Abstract
The classification of undifferentiated pleomorphic sarcoma (UPS) has been evolving with advances in immunohistochemistry and genomic profiling over the past 20 years. There is a lack of current information on UPS incidence. Due to the lack of designated histology codes for UPS in the Surveillance, Epidemiology, and End Results (SEERs) program, we estimated UPS incidence by three different definitions based on clinical opinions using the 2000-2020 data from 22 registries of the SEER program. The incidence varied widely across the three definitions with 0.06 per 100,000 persons for the least inclusive definition and 0.67 per 100,000 persons for the most inclusive definition in 2016-2020, making it challenging to estimate the exact incidence of UPS. Regardless, all the incidences decreased between 2000 and 2020. Guidelines in UPS diagnosis and classification need to be better implemented in the US.