[A Case of Anorectal Amelanotic Malignant Melanoma]

We report a case of primary anorectal amelanotic malignant melanoma (MM), treated with a laparoscopic abdominoperineal resection (APR). A 75-year-old woman was referred to our hospital because of anal bleeding and pain. A finger-tip sized, dark reddish tumor, which protruded from her anus, was observed. After a tumor biopsy, the diagnosis was MM. No cutaneous pigmented regions were observed, and distant metastases and regional lymphadenopathy were not detected by computed tomography and magnetic resonance imaging. Therefore, we performed laparoscopic APR in order to relieve her symptoms. The resected specimen showed a partially pigmented tumor with a diameter of 6.0×4.1 cm in the anorectal junction. Histopathological examination of the specimen showed an amelanotic MM (negative for melanin pigmentation, and positive for HMB-45, Melan-A, and S-100). The post-operative course was uneventful, and we could not find any obvious evidence of recurrence of the disease 11 months after surgery. The prognosis of anorectal MM is extremely poor, thus a less invasive surgical procedure is recommended for treatment of anorectal MM. Laparoscopic APR is useful for treating anorectal MM due to its minimally invasive nature.