Colonoscopic detection of a malignant melanoma metastatic to a tubular adenoma of the colon: report of a case.

PURPOSE: Metastasis of a malignant neoplasm to a benign tumor is an infrequent event. Adenomatous polyps of the colon have only been reported on three occasions acting as host tumors. We report the first case where a malignant melanoma metastasizing to a tubular adenoma in the colon was detected during colonoscopy.

METHODS

An 85-year-old male with dementia and right hemiparesis presented to the hospital with change of bowel habits, signs and symptoms of anal incontinence, and a perianal ulcer. On physical examination, multiple subcutaneous nodules over his chest and abdominal wall were detected, and laboratory data revealed iron deficiency anemia. A colonoscopy was performed.

RESULTS

During colonoscopy, a pedunculated, greenish-brown-colored polyp measuring 2 cm in greatest dimension located in the transverse colon was identified and snared. Pathology of this polyp was consistent with a malignant melanoma infiltrating into a tubular adenoma. Subsequently, one of the subcutaneous lesions was biopsied and this revealed malignant melanoma cells metastatic in soft tissue. The primary site of the malignant melanoma could not be identified.

CONCLUSION

Polyp-cancer sequence is well documented in colorectal cancer. The vast majority of malignant lesions found in adenomatous polyps in the colon are the consequences of malignant transformation from benign neoplastic colonic adenomatous tissue. The possibility that colonic adenomatous polyps could harbor metastatic tumors should also be entertained. This could change the treatment and may require an appropriate workup.