Monophasic epithelial synovial sarcoma initially diagnosed as metastatic adenocarcinoma of unknown primary

Monophasic epithelial synovial sarcoma is extremely rare, and its existence has been debated. We report a case of monophasic epithelial synovial sarcoma, which was initially diagnosed as metastatic adenocarcinoma. A 45-year-old woman presented with a tender pelvic mass, whose morphology was consistent with a glandular neoplasm. The mass was misdiagnosed as metastatic adenocarcinoma of unknown primary, and was treated as such. The tumor recurred 12 years later; it showed biphasic morphology including glandular and spindle sarcomatoid components. TLE1 immunoreactivity of the second tumor supported diagnosis of biphasic synovial sarcoma. The original lesion was reexamined and revealed very focal minor spindle tumor foci admixed with an extensive glandular background. The TLE1 immunostain was strongly positive in the initial lesion, thus confirming diagnosis of epithelial-predominant synovial sarcoma. This case emphasizes the importance of generous sampling and the use of TLE1 staining when soft tissue tumors show predominantly epithelial architectures without an apparent primary site.