Undifferentiated pleomorphic sarcoma as a rare extracolonic manifestation in Lynch syndrome with MLH1 germline mutation: a case report

Undifferentiated pleomorphic sarcoma (UPS) is a rare and highly aggressive soft tissue sarcoma that typically occurs in the extremities but is rarely found in the retroperitoneal cavity. Despite the documented cases of sarcomas in patients with Lynch syndrome (LS), the relationship between these malignancies and LS has not yet been fully elucidated. In this report, we present the case of a patient with LS who harbored a germline MLH1 loss-of-function mutation (c.2171T>A) and developed synchronous malignancies, including colorectal cancer, cholangiocarcinoma, and retroperitoneal UPS. Consequently, molecular testing revealed mismatch repair deficiency (dMMR) among two cancers, thereby confirming the diagnosis of LS. This case suggests that UPS may represent a rare extracolonic manifestation of LS and highlights the importance of considering MMR/MSI testing in sarcomas of uncertain origin, which may have implications for diagnosis and personalized treatment strategies, including the use of immune checkpoint inhibitors.