• Age ≥ 18 years old.

• Life Expectancy \> 12 weeks.

• Subjects must have signed and dated an IRB/IEC approved written informed consent form in accordance with regulatory and institutional guidelines. This must be obtained before the performance of any protocol related procedures that are not part of normal subject care.

• • Subjects must be willing and able to comply with scheduled visits, treatment schedule, laboratory testing, and other requirements of the study.

• Histologically confirmed malignant melanoma with measurable metastases in the brain (≥ 0.5 cm).

• At least one measurable intracranial target lesion, which previously was not treated with local therapy (no prior SRS to this lesion).

• • Largest diameter of ≥ 0.5cm, but ≤ 3cm as determined by contrast-enhanced MRI.

• Representative formalin-fixed paraffin-embedded (FFPE) tumor specimens in paraffin blocks (blocks are preferred) OR at least 4 unstained slides, with an associated pathology report, for testing of tumor PD-L1 expression:

‣ Tumor tissue should be of good quality based on total and viable tumor content.

⁃ Patients who do not have tissue specimens may undergo a biopsy during the screening period. Acceptable samples include core-needle biopsies for deep tumor tissue or excisional, incisional, punch, or forceps biopsies for cutaneous, subcutaneous, or mucosal lesions. Fine Needle Aspirations (FNA) will not be considered acceptable for tissue procurement.

⁃ Tumor tissue from bone metastases is not evaluable for PD-L1 expression and is therefore not acceptable.

⁃ However, if repeat biopsy is not feasible, and no archival tissue available patient still may be enrolled.

• Prior SRT and prior excision of up to 5 MBM is permitted if there has been complete recovery, with no neurologic sequelae, and measurable non irradiated lesions remain.

• Growth or change in a lesion previously irradiated will not be considered measurable. Regrowth in cavity of previously excised lesion will not be considered measurable.

• • Any prior SRT to brain lesions or prior excision must have occurred ≥ 1 weeks before the start of dosing for this study.

• Radiation to NON-CNS lesions. Prior radiation to NON-CNS is allowed and does not require a washout period for treatment initiation.

⁃ Subjects must be free of neurologic signs and symptoms related to metastatic brain lesions and must not have required or received systemic corticosteroid therapy in the 5 days prior to beginning protocol therapy.

⁃ ECOG performance status ≤1.

⁃ Adequate organ function as described below.

⁃ Women of child-bearing potential (WOCBP) must not be breastfeeding and must have a negative pregnancy test within 3 days prior to initiation of dosing. She must agree to use an acceptable method of birth control from the time of the negative pregnancy test, through the duration of treatment with the study combination plus 5 half lives of study treatment for a total of 5 months post -treatment completion. WOCBP must agree to adhere to the contraceptive guidance in Appendix 5. NOTE: A female participant is eligible to participate if she is not a woman of childbearing potential as defined Appendix 4.

⁃ All associated toxicity from previous or concurrent cancer therapy must be resolved (to ≤ grade 1 or baseline) prior to study treatment administration. This excludes non-serious toxicities. However, stable endocrinopathies requiring replacement therapy will be allowed.

⁃ Steroids for physiological replacement are allowed.