A 37 year old male was referred to our department for severe anal pain over the last month and periodical rectal bleeding without changes in bowel movements. The pain was partially relieved by taking paracetamol at usual doses. The patient was afebrile and had no significant medical history or previous operations. Rectal examination revealed a painful, tender bluish mass at the 2 to 5 o'clock position (Figure 1a). The rest of the physical examination was normal. Initial laboratory tests showed no significant changes except for a slightly decreased hemoglobin level of 12.1 g/dL (normal, 13.0–17.0). Sigmoidoscopy was performed which revealed a friable flat lesion extended 3 cm proximally from the anal verge. The rest of the rectum and the sigmoid colon were normal. Biopsy specimens were taken.

Histologic examination of the specimens showed diffuse infiltration by hyperchromatic neoplastic cells (Figure 1b). The neoplastic cells had variably prominent eosinophilic nucleoli (long arrows), intranuclear inclusions (short arrow) and dusty melanin pigment granules (dashed arrow) (Figure 1c). The epithelial marker cytokeratin 8/18 cocktail was staining strips of colonic epithelium but was negative in the neoplastic cells (Figure 1d). The tumor cells were immunohistochemically positive for S-100 protein as well as scattered melanocytes in the overlying squamous epithelium (Figure 2a). HMB-45 an anti-melanoma antibody was positive in the neoplastic cells in the stroma and in rare intraepithelial melanocytes (Figure 2b). Melan-A/MART-1 was positive in the neoplastic cells in the stroma (Figure 2c). Microphthalmia transcription factor (MITF) was positive in the neoplastic cells and negative in the overlying squamous epithelium (Figure 2d). The diagnosis of melanotic malignant melanoma was made.

Anorectal melanoma is a rare and aggressive tumor with an unfavourable prognosis. It represents approximately 1% of all anorectal malignancies, and is more common in females and affects all ages, with the highest incidence during the sixth and seventh decade. The anal canal is the most frequent site of melanoma after the skin and retina, represtenting about 1% of all melanomas. It arises from melanocytes present in the transitional zone of the anal canal. Presenting symptoms are non specific and most patients complain for rectal pain, tenesmus, bleeding, bowel habit changes and weight loss. Thirty per cent of the anorectal melanomas are amelanotic and more difficult to be recognized, their diagnosis depends on demonstration of melanin pigment by immunohistochemistry. At early stage, the lesion looks like a polyp or a thrombosed haemorrhoid as in our case.

Early infiltration and distant metastases are common during diagnosis resulting in poor overall prognosis despite surgery, chemotherapy, radiotherapy and immunotherapy. The 5-year survival rate despite multimodal treatment is less than 20%.