Primary amelanotic anorectal melanoma: an uncommon neoplasia with poor prognosis

Anorectal melanoma is a rare and aggressive mucosal cancer. There is usually a delay in diagnosis because about 30% of these cancers are amelanotic and are often mistaken for benign conditions. Herein, we report a case of amelanotic anorectal malignant melanoma with an unusual metastatic deposit in the vulva and also review the literature.

A 67-year-old woman presented with a history of prolapse of an anal tumour. Clinical examination showed a pedunculated and ulcerated amelanotic tumour associated with three other nodules, 1 cm in diameter, localized in the vulval mucosa. A left inguinal node was palpable. Histological examination and immunohistochemical staining of all tumours demonstrated malignant melanoma. Radiological diagnostic procedures revealed no evidence of metastases.

Nine cases of amelanotic malignant melanoma have been reported in the literature. The age at diagnosis ranged from 45 to 77 years. Females appear to be far more frequently involved than males (F/M = 7/2). Anorectal melanoma is most common in the rectum, followed by the anal canal. Metastases occur early. Our case is the tenth case of amelanotic anorectal melanoma and probably corresponds to multiple synchronous primary melanomas of the anorectal region and the vulva, with the possibility that one of the lesions is a primary melanoma and the others are satellite lesions. Wide local excision where negative margins can be achieved is the preferred treatment.