Outcomes of a Single Isocenter Brain Multi-Metastases Linear Accelerator Delivered Stereotactic Radiosurgery (SRS).

PURPOSE/OBJECTIVE(S): The management of brain metastases has evolved from using 2D whole brain radiotherapy (WBRT) to more complex techniques like stereotactic radiosurgery (SRS) for patients with limited disease. Long-term control of lesions is challenging with WBRT techniques but treating multiple lesions with traditional SRS, where each lesion is treated on its own isocenter, can be time-consuming and difficult on patients, especially those with claustrophobia. Single Isocenter Multiple Metastases (SIMM) SRS has emerged as an option to deliver ablative SRS doses simultaneously to multiple brain metastases using a single isocenter, thereby limiting the duration of treatments for patients. Though appealing, SIMM SRS adds technical complexity and could potentially lead to worse outcomes or more complications relative to traditional SRS treatments. Given the current paucity of clinical evidence supporting SIMM SRS, we sought to retrospectively review our institution's outcomes and complications for patients treated with SIMM SRS to determine the efficacy and safety of this approach in our hands.

MATERIALS/METHODS: Patients treated at our institution with SIMM SRS with at least one post-treatment brain MRI were identified. Date on patient clinical characteristics, planning, and treatment characteristics, and outcomes were retrospectively collected. Post-treatment tumor control was evaluated with follow-up MRI imaging based on RANO criteria. Correlation between tumor control and toxicity was done by assessing radiation doses, PTV coverage, and normal brain V12 constraints.

RESULTS: A total of 27 patients received SIMM SRS from January 2015 to February 2022. The median age at first SIMM SRS was 61 (range: 38-87). The most common disease sites were lung (63.0%), breast (18.5%), and GI (7.4%). The 27 patients had 47 SIMM SRS treatments of 163 lesions total. The median number of lesions treated per isocenter was 3 (range: 2-9). 5 patients had 2 SIMM SRS isocenters treated on the same day, treating clusters of lesions (ranging from 5-11 lesions treated on that day). The most common locations involved were frontal, cerebellar, and parietal lobes (32.52%, 21.47%, and 15.34%). The modal dose was 22 Gy (range: 18-24 Gy). Median OS from initial primary diagnosis was 23.23 months, and 9.92 months after the first SIMM SRS treatment. The median imaging follow-up was 9.8 months per lesion, and the local control rate was 95.03%. 2 lesions (1.23%) developed radiation necrosis and the median time to RT necrosis among those lesions was 5.7 months after treatment.

CONCLUSION: The utilization of SIMM SRS demonstrates acceptable efficacy and safety as it has been implemented at our institution. Further studies to evaluate this planning modality are warranted to establish suitable candidates for SIMM SRS as well as evaluate the long-term outcomes for these patients.