Background: Resection (R) followed by single or multi-fraction stereotactic radiosurgery (SRT) of brain metastases lowers resection bed recurrence compared to R alone. Nevertheless, for larger (>2.5cm) brain metastasis, 12-month recurrence rates after R+SRT can exceed 20–30%. Aiming to improve outcomes, a permanently implanted collagen tile brachytherapy device (GammaTile or GT, GT Medical Technologies, Tempe, AZ) utilizing Cs-131 seeds embedded within a bioresorbable collagen tile was developed and is described as Surgically Targeted Radiation Therapy (STaRT) to distinguish it from external beam radiation therapy. It is hypothesized that immediate adjuvant radiotherapy (RT) and/or RT dose intensification could improve outcomes. The device is FDA-cleared for this indication and early commercial use is demonstrating favorable safety and efficacy outcomes. STaRT allows rapid dose delivery of radiation therapy directly to the tumor bed with predictable dosimetry immediately at the time of resection, and an intense but localized radiation treatment, which may confer a reduced risk for radiation necrosis compared to other therapies. The device is easily placed with minimal additional operative time and limited staff radiation exposure. It is hypothesized that R+ STaRT will increase the surgical bed recurrence-free survival, while reducing the impact on functional and neurocognitive status compared to R+SRT. Methods: Multicenter, randomized, comparison trial of patients with resectable, previously untreated “index” brain metastases measuring ≥2.5–5 cm, and 0–3 other tumors, will be preoperatively randomized 1:1 to undergo either R+ SRT or R+STaRT to the index lesion; unresected tumors in both groups will receive SRT. Planned sample size is 180 from 13 sites. Enrollment will open in Q1. Primary endpoint is surgical bed-recurrence free survival. Secondary endpoints include overall survival, quality of life (Functional Assessment of Cancer Therapy-Brain, Linear Analog Self-Assessment), neurocognition (Hopkins Verbal Learning Test, Trail Making Tests, Controlled Oral Word Association), functional status (Karnofsky Performance Scale, Barthel-ADL), and adverse events. Follow-up will be at 1,3,6,9, and 12 months, then every 6 months through 24 months. This will be the first randomized trial comparing R+SRT versus R+STaRT delivered by Cs-131 sources in permanently implanted resorbable collagen tile carriers. Primary and secondary outcome measures will be captured to elucidate the potential risks and benefits of these two differing post-operative RT delivery methods in the setting of newly diagnosed metastatic brain tumors. Clinical trial information: NCT04365374