Anorectal melanoma is a rare tumor that develops in melanin producing pigment cells of the anus and rectum. As a rare entity there is not much literature regarding the management of this pathology. It`s a difficult diagnosis and the majority of patients present metastases. Therefore we presented a case of probable anorectal melanoma submitted to 18F-FDG PET/CT.

76 years old, male with history of anal pain and sporadic bleeding after bowel movement in the last 6 months. Bowel habits of 3-4 times daily and weight loss not quantified in the period. Digital rectal examination revealed a vegetative, friable and painful lesion, found 1 cm from anal verge, in the anterior wall, extending for 3 cm and presence of blood. Colonoscopy revealed an infiltrative, multilobular, friable and non stenosing lesion, measuring 5 cm and including pectineal line and anterior wall of the anus. Biopsy revealed colorectal mucosa with atypical cellular proliferation and ulceration and immunohistochemistry demonstrated markers that confirmed the diagnosis of melanoma (primary lesion or metastatic). Chest and abdominal CT scans demonstrated lung, liver and left adrenal lesions. 18F- FDG PET/CT was performed for primary staging. PET scan revealed hypermetabolism in anorectal wall thickening, that might be the primary lesion and in multiple hypermetabolic lesions in thyroid, lungs, liver, left adrenal, stomach lesser curvature, retroperitoneal and pelvic nodes, peritoneal implants and bones, suggesting metastatic involvement.

This report shows a patient with a clinical picture, physical examination and complementary exams compatible with anorectal neoplasia. Immunohistochemistry confirms the diagnosis of melanoma. Anorectal melanomas are extremely rare and aggressive. Lymphatic dissemination of anal melanomas results in distant metastases to the liver and lungs by up to 90% of cases. These findings are in line with the FDG PET/CT reported in our study. 18F-FDG PET/CT may be useful in the primary staging of anal melanoma patients and in identifying lesions missed by other conventional radiological methods.