Primary anorectal melanoma on FDG PET/CT.

Primary anorectal melanoma is an extremely rare, aggressive malignancy with a poor prognosis. A 76-year-old man with a 2-month history of rectal bleeding, anal pain, and tenesmus was referred for an F-FDG PET/CT examination, which revealed an abnormal focal FDG uptake on the lower rectum with multiple other intense FDG-avid regions in locoregional pelvic lymph nodes, bones, liver, and lung. It was highly suggestive of malignancy, and a provisional diagnosis of advanced rectal carcinoma was made. However, a following tissue biopsy of the lesion on the lower rectum performed by colonoscopy histopathologically indicated primary anorectal melanoma.