PAX8-positive Biphasic Synovial Sarcoma Expressing Hormonal Receptors

PAX8, estrogen receptor-α (ERα) and progesterone receptor (PR) are markers usually expressed in neoplasms of müllerian origin. We report a subdiaphragmal mass in a 41-year-old woman corresponding to a malignant biphasic tumor with nests of epithelial-like cells forming variably sized cyst-like spaces alternating with spindle cells forming intersecting fascicles. The later were juxtaposed to coalescent densely cellular nodules of spindle cells with appreciable cytologic atypia and mitotic counts up to 30/10 high-power fields. The tumor cells were AE1/AE3, EMA, ERG, ERα, PR, and PAX8 positive whereas spindle cells showed reduced immunopositivity for these markers, especially marked in coalescent nodular areas, with notable exception of PAX8, which was diffuse and strongly positive. The possibility of an endometrioid carcinoma with spindle cells was considered by the referring pathologist, but fluorescent in situ hybridization showed rearrangement of SS18 gene in 48 of 50 tumor nuclei, rendering a diagnosis of biphasic synovial sarcoma, the first reported in the English literature to the best of our knowledge expressing PAX8, ERα, and PR. Further studies evaluating the expression of these markers in synovial sarcoma and other sarcomas are needed, as sometimes the findings may lead to misdiagnosis as other neoplasms including those of the female genital tract. Additional molecular tests may be helpful to determine the molecular mechanism of this aberrant immunoprofile, which could be directly or indirectly related to t(X:18).