Frameless Fractionated Stereotactic Radiosurgery for Meningioma and Schwannoma: An Institutional Series of 126 Cases

Abstract

<h3>Purpose/Objective(s)</h3> Cobalt-60 stereotactic radiosurgery (SRS) typically employs frame-based immobilization and single-fraction treatment. In some circumstances, including close proximity to critical structures, large tumor size, and retreatment, SRS must be fractionated, with mask for immobilization. We present a large institutional experience with mask-based fractionated SRS for meningioma and schwannoma. <h3>Materials/Methods</h3> All patients treated with frameless fractionated SRS for meningioma or schwannoma between March 2017 and July 2021 at one institution were identified. Patient, tumor, and treatment variables were abstracted. The primary outcomes were one- and two-year local control (LC), which was analyzed via Kaplan-Meier analysis. This study was approved by the institution's IRB. <h3>Results</h3> One hundred and five patients with a total of 126 tumors were treated. The median follow-up time was 18.7 months. The median age at treatment was 66 years. Seventy-one percent of patients were female. Eighty-six percent of tumors (n=108) were meningioma, and the remaining 14% (n=18) were schwannoma. Of the 61 meningiomas for which pathological information was available, 33 were grade 1, 26 were grade 2, and 2 were grade 3. The most common indication for fractionation was proximity to critical structures (55.6% of tumors); other indications were retreatment (23.0%) and large size (21.4%). For the entire cohort, the mean maximal linear size was 34.6 mm; for tumors fractionated due to size, the mean size was 44.4 mm. The median total dose was 2,500 cGy (range: 1,950-3,000), and the median dose per fraction was 500 cGy (range: 500-700); the most common dose-fractionations were 2,500 cGy in 500 cGy per fraction (67.5% of patients) and 2,750 cGy in 550 cGy per fraction (17.5% of patients). The mean target volume was 8.62 cc (range: 0.13-120.0), and the mean maximum dose was 4,716 cGy (range: 3,200-6,000). The prescription isodose line was 50 to 59% for 84% of treatments (mean % IDL, 55%). Target coverage was 100% in all 126 cases. For lesions in proximity to the brainstem, mean brainstem maximum point dose (MPD) was 24.9 Gy and mean brainstem mean dose was 6.7 Gy. For lesions in proximity to the optic pathway, mean optic nerve MPD was 15.7 Gy, mean optic nerve mean dose was 6.4 Gy, mean optic chiasm MPD was 17.3 Gy, and mean optic chiasm mean dose was 7.5 Gy. One-year LC was 97.6% and two-year LC was 93.1%. Twenty-one percent of patients experienced acute grade 1-2 toxicity; there were no grade 3 or higher acute toxicities. Thirteen 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 benign brain tumors offers excellent local control and minimal toxicity. Frameless fractionated SRS is technically feasible, offers excellent sparing of critical structures, and is well-tolerated. Frameless fractionated SRS is an excellent treatment option for patients with large, retreated, or critically located meningioma or schwannoma.