ANORECTAL MUCOSAL MELANOMA: A RARE BUT POTENTIALLY FATAL CAUSE OF RECTAL BLEEDING R Mahat MD, G Echefu MD, D Kantamani MD, MP Stagg MD Department of Medicine, Baton Rouge General Medical Center, Baton Rouge, LA
INTRODUCTION: Rectal bleeding is a common complaint with many relatively benign causes including hemorrhoids, anal fissures, and colonic diverticula. However, there are more insidious causes of rectal bleeding that can potentially prove to be fatal without early intervention.
CASE: A 71-year-old male presented to the gastroenterology office for 1-2 years history of bright red blood per rectum three to four times a week. Bleeding was associated with rectal pain and itching. He denied abdominal pain, weight loss, anorexia, altered bowel habit, or change in stool caliber. He was evaluated by his PCP in the past and was treated for hemorrhoids. Screening colonoscopy done 6 months earlier revealed nonbleeding thrombosed external and internal hemorrhoids, and sessile polyp in the rectum. The polyp was completely resected, and biopsy revealed tubular adenoma. Digital rectal examination showed grade II internal hemorrhoids. Flexible sigmoidoscopy showed 14 mm oozing, polypoid, semi pedunculated lesion in the distal rectum. Initial Computed Tomography scan of abdomen and pelvis showed ill-defined soft tissue thickening of the inferior rectum. The patient underwent trans anal resection of the mass and biopsy with histopathology revealed nodular malignant melanoma. Immunohistochemical stain was positive for S100 and Sox 10. Staging Positron Emission Tomography (PET) scan revealed metastasis in the left internal iliac chain lymph nodes without evidence of distal metastasis. He was staged as stage II ARM. After multidisciplinary discussion in a tumor board the patient was discharged home with a plan of outpatient immunotherapy. He completed a total of 3 cycles of immunotherapy with pembrolizumab. Follow up PET scan after 5 months showed multiple lung and liver metastasis. DISCUSSION: Anorectal mucosal melanoma (ARM) is rare, accounting for less than 1% of all melanomas, 24% of all mucosal melanomas. Unlike other subset of mucosal melanomas, ARM is increasing. Patients presents with rectal bleeding, a mass, anorectal pain, or a change in bowel habits. Approximately 20–30% of ARMMs are amelanotic, resembling benign polypoid lesions endoscopically. Pathology with immunohistochemistry is the gold standard for diagnosis. By the time of diagnosis, the majority of cases (61%) are locally progressed or have distant metastasis. Surgical excision remains the cornerstone of therapy. The prognosis is often dismal, with a 5-year survival rate of only 20%.
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