Anal melanoma is an aggressive and uncommon cancer. Even with radical surgery, the prognosis for anal melanoma is poor, with a local relapse incidence of 45.8%. This may in part be due to delayed diagnosis, with clinical differential diagnoses including haemorrhoids or benign polyps. Early metastases also contribute to the poor prognosis. Unlike for melanoma of the skin, sun exposure is not a known risk factor for anorectal melanoma.

Pre-operative staging influences the treatment schedule of surgery and/or adjuvant therapy. Whilst limited information is available about PET-CT in anorectal melanoma, the modality has improved sensitivity, specificity and accuracy for cutaneous melanoma metastases, as compared to FDG-PET alone. It has limitations in the detection of early-stage disease, small lung nodules, and brain metastases. Most false-negatives are due to lesions smaller than 10mm, for which sentinel lymph node biopsy still has greater accuracy. False-positives may be due to postsurgical or other inflammatory changes, or benign lesions. For baseline staging of melanoma of unknown primary site, serum s100 protein and whole body FDG-PET have been shown to be useful. Further research in the utility of PET-CT for assessment of anorectal melanoma metastases is required.