Synovial sarcoma is a mesenchymal tumor composed of spindle cells that are intermixed with elements that feature epithelial differentiation, hence its characteristic biphasic appearance. It can occur at any age, however its peak incidence is in the third decade. Synovial sarcomas have a rather equal male:female incidence ratio. Although it typically occurs in the extremities, usually near joints, it has also been reported in the trunk and head and neck regions.[2, 3] Rarer sites of involvement include kidney, upper digestive tract, heart, male and female internal and external sex organs,[7, 8] among others.
Synovial sarcomas generally present as a tender mass that most of the times is larger than 5 cm and grossly shows areas of hemorrhage or myxoid change admixed with tan-gray or pink areas that vary from soft to firm in consistency. Microscopically these tumors are composed, of two main elements present in different proportions; one, a relatively monotonous population of spindle cells with granular chromatin, no visible nucleoli and high nuclear-to-cytoplasm ratio; and an epithelial cell population forming tubular, glandular or cord-like structures or present as solid nests. The epithelial cells generally present cuboid or columnar morphology with ovoid nuclei, vesicular chromatin and eosinophilic cytoplasm.
Several malignant mesenchymal tumors such as liposarcomas, leiomyosarcomas, neurofibrosarcomas, and malignant fibrous histiocytoma have been reported involving the pelvis generally presenting as large tumors sometimes extending to pelvic viscera, representing true therapeutical challenges to surgeons and oncologists. Although cases of synovial sarcoma occurring in the pelvis have also been reported[12, 13] other tumors occur in much higher frequency at this site in female patients and should therefore be considered in the clinical and radiological differential diagnosis. Such tumors include ovarian neoplasms of either germinal, surface epithelium, or sex-cord stromal cell origin, malignancies arising from the cervix and corpus of the uterus such as cervical carcinoma or endometrial stromal tumors, or less frequently in this age group malignant mixed müllerian tumors.
In our case the morphological features appreciated on routine H&E sections made the most frequent ovarian tumors, cervical carcinoma, and endometrial stromal tumors unlikely possibilities. The distinction between a possible malignant mixed müllerian tumor and a neoplasm of mesenchymal origin was achieved through the use of immunohistochemical stains that were negative for CD10, PAX8, WT1 and ER decreasing the probabilities of a mullerian origin,[14-17] and positive for CD99 which pointed to a potential soft tissue origin for the lesion. The combination of CD99 positivity in the spindle cells and cytokeratin (AE1/AE3, CAM 5.2) and non-cytokeratin (Ber-EP4 and EMA) markers of epithelial differentiation positive in the remainder component of the tumor enhanced its biphasic nature, putting synovial sarcoma high in the list of differential diagnoses and distinguishing it from the monophasic variant of this tumor that generally shows only focal positivity for keratins. Finally, the FISH results showed SS18 (SYT) gene rearrangement that is associated to the characteristic t(X;18) (p11.2; q11.2) translocation, found exclusively in synovial sarcomas providing optimal proof of this tumor identity.
Contributed by Cecilia Lezcano, M.D. and David J. Dabbs, M.D.