is important to consider tumor growth velocity and concurrent therapy when assessing true progression after SRS treatment of brain metastases.