The anus is the final 4 cm of the large bowel that opens to facilitate defaecation. While abnormal cells in the anus can sometimes be harmless, they may progress into cancer. Anal cancer is highly treatable if detected early, and the outlook for patients with anal cancer is often better than for other types of bowel cancer. Precancerous and cancerous changes in the anal canal are similar to cervical cancer.
Anal cancer is rare and accounts for less than 2.5% of all cancers diagnosed in the gastrointestinal tract. Around 250 new cases of anal cancer are detected each year in New Zealand and age is a significant risk factor in its development. In 2015, no cases were reported in New Zealand for people under 29 years and the highest rates were seen in the 70+ age group. After age 50, anal cancer is slightly more common in women.