Abstract

Ewing sarcoma is a fusion defined small round blue cell tumor characterized by gene family FET::ETS rearrangements. Although typically originating in bone, approximately 12% of patients may have a tumor of extraskeletal origin and even fewer have tumors primarily situated in the skin. We report a case of cutaneous/superficial Ewing sarcoma in a 64-year-old female on the shoulder with no prior history or bone involvement. Histologically, the tumor consisted of monotonous basaloid cells with duct-like structures and eosinophilic secretions in an ill-defined nodular configuration. These cells were positive for CD99 and NKX2.2 stains. FISH was positive for a EWSR1::FLI1 fusion. We also review cases of Ewing sarcoma situated in the deeper soft tissues that presented clinically as a benign skin lesion such as a cyst or lipoma. Ewing sarcoma is a malignancy with a moderate cure rate for localized disease, but cases that metastasize have guarded prognoses. Patients with cutaneous limited disease typically have a better prognosis than patients with bone involvement. Separating this entity from morphologically similar entities including lymphoma, melanoma, CIC::DUX4 sarcoma, or other sarcomas is critical for optimal patient management and diagnosis.