S1742 Not as It Appears: A Case of Amelanotic Anorectal Melanoma

Abstract

INTRODUCTION: Primary anorectal melanoma (ARM) is a rare entity with anorectal presentation accounting for 0.4- 1.6% of all primary melanomas. ARM has an incidence of 0.5-4% of all anorectal malignancies. It presents after the fifth decade of life with nonspecific complaints. In patients taken for colonoscopy, diagnosis and resection can be difficult as lesions may be amelanotic and spread along submucosal planes. With incidence increasing, it is especially important to recognize ARM as a source of morbidity and mortality. CASE DESCRIPTION/METHODS: Our patient is an 89 yo female with Non-Hodgkin lymphoma presenting with left lower quadrant abdominal pain. Her pain was associated with loose stools and hematochezia attributed to hemorrhoids. The patient endorsed colonoscopy 10 years ago and had no polyps identified on any prior studies. Her labs demonstrated WBC of 71, Hgb of 12 with CT A&P demonstrating thickening and fat stranding at the rectum and splenic flexure. Physical exam was notable for tenderness on palpation of the LLQ, and rectal exam demonstrated a palpable mass proximal to the anal verge. The patient was taken for flexible sigmoidoscopy. Sigmoidoscopy demonstrated a non-bleeding malignant appearing 5 cm mass within the rectum which was non-circumferential, just above the dentate line, with multiple cold forceps biopsies taken. Pathology returned as suspicious for malignant melanoma with sections demonstrating an epithelioid malignancy with scattered pigmented cells. This sample was sent for further evaluation, and demonstrated a subepithelial proliferation of poorly differentiated malignant cells in a sheet like configuration. The nuclei displayed severe pleomorphism and the cytoplasm was granular and eosinophilic with questionable, focal pigmentation. By immunohistochemistry, tumor cells showed Mart-1 and CD31 coexpression with focal S-100 staining. The results were thought to represent a poorly-differentiated malignant melanoma. These findings were discussed with the patient, and it was determined she had a poor prognosis with no option for aggressive treatment. Patient did elect for hospice. DISCUSSION: ARM carries a poor prognosis. This is due to misdiagnosis as hemorrhoids, polyps and other rectal cancer. At the time of diagnosis only 37% of ARM is confined at the anorectal area. Surgical resection is the mainstay of treatment for those with local disease. As ARM spread along submucosal planes and are often beyond complete resection, early recognition of ARM is of the utmost importance.Figure 1.: Rectal mass.Figure 2.: Rectal mass.