Primary cutaneous synovial sarcoma—Sometimes the hoof beats are zebras

Abstract Synovial sarcoma accounts for 5% to 10% of soft tissue sarcoma and it typically presents as a deep soft tissue mass. Primary cutaneous presentation is exceptionally rare, with only isolated case reports. We report a case of primary cutaneous synovial sarcoma in a 58‐year‐old woman that presented as a nodule involving the left occipital scalp. A complete radiologic evaluation of the patient failed to reveal any other mass lesion. Histologic sections showed a densely cellular, diffuse spindle cell proliferation within the subcutis. The lesion was composed of uniform, plump spindled cells with nuclei and vesicular chromatin, arranged in haphazard fascicles. There was admixed hemorrhage and a hemangiopericytoma‐like vasculature. Immunohistochemically, the spindled cells showed focal strong positivity for cytokeratin (CK) OSCAR, CK5/6, CK34BE12, and pan‐CK. Fluorescence in situ hybridization was positive for a rearrangement of SYT (SS18) , confirming the diagnosis of monophasic synovial sarcoma. Synovial sarcoma should be considered in the differential diagnosis of monomorphous spindle cell tumors, especially if the tumors have a hemangiopericytoma‐like vasculature or express keratins. In such cases, confirmatory molecular testing should be performed to confirm the diagnosis.