Primary malignant melanoma without melanosis of the colon

Primary malignant melanoma (PMM) of the gastrointestinal tract is rare. Reported cases of PMM of the lower gastrointestinal tract typically describe anal and rectal involvement rather than colonic lesions. This report describes a rare case of a 50-year-old woman with PMM originating in the colon. The patient presented to Inje University Busan Paik Hospital with a 3-day history of blood-tinged stools. She underwent colonoscopy for a diagnosis of hematochezia. The colonoscopic examination revealed a large-sized semi-pedunculated sigmoid colon polyp with a reddish-colored mucosal surface. Endoscopic mucosal resection was performed, and the final histopathological findings were consistent with a diagnosis of malignant melanoma. Systemic work-up was performed for assessment of metastasis and to identify the primary tumor considering the high metastatic rate of gastrointestinal malignant melanoma; however, no other malignant lesion was detected. Thus, she was diagnosed with colonic PMM. She underwent laparoscopic low anterior resection and lymph node dissection and has been recurrence-free for > 2 years.