Stereotactic radiosurgery (SRS) confers excellent local control for melanoma brain metastases (MBM). Factors impacting the rate of volumetric regression or recurrence in the follow-up period, including the nature and timing of immunotherapy, are poorly understood. This study examines the MRI volumetric tumor response over time of melanoma brain metastases following Gamma Knife SRS, and aims to synthesize a predictive model of volumetric change following treatment.

A retrospective single-institution analysis was performed of patients who received single-fraction Cobalt-based SRS for melanoma brain metastases from January 2016 to June 2019. Predictive factors relating to patient characteristics, tumor factors, SRS dose, volume and systemic therapy treatment factors such type (CTLA-4, PD-1 and/or BRAF/MEK inhibitors) and timing of treatment were collected. Response assessment was characterised radiologically using MRI on pre-contrast 3-dimensional (3D) T1 and post-contrast 3D T1-Gadolinium sequences with 1-1.5mm isotropic resolution as per ‘Standardized Brain Tumor Imaging Protocol’ (BTIP) guidelines (1). Lesion volume...

101 patients with 425 MBM were treated with SRS in the study period. Median follow-up was 29.2 months (IQR 19.7-39.8). Median dose was 20Gy (IQR 18-20). Median baseline volume and lesion diameter were 0.24cc (IQR 0.06-1.02) and 7.7mm (IQR 4.8-12.4) respectively. 53% of patients were BRAF mutant with 65% having failed BRAF inhibitors at time of SRS. 77% of patients received concurrent immunotherapy (2 weeks prior to 4 weeks post-SRS). Normalised to baseline, median interpolated lesion size at 3, 6, 9, 12- and 15-month was...

This study demonstrates a significantly greater volumetric regression with concurrent immunotherapy and SRS in MBM in the initial 6 months following treatment. Combined modality immunotherapy and SRS provides a high degree of local control with a low rate of symptomatic radionecrosis.

1. Ellingson BM, Bendszus M, Boxerman J, Barboriak D, Erickson BJ, Smits M, et al. Consensus recommendations for a standardized brain tumor imaging protocol in clinical trials. Neuro-oncology. 2015;17(9):1188-98.