Anal cancer accounts for only 3-5% of all large bowel malignancies. Most anal tumours arise from the epidermal elements of the anal canal lining (squamous cell—85% of anal tumours), though some arise from the glandular mucosa of the uppermost part of the anal canal or from the anal ducts and glands (adenocarcinomas—10% of anal tumours). Malignant melanoma of the anus is very rare (<5% of anal tumours) and carries a poor prognosis.

Traditionally, the anal region is divided into the anal canal and the anal margin or verge. The boundaries of these areas differ from one group of authors to another, making interpretation of studies difficult.

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