Primary Amelanotic Anorectal Melanoma with Upper Gastrointestinal and Brain Metastases

Abstract

Background

The anorectum is a rare anatomic location for primary melanoma. In many cases the disease is undetected or is misdiagnosed as a benign polyp or hemorrhoids until it reaches an advanced state. The prognosis is poor, due to delayed diagnosis and early distant metastasis. Involvement of the upper gastrointestinal (GI) tract is very rare in anorectal melanoma and has been described in only a few case reports.

Case report

An 83-year-old man presented with a 24-hour history of confusion, slurred speech and gait difficulty of abrupt onset. A computed tomography (CT) scan of the brain revealed three hemorrhagic lesions of 18mm, 14mm and 11mm in their greatest dimension in the cerebral hemispheres, specifically the left and the right frontal regions and the left temporal lobe, which were considered to be metastases. Subsequently, upper GI endoscopy revealed multiple small nodules involving the stomach and the duodenum. A non-pigmented anorectal mass 2cm in diameter was found on colonoscopy.

Results

Biopsy of the lesions in the stomach and duodenum revealed that they shared the same histopathological and immunohistochemical characteristics as the anorectal tumor. The tumor cells were diffusely positive for S-100 protein, HMB-45, Melan-A and MITF-1 antigens. Based on these findings, the diagnosis was made of primary anorectal melanoma with secondary metastases in the upper GI tract.

Conclusion

Primary anorectal melanoma is a rare aggressive malignancy with a tendency to metastasize early, due to its abundant vascular system, although metastasis to the stomach and duodenum is rare. The diagnosis of melanoma is based on the histological features and immunohistochemical stains.