P03.06.A FRAMELESS FRACTIONATED STEREOTACTIC RADIOSURGERY FOR BRAIN METASTASES: AN INSTITUTIONAL SERIES OF 145 CASES

Abstract

Abstract BACKGROUND Cobalt-60 stereotactic radiosurgery (SRS) typically involves single fraction treatment with frame immobilization. However, large tumor size, proximity to critical structures, and prior radiation treatment sometimes necessitate fractionated SRS with mask immobilization. We present a large institutional experience with fractionated mask-based SRS for brain metastases. MATERIAL AND METHODS In this single-institution, IRB-approved study, all patients treated with mask-based fractionated SRS for brain metastases from March 2017 to January 2023 were identified. The primary outcomes were 1- and 2-year local control (LC) by Kaplan-Meier method. RESULTS 118 patients with a total of 145 metastases were treated. The median follow-up time was seven months. The median age at treatment was 64.1 years (range: 26-95 years). 55.9% of patients were female. The most common primary tumors were breast (25.5%), non-small cell lung (23.4%), small-cell lung (8.3%), and melanoma (8.3%). For most cases (59.3%), the indication for fractionation was retreatment. Large size (28.3%), critical location (9.7%), and medical comorbidity (2.1%) were other indications. For all cases, the mean maximal linear size was 34.9 mm and mean target volume was 15.6 cc. For cases fractionated due to size, the mean size was 43.9 mm and mean target volume was 23.8 cc. Median total dose was 2,700 cGy (range: 1,620-3,000), and median dose per fraction (fx) was 600 cGy (range: 405-900). The most common prescriptions were 3,000 cGy/5 fx (40.0% of patients) and 2500 cGy in 500 cGy per fraction (37.2% of patients). Mean maximum dose was 4,833 cGy (range: 2,920-7,500). For 75.2% of treatments, the prescription isodose line was 50 to 59% (mean, 56.9%). Target coverage was 100% in all but one case (99%). For lesions near the brainstem, mean brainstem maximum point dose (MPD) was 9.3 Gy ± 9.8 Gy and brainstem mean dose was 3.3 Gy ± 3.3 Gy. For lesions near the optic pathway, mean optic nerve MPD was 14.4 Gy ± 9.2, optic nerve mean dose was 6.4 Gy ± 5.4 Gy, mean optic chiasm MPD was 11.7 Gy ± 7.9 Gy, and optic chiasm mean dose was 5.4 Gy ± 4.7 Gy. 1-year LC was 88.2% and 2-year LC was 80.4%. When retreatments were excluded, 1-year LC was 98.0% and 2-year LC was 98.0%. 18% of patients had acute grade 1-2 toxicities (fatigue, headache, nausea, and/or alopecia), and one patient had acute grade 3 fatigue. There were no other grade 3+ acute toxicities. 14% of patients had grade 1-2 radiation necrosis (RN); there were no cases of grade 3+ RN. CONCLUSION Cobalt-60 frameless fractionated SRS for brain metastases offers excellent local control, rigorous sparing of critical structures, and minimal toxicity. Frameless fractionated SRS should be considered for large, retreated, or critically located metastases.