Anorectal Melanoma Masquerading as Common Hemorrhoids

Abstract

Introduction: Primary melanoma of the anorectum is a very rare and aggressive malignancy that is found to contribute to 1% of all anorectal malignancies. Usually, it affects women during the fifth and sixth decade of life and patients complain of rectal bleeding that is associated with changes in their bowel habits. Our case report describes a patient with hematochezia who was thought to have benign hemorrhoids but was diagnosed with anorectal melanoma. Case: An 83-year-old Caucasian female with a history of hyperlipidemia and HTN, presented to our office with a chief complaint of intermittent hematochezia for the past 2-3 months. Review of systems was pertinent for vague abdominal discomfort and change in bowel habits on several occasions. On physical exam, vitals were unremarkable; the abdomen was soft with minimal tenderness in the left lower quadrant. There were no masses or enlarged viscera. A digital rectal exam revealed prolapsed internal hemorrhoids. Colonoscopy revealed scattered sigmoid diverticulosis and an equivocal raised, nonpigmented, anorectal lesion that measured 2x1 cm, within the left lateral anal canal. The lesion was thought to be a thrombosed hemorrhoid. Biopsies were taken and results were consistent with malignant melanoma. A CT of chest, abdomen and pelvis demonstrated metastasis to the adrenals, pancreas and kidney. A bone scan performed was negative and an MRCP showed dilatation of pancreatic duct in the tail of pancreas. The patient underwent a transanal excision of anal tumor without any complications. Discussion: Primary melanoma of the rectum is a very rare condition that carries a dismal prognosis. The most common complaint is rectal bleeding but some patients present with anal pain, anal pruritus, tenesmus, and change in bowel habits. Unlike cutaneous melanoma, anal melanoma has no known risk factors. The diagnosis is often delayed and easily confused with common benign hemorrhoids. The median survival rate is 24 months, and a 5-year survival rate of only 15%. Because of the rareness of this condition, there are few guidelines in the approach for treatment depending on the size of the lesion. Radical abdominoperineal resection is effective when the lesion is less than 2 mm wide, whereas wide local excision is applicable with lesions greater than 2 mm. Thus, it is imperative that the diagnosis be made timely so that prompt treatment may be applied. Chemotherapy, radiation therapy, and immune therapy have a limited role. In conclusion, physicians need to be aware of this devastating malignancy that can easily be confused with benign hemorrhoids.