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Anorectal malignant amelanotic melanoma: Report of a rare aggressive primary tumor

Malignant melanoma of the anorectal region is a very rare aggressive malignant neoplasm and it constitutes 1% of all malignant lesions of this area. About 70% of these lesions are pigmented, whereas 30% are amelanotic. Demonstration of immune markers of melanoma by immunohistochemistry (IHC) is required for confirming the diagnosis of amelanotic malignant melanoma. Here, we report a case of anorectal malignant amelanotic melanoma in a 65-year-old male with no medical comorbidities, who presented with chief complaints of bleeding per rectum associated with prolapsing mass per rectum of 7 months duration. On external examination and proctoscopy, three prolapsed pedunculated fungating masses were seen externally protruding out of the rectum approximately 4 cm from the anal verge. Contrast-enhanced computed tomography of the whole abdomen and pelvis was suggestive of moderately enhancing lobulated anorectal mass with large polypoidal intraluminal component arising from anorectal walls and extension into mid-lower rectum with liver and locoregional lymph nodes metastasis. The patient was taken up for palliative local excision. Per-operatively, three large irregular highly vascular pedunculated rectal growth was seen. The growth was excised and sent for histopathological examination. Microscopic examination of mass show spindle-to-ovoid tumor cells with hyperchromatic central to eccentric nuclei arranged in intersecting fascicles with a focal alveolar pattern. The large number of atypical mitotic figures (40-50/10 High Power Field (HPF)) was seen along with areas of necrosis and the presence of few bizarre binucleated and multinucleated giant cells. A differential diagnosis of malignant amelanotic melanoma was given along with undifferentiated carcinoma, gastrointestinal stromal tumor , and Non-Hodgkin's lymphoma. On IHC, the tumor cells were reactive for HMB45, S-100, and SOX-10. Thus a diagnosis of malignant amelanotic melanoma was confirmed. The patient had symptomatic improvement.