Anorectal Melanoma: A Rare Cause of Large Bowel Obstruction

Abstract

Anorectal melanoma is a rare and aggressive malignancy with a challenging diagnosis and management. We present the case of a 69-year-old male with a history of chronic constipation and recent weight loss, who presented with symptoms suggestive of anorectal pathology. Despite initial diagnostic challenges, including an unsuccessful colonoscopy due to inadequate bowel preparation, the subsequent radiographic evaluation revealed a large bowel obstruction secondary to a protruding anorectal mass. Abdominal X-ray demonstrated significant colonic dilation while contrast-enhanced CT scan revealed a large hypodense mass protruding from the anal canal. Surgical intervention led to the excision of the mass, and histopathological examination confirmed malignant melanoma. Immunohistochemistry markers, including HMB 45, Melan A, and S100, supported the diagnosis. This case underscores the importance of considering anorectal melanoma in patients presenting with atypical anorectal symptoms, despite its rarity. Early recognition and intervention, supported by appropriate imaging modalities, are critical for optimizing patient outcomes in such cases.

Keywords: abdominal perineal resection; anorectal melanoma; large bowel obstruction; malignant melanoma; melanoma.

Figure 1. Protrusion of the anorectal mass

The protruding mass from the anal canal shows black pigmentation.

Figure 2. Coronal section showing a large bowel obstruction

The black arrow indicates a fully loaded transverse colon

Figure 3. The transverse section shows a mass at the anorectal junction

The black arrow indicates a mass in the anorectal junction.

Figure 4. Pedenculated rectal mass below the dentate line

The image indicates a mass raising from the left lateral wall of the anal canal.

Figure 5. Excised specimen of the anorectal mass

The medial surface of the tumor shows black pigmentation.

Figure 6. Histopathological examination

The black arrow indicates brown pigment below the stratified squamous epithelium. Neoplastic cells are arranged in the sheets and nests pattern.

Figure 7. Melan A positive

Melan A is a cytoplasmic stain that detects a melanocyte-specific cytoplasmic protein. This image indicates Melan A is positive for this tumor.