Colonic malignant melanoma, primary or metastatic? Case report.

Gastrointestinal malignant melanomas, either primary or metastatic, are rare and overlooked tumors. There is also controversy regarding the actual existence of primary melanoma in the gastrointestinal tract apart from the esophagus and anorectal regions, where melanocytes normally exist. A case of malignant melanoma in the cecum is presented. The patient was a 30- year-old male who presented to the hospital for abdominal pain and diarrhea. The tumor was located mainly in the submucosa and measured 14x11x4.5 cm. The cut surface was solid, gray-white and fleshy. Histologically, tumor cells were arranged in compact nests or wide cords surrounded by fibrous stroma. The tumor cells had pleomorphic nuclei and quite rich cytoplasm; multinucleated, giant tumor cells were intermingled. Although no tumor cells contained apparent brown pigment, most were found to be positive for S-100 protein, HMB-45, Melan-A, and vimentin. The possibility of a metastatic lesion was considered. While the patient had a history of a pathologically examined dorsal nevus excision two years before, there was no evidence of either cutaneous or ocular primary melanoma at the time of diagnosis. Moreover, a thorough postoperative investigation did not reveal any other lesion in any other site favoring a metastatic spread. There was also no evidence of recurrent disease or metastasis one year after the surgery. This case is presented in view of its rare occurrence in the cecum. The difficulties in the diagnostic course are discussed, together with a literature review on distinguishing a primary mucosal melanoma from a metastatic one from an unknown or regressed cutaneous primary tumor.