Amelanotic malignant melanoma of the anorectum: Report of a case and a review of Japanese literature

A 60-year-old man was admitted to our hospital, complaining of anal bleeding for a month. An elastic soft mass, which occupied half circumference of the anal canal, was palpated. Colonoscopy on admission showed a slightly dark reddish tumor, which was covered with thick viscous fluid and was surrounded by small protrusions at its base. Roentgenography revealed lobulated nodules at the anorectum. Computed tomography showed a wall thickening of the lower rectum and a swelling of lymph node in the pelvic region. Magnetic resonance imaging revealed the masses, low intensity in the T1 emphasis and high intensity in the T2 emphasis, at the lower rectum and in the pelvic region. Histology of biopsy specimens including the immunohistochemical analysis was compatible with amelanotic malignant melanoma. We treated him by surgical operation. Histopathological examination of the resected tumor, measuring 5 × 2.6 × 2.6cm, showed that the neoplastic cells had pleomorphic, large and vesicular nuclei with prominent nucleoli. Mitotic figures were frequent. Immunohistochemically, tumor cells were positive for S-100, vimentin and HMB-45 and were negative for cytokeratin (clone AE₁/AE₃) and leukocyte common antigen (CD45RB). Diagnosis of an amelanotic malignant melanoma at the anorectum with lymph node metastasis in the pelvic cavity was confirmed. He developed multiple metastasis in many organs and died after sixteen months postoperatively.