Patients diagnosed with metastatic sarcoma have limited options for achieving both local and distant tumor control. While SBRT can achieve local control, distant response rates remain low. There is limited evidence demonstrating the safety and efficacy for combining SBRT with concurrent PD-1 checkpoint blockade in metastatic sarcoma. In this prospective case-series, we examined five patients with metastatic sarcoma on pembrolizumab treated concurrently with SBRT from July 1, 2016–October 30, 2018. Acute and chronic toxicity were recorded using Common Terminology Criteria for Adverse Events (CTCAE, version 5.0). SBRT-treated tumor control was assessed using Response Evaluation Criteria in Solid Tumors (RECIST version 1.1). With median follow-up of 14.9 months, three patients with undifferentiated pleomorphic sarcoma, one with intimal, and one with chondroblastic osteosarcoma received SBRT with concurrent pembrolizumab to 10 sites of metastatic disease. No grade 5 toxicities were observed. There was a single incidence of transient grade 4 lymphopenia which resolved without intervention. Grade 3 toxicities included anemia, thrombocytopenia, lymphopenia and colitis. One tumor demonstrated local progression after SBRT, and all others remained stable or with response. In conclusion, combining SBRT with PD-1 inhibition appeared to be safe in this patient population. Expected high rates of treated-tumor local control after SBRT were observed. Two of five patients demonstrated either enhanced local tumor regression, or possible abscopal effect.

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