It is important for doctors to know whether a cancer is in the anal canal or anal margin as the treatment of the two types of tumour can sometimes be different. Most anal canal tumours are treated with a combination of radiotherapy and chemotherapy. Small cancers in the anal margin can often be treated with a minor operation.

The anus is the name for the muscular area at the end of the large bowel. It includes the muscle, which opens and closes to control bowel movements, and is where the bowel opens to the outside.

The internal part of the anus is called the anal canal. It is a cylinder of tissue about 2-3 centimetres long (about one inch). The patch of skin immediately around the opening of the anus is known as the anal margin. Sometimes doctors talk about the point where the anal canal and anal margin meet one another as being the anal verge. Most of these anal cancers develop in the anal canal but about 1 in 5 are in the anal margin.

Reference

• Clark M et al. 2004 Cancer of the anal canal. Lancet Oncology 5(3);149-157