Background: Combining HT-RT with anti PD-L1 therapy may enhance local and distant tumour control in mSTS. This was a Phase 1 trial to evaluate the safety, tolerability, and efficacy of avelumab in patients with mSTS. Methods: This was a single centre phase 1 trial of avelumab with HT- RT in patients with mSTS (NCT03602833). Patients > 18 years with at least 2 pulmonary metastases, ECOG performance status 0-1 were eligible between Oct 2018-Apr 22. Patients received HT-RT (36 Gy in 12 fractions), with concurrent intravenous avelumab (10 mg/kg q14d) until disease progression. The primary end point was safety of avelumab with HT-RT. Secondary endpoints included local control rate at 3 months. Dose limiting Toxicity (DLT) assessed from start of HR-CT and avelumab up to Cycle 7 avelumab and defined as: ≥ G2 pneumonitis, ≥ G2 myelitis, ≥ G3 non-haem toxicity, ≥ any other G4 and/or HT-RT interruption > 5 days. Response was evaluated using the Response Evaluation Criteria in Solid Tumors (RECIST). Results: 12 patients were treated in this trial. Histological subtypes: leiomyosarcoma (n = 2), undifferentiated pleomorphic sarcoma (n = 2), and spindle cell sarcoma (n = 2). The median age was 57.5 (37-68) years, 7 were female (58%). All patients had lung metastases. 1 previously received SBRT to the lung, and 5 (42%) patients had received prior systemic treatment. 12 patients received at least one cycle of avelumab, median 9 (range: 1-37), and treatment was discontinued in 11 (92%) patients; 10 (83%) stopping for progressive disease, 1 (8%) patient for G4 immune-mediated hepatitis after DLT period and 1 patient completed and continues on compassionate-use avelumab. Eleven (92%) completed radiotherapy (36 Gy, 12 fractions). No DLTs or treatment-related deaths were observed. HT-RT toxicity: 3 G2 acute (1 oesophagitis, 2 skin) noted up to 11 weeks. 13 G1 late toxicities (5 pneumonitis, 5 skin, 1 cardiac, 1 myelitis, 1 oesophagitis) reported. Avelumab toxicities: 2 G3 (1 ALT rise, 1 viral infection) and 3 G4 (1 AST rise, 1 hepatitis, 1 sepsis) reported after DLT observation period. The median follow-up was 21.8 (range 3.2-31.9) months with a 3-month local control rate of 50% (95% CI, 21%-74%). RECIST responses at 3 months were 0 CR, 1 (8%) PR, 5 (42%) SD and 5 (42%) PD. Conclusions: Avelumab in combination with thoracic radiotherapy was safe with encouraging anti-tumour activity in patients with metastatic soft-tissue sarcomas. Additional molecular biomarker analyses are in progress. Clinical trial information: NCT03602833.