Biphasic synovial sarcoma with myoepithelial features: a distinctive variant with a predilection for the foot

Synovial sarcoma (SS) is a tumor known for its classic monophasic spindle cell or biphasic morphology. However, it exhibits a wide range of histologic variations, leading to diagnostic challenges. Here, we present four cases of molecularly confirmed, biphasic SS originating in the feet and displaying myoepithelial differentiation. The patients were two men and two women with an age range from 19 to 71 years (mean, 45 years). Each tumor showed foci with conventional spindle cell morphology. The epithelial components included areas with nests and cords of epithelioid cells set within a hyalinized and sclerotic stroma. The cytoplasm was clear to pale and eosinophilic. The nuclei were ovoid-round with fine chromatin and small to inconspicuous nucleoli. Mitotic figures were present (2-13 per 10 high-power fields; mean, 6.5). Immunohistochemical studies showed variable staining of the myoepithelial-like regions for low molecular weight keratins, EMA, p63, and S100 protein. Molecular studies confirmed the presence of SS18::SSX1/2 fusion in all four tumors. These cases highlight an unusual variant of synovial sarcoma with an apparent predilection for the distal lower extremity and suggest that differentiation of biphasic synovial sarcoma may be impacted by the anatomic site. Awareness of this variant is important to avoid misclassification and potential treatment and prognostic implications.