Epithelioid melanoma of the rectum: reappraisal of a deceptive microscopic simulator

We report 2 cases of rectal melanoma. The patients were a 48-year-old man and an 88-year-old woman. In both cases, the tumor presented as polypoid and necrotic masses, making initial diagnostic assessment difficult because of the regressive tissue changes. Microscopically, tumors were mostly composed of epithelioid, highly atypical cells mimicking poorly differentiated carcinoma or large cell lymphoma. Tumor cells were negative for keratin, leukocyte common antigen, and epithelial membrane antigen and positive for S100 protein and several melanoma markers, although in endoscopic biopsy specimens, the positive reaction was focal due to the limited viable tissue. One patient was treated with abdominoperineal resection. Pathologic examination of the surgical specimen revealed a polypoid mass infiltrating the muscle coat of the rectum. Local lymph nodes contained metastatic deposits. Microscopic examination of the rectal mucosa adjacent to the infiltrating neoplasm revealed colonization of the intestinal crypts by atypical melanocytes. This patient is alive 1 year after surgery with no evidence of recurrent disease. The other case was not deemed amenable to radical surgery because of the patient's advanced age and evidence of diffuse metastatic disease. She was alive 6 months after the initial diagnosis but was lost to follow-up. Data indicate that the diagnosis of melanoma, although feasible in large resection specimens, may be problematic in endoscopic biopsy, especially when only small tissue fragments are available and tumor regression hampers the accuracy of immunohistochemical stains. Documentation of intracryptic atypical melanocytes in nonneoplastic mucosa conceptually corroborates claims that rectal melanoma is related to preinvasive precursors as in cutaneous melanoma.