Superficial Ewing Sarcoma of the Rectum: A Case Report and the Utility of Molecular Diagnostics.

Ewing sarcoma is an undifferentiated small round cell sarcoma that most commonly presents as a malignant bone tumor in pediatric and young adult patients. The diagnosis is typically confirmed by molecular genetic identification of a fusion protein, most commonly involving members of the FET and ETS gene families. The most frequent translocation is EWSR1::FLI1 which occurs in approximately 85% of cases. Rare extraosseous cases of superficial Ewing sarcoma have been described, including one molecularly confirmed tumor involving the rectum. Herein, we report the second molecularly confirmed case of a superficial, rectal primary tumor arising in a 38-year-old male who initially presented with bleeding. Physical exam noted an ulcerated rectal polyp near the dentate line. Excisional biopsy of the lesion revealed a monotonous round cell malignancy involving the submucosal tissue with necrosis and marked mitotic activity. Immunohistochemical studies showed reactivity for CD99 and NKX2.2 with weak, focal expression of synaptophysin. Next-generation sequencing revealed an EWSR1::FLI fusion, confirming the diagnosis of Ewing sarcoma. The fusion variant junction was located in exon 7 for EWSR1 and exon 6 for FLI1. Subsequent imaging revealed no evidence of metastatic disease, and re-excision showed no residual tumor. Primary superficial Ewing sarcoma is extremely rare, and this is only the second molecularly confirmed case of rectal origin.