TPS9600 Background: Nivolumab combined with ipilimumab is active in melanoma brain metastases, with intracranial response rates > 55% and durable survival in treatment naïve patients (pts) (Long GV et al Lancet Onc 2018; Tawbi H et al NEJM 2018). We seek to determine if the addition of stereotactic radiotherapy (SRS) results in improved intracranial outcomes. Methods: This is a multisite, open-label, phase 2 trial in systemic treatment-naïve pts with melanoma brain metastases. Pts must have ≥1 asymptomatic brain metastases that are ≥5mm and ≤40mm as per modified RECIST 1.1, on gadolinium-enhanced MRI, and no history of previous treatment with SRS. Eligible pts are randomly assigned to either receive nivolumab plus ipilimumab with SRS or nivolumab plus ipilimumab alone. Nivolumab (1mg/kg) and ipilimumab (3mg/kg) are given every 3 weeks for 4 doses. Following induction, 480mg nivolumab is given every 4 weeks until progression, unacceptable toxicity, or a maximum of 2 years. SRS is administered as single fraction of 16-22Gy, or hypofractionated for larger lesions (24-27Gy in 3 fractions), within 7 days of immunotherapy commencement. Pts will be evaluated for intracranial and extracranial tumour response, and overall response, every 6 weeks to week 24 and 12 weekly thereafter until overall disease progression or death. The primary endpoint is neurologic specific survival (NSS) at 12 months. Secondary endpoints include intracranial response rate, intracranial PFS, overall PFS, overall progression free survival, overall survival, neurocognitive function and incidence of radiation necrosis. 109 patients in each cohort (218 total) will achieve > 80% power at the significance level (alpha) of 0.10 to detect a minimum absolute increase of 9% in the NSS rate at 12 months. Clinical trial information: NCT03340129.