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Purpose: Radiation-induced sarcomas (RISs) have poor prognosis and lack effective treatments. Its genome and tumor microenvironment are not well characterized and need further exploration. Experimental Design: Here, we perform whole exome (WES) and mRNA sequencing (mRNA-seq) on patient with RISs and primary sarcomas (WES samples 46 vs 48, mRNA-seq samples 16 vs 8, mainly in head and neck), investigate the anti-tumor effect of PD-1 blockade in RIS-PDX models, and analyze clinical data of RIS patients treated with chemotherapy alone or combined with an anti-PD-1 antibody. Results: Compared to primary sarcomas, RISs manifested different pattern of copy number variations, significantly higher number of predicted strong MHC-binding neoantigens, and significantly increased immune cell infiltration. Clinical data showed that the combinatorial use of chemotherapy and PD-1 blockade achieved a higher overall response rate (ORR) (36.67% vs 8.00%, p = 0.003), longer overall survival (31.9 months vs 14.8 months, p = 0.014) and longer progression free survival (4.7 months vs 9.5 months, p = 0.032) in RIS patients compared to single chemotherapy. Conclusion: Elevated genomic instability and higher immune cell infiltrations was found in RISs than primary sarcomas. Moreover, higher efficacy of chemotherapy plus PD-1 blockade was observed in animal experiment and clinical practice. These evidence indicated the promising application of immune checkpoint inhibitors in the treatment of RISs.