Papillary clear cell adenocarcinoma of the groin arising from endometriosis

A 67-year-old woman presented with rapidly enlarging right anterior thigh mass. Clinical impression was of a primary sarcoma, but the histologic analysis of an incisional biopsy specimen showed adenocarcinoma with clear cell differentiation. Initially thought to be a metastasis, the malignant tumor appears to have arisen from extraabdominal endometriosis of the right groin. This case shows clinical, radiographic, and histologic findings that may help the orthopaedic surgeon recognize an unusual complication of extrapelvic endometriosis.