Evolutionary pressures shape undifferentiated pleomorphic sarcoma development and radiotherapy response.

Although radiotherapy plays a crucial role in treating many cancers, the effect of radiation on tumor evolution remains unclear. We integrated temporal genomic profiling of 120 spatially distinct tumor regions from 20 patients with undifferentiated pleomorphic sarcomas (UPS), longitudinal circulating tumor DNA (ctDNA) analysis, and evolutionary biology computational pipelines to study UPS evolution during tumorigenesis and in response to radiotherapy. Most unirradiated UPS displayed initial linear evolution followed by subsequent branching evolution with distinct mutational processes during early and late development. Using metrics of genetic divergence between regions, we demonstrated evidence of strong selection pressures during UPS development that further increased during radiotherapy. We observed changes in subclone abundance following radiotherapy with subclone contraction tied to alterations in calcium signaling and demonstrated that inhibiting calcium transporters can radiosensitize sarcoma cells. Finally, ctDNA analysis accurately measured subclone abundance and enabled non-invasive monitoring of subclonal changes. These results demonstrate that radiation exerts selective pressures on UPS and suggest that targeting radioresistant subclonal populations could improve outcomes after radiotherapy.