ANAL MELANOMA: CASE REPORT

A 67-year-old white female with chronic kidney disease and grade III obesity was referred to a coloproctologist due to hematochezia, pain, pruritus, and progressive anal protrusion over 8 months. Upon anal inspection, a vegetative, ulcerated, friable lesion with necrotic areas, irregular borders, and apparent infiltration into deeper layers was found on the left posterior-lateral anal edge, measuring approximately 4cm cranio-caudally. On palpation, a lesion with the same characteristics was felt up to 3cm from the anal edge. A colonoscopy performed 3 months earlier showed colonic diverticula and an erosive anal plica. Pelvic magnetic resonance imaging revealed an expansive, lobulated lesion in the posterior anal canal measuring 56x30mm, with an oval-shaped inguinal lesion on the left side measuring 25x19mm. The patient underwent partial excision of the lesion, with histopathology and immunohistochemistry confirming the diagnosis of anal canal melanoma, with positive S100 and MELAN-A markers. Two months after diagnosis, the patient passed away at an external oncology service. Anal melanoma is the third most common site for mucosal melanoma and the primary site in the gastrointestinal tract. It is more frequent in white women, with an average age of 55 years. At diagnosis, more than 60% of patients have metastases, a situation often due to tumor aggressiveness and delayed diagnosis caused by nonspecific symptoms resembling benign anorectal diseases. Anoscopy can help evaluate the “ABCDE” of melanoma, but lesions generally appear polypoid, ulcerated, and range from 1 to 6 cm. A definitive diagnosis is made by biopsy with immunohistochemistry, showing positive markers such as S100 and MELAN-A. For loco-regional and distant staging, positron emission tomography (PET) is superior to contrast-enhanced computed tomography due to the multiple lymphatic drainage areas in the region. Treatment involves local excision or abdominoperineal amputation, often with adjuvant therapies such as chemotherapy, radiation, and more recently, immunotherapy. The prognosis is poor, with a 5-year survival rate of approximately 20%. This case report supports the epidemiology of anal mucosal melanoma and emphasizes the importance of thorough anorectal evaluation for early diagnosis of this aggressive tumor.

No conflict of interest has been declared by the author(s).

Publication History

Article published online:
25 April 2025