[Anorectal melanoma. A rare and highly malignant tumor entity of the anal canal].

Anorectal melanomas represent a very small group of mucosal melanoma with unknown etiology and poor prognosis. In view of their location, a history of sun exposure is not likely to have had an impact on their development. Recent epidemiologic data indicate a bimodal age distribution. To date there is no information whether an infection with the human papilloma virus plays a role in the tumorigenesis of anorectal melanoma. The lesions can be misdiagnosed as hemorrhoids on clinical examination. On histological examination amelanotic types have been misdiagnosed as lymphoma, sarcoma,and undifferentiated carcinoma. Useful immunohistochemical markers are S 100 protein, HMB-45, Melan A, and MiTF (microphthalmia-transcription-factor). Therapy includes local excision or abdominoperineal resection followed by optional inguinal and parailiac lymph node dissection, and consecutive chemo- and immunotherapy. The poor long-term prognosis of anorectal melanomas correlates with their advanced tumor size and depth of infiltration at diagnosis. The overall 5-year survival rates range between <5 and 22% in different series.