Frameless Fractionated Stereotactic Radiosurgery for Brain Metastases: An Institutional Series of 89 Cases

Abstract

<h3>Purpose/Objective(s)</h3> Cobalt-60 stereotactic radiosurgery (SRS) conventionally involves frame-based immobilization and single-fraction treatment. However, certain anatomical and technical factors, including large lesion size, close proximity to critical structures, and prior radiation treatment, necessitate fractionated SRS, which requires mask-based immobilization. We present a large institutional experience with mask-based fractionated SRS for large, retreated, or critically-located brain metastases. <h3>Materials/Methods</h3> All patients treated with frameless fractionated SRS for brain metastases between March 2017 and August 2021 at a single institution were identified. Patient, tumor, and treatment variables were abstracted. The primary outcome was one-year local control (LC), which was analyzed with the Kaplan-Meier method. This study was approved by the institution's IRB. <h3>Results</h3> Seventy-four patients with a total of 89 brain metastases were treated. The median follow-up time was seven months. The median age at treatment was 64 years. Sixty-two percent of patients were female. The most common primary tumors were breast (24.7%), non-small cell lung (24.7%), small-cell lung (7.9%), renal cell (6.7%), and melanoma (6.7%). For the majority (62.9%) of tumors, the indication for fractionation was retreatment; other indications were large size (21.3%), critical location (12.4%), and medical comorbidity (3.4%). For all tumors, the mean maximal linear size was 35.5 mm; for tumors fractionated due to size, the mean size was 45.8 mm. The median total dose was 2,750 cGy (range: 1,620-3,000), and the median dose per fraction was 600 cGy (range: 405-900); the most common dose-fractionations were 3,000 cGy in 600 cGy per fraction (42.7% of patients) and 2500 cGy in 500 cGy per fraction (31.5% of patients). The mean target volume was 17.6 cc (range: 0.13-120.0), and the mean maximum dose was 4,980 cGy (range: 3,240-6,000). For 91% of treatments, the prescription isodose line was 50 to 59% (mean, 54%). Target coverage was 100% in all but one case, where it was 99%. For lesions near the brainstem, mean brainstem maximum point dose (MPD) was 9.78 Gy and mean brainstem mean dose was 3.56 Gy. For lesions near the optic pathway, mean optic nerve MPD was 14.39 Gy, mean optic nerve mean dose was 6.2 Gy, mean optic chiasm MPD was 11.7 Gy, and mean optic chiasm mean dose was 5.4 Gy. LC at 12 months was 91%. Nineteen percent of patients experienced acute grade 1-2 toxicity; one patient experienced acute grade 3 fatigue, and there was no other grade 3 or higher acute toxicities. Seventeen percent of patients had grade 1-2 radiation necrosis (RN); there were no cases of grade 3 or higher RN. <h3>Conclusion</h3> Cobalt-60 frameless fractionated SRS for brain metastases offers excellent local control. This treatment modality is technically feasible, offers excellent sparing of critical structures, and has minimal toxicity. Frameless fractionated SRS should be considered for large, retreated, or critically located metastases.