Two Cases of 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 adults. Primary SS in the pelvis is extremely rare with scattered case reports. It often causes a diagnostic challenge in small biopsy and/or with aberrant expression of immunohistochemical markers. Here, we report 2 unusual cases of SS in the pelvis. Microscopically both cases present with biphasic morphology including spindle and epithelioid cells. In addition, the tumor cells in both cases expressed PAX8 and estrogen receptor. PAX8 is a transcription factor usually expressed in tumors of thyroid gland, kidney, and Müllerian system origin. The expression of PAX8 especially with co-expression of estrogen receptor can be misleading and result in a diagnosis of Müllerian tumors in female patients with pelvic masses. The diagnosis of SS for both cases was confirmed either with the fluorescence in situ hybridization or reverse transcription polymerase chain reaction showing a SS18 (SYT) (18q11) gene rearrangement. It is imperative to include SS in the differential diagnosis for malignant neoplasms exhibiting monotonous spindle cells (monophasic SS) and biphasic mixed monotonous spindle and epithelioid tumor cells in female patients with a pelvic mass. Molecular study for SS18 translocation is essential for the diagnosis in such cases.