Two Cases of Pelvic Biphasic Synovial Sarcoma With Expression of PAX8 and ER: A Diagnostic Pitfall.

Synovial sarcoma (SS) is a high-grade malignant neoplasm frequently arising in the deep soft tissue of the lower and upper extremities of young adult. Primary SS in pelvis is extremely rare with scattered case reports. It usually raises diagnostic challenge in small biopsy and/or with aberrant expression of immunohistochemical markers. Here, we report 2 unusual cases of SS in pelvis. Microscopically both cases present with biphasic morphology including spindle and epithelioid cells. In addition, the tumor cells in both cases express PAX8 and estrogen receptor. PAX8 is a transcription factor usually expressed in tumors of thyroid gland, kidney, and Müllerian system. The expression of PAX8 especially with co-expression of estrogen receptor can mislead to a diagnosis of Müllerian tumors for pelvic mass in female patients. The diagnosis of SS for both cases was confirmed either with the fluorescence in situ hybridization or reverse transcription polymerase chain reaction by positive SS18 (SYT) (18q11) gene rearrangement. It is imperative to include SS in the differential diagnosis for malignant neoplasm exhibiting monotonous spindle cells (monophasic SS) and biphasic mixed monotonous spindle and epithelioid tumor cells for pelvic mass in female patients. Molecular study for SS18 translocation is essential for the diagnosis in such cases.