Abstract

Primary cutaneous Ewing sarcoma (pcES) is an extremely rare clinical presentation of Ewing sarcoma (ES), with less than 100 cases reported in literature. Extraskeletal ES presents as a palpable soft tissue mass, usually <5cm, with a wide anatomical distribution, most commonly in children and young adults. It has a favorable prognosis with a 10-year survival rate of approximately 91%, which distinguishes it from other small round blue cell tumors, and is defined by characteristic molecular aberrations. We report a case of pcES occurring on the lateral thigh of an 11-year-old male clinically suspected to be a post-traumatic cystic hematoma or abscess. The mass was raised, with associated ecchymosis. Imaging revealed a subcutaneous well-circumscribed, complex cystic, hemorrhagic and septated mass with nodular enhancement. Surgical excision revealed a gelatinous 5.5 x 5.0 cm soft tissue mass. Histopathologically it was comprised of monomorphic small round blue cells with fine chromatin, absent nucleoli, arranged almost entirely in rosettes with prominent hemorrhagic cystic spaces. CD99 showed strong, diffuse membranous expression and diffuse S100 expression was also present. Tumor cells were negative for ERG, desmin, synaptophysin, chromogranin, and HMB45. Molecular testing demonstrated EWSR-FLI1 fusion, confirming the diagnosis. Staging imaging studies were negative, reaffirming that this was a pcES. In summary, this is a rare case of pcES in a child with EWSR-FLI fusion demonstrating prominent cystic changes and mimicking a benign post-traumatic lesion both clinically and radiologically.

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