What is anal cancer?

Anal cancer is a rare cancer that starts in the anus. This is the opening at the end of the bowel. 

Cancer is when abnormal cells start to divide and grow in an uncontrolled way. The cells can grow into surrounding tissues or organs and may spread to other areas of the body. 

The anus

The anus is the part of the large bowel that opens to the outside of the body. It is the tube that your poo (stool) passes through when you go to the toilet to empty your bowel. The anus makes up part of your digestive system. It is about 3 cm long, and is at the end of your back passage (rectum).

The parts of the anus include the:

  • transitional zone - this is where the rectum meets the anal canal
  • anal canal, which connects the rectum to the anus
  • anal margin - this is the lower part of the anal canal and it contains muscles called the anal sphincters

You have an internal and external anal sphincter. They are the muscles that control your bowel movements. 

Diagram showing the anatomy of the anus

Where does anal cancer start?

Anal cancer can start in any part of the anus. And it can start in different types of cell. There are different types of anal cancer depending on which type of cell the cancer starts in. The cell type and position of your cancer can affect what treatment you have.

Parts of the anus

Anal cancer can start in the anal margin, the anal canal or the transitional zone.  

Anal margin cancers are more common in men than women. Cancers of the anal margin usually look more like normal cells. Doctors call these well differentiated tumours.

Cancers that start higher up in the anal canal are more common in women.

Cell type

Cancer can start in the following cell types:

  • squamous cell cancers start in squamous cells that make up the lining of the anal canal  - most anal cancers are squamous cell cancers
  • adenocarcinoma of the anus starts in glandular cells that make the mucus that helps stools (faeces) pass through the anus smoothly – adenocarcinoma of the anus is less common
  • melanoma starts in cells in the skin called melanocytes – this is another rare type of anal cancer

Anal intraepithelial neoplasia (AIN)

Anal intraepithelial neoplasia means there are abnormal cells in the anus.

It is not cancer but might develop into cancer.

The cells might be inside your anal canal or in the skin around the back passage (perianal skin).

How common is anal cancer?

Anal cancer is rare. Around 1,500 people are diagnosed in the UK each year. That’s around 4 cases diagnosed every day in the UK.

Anal cancer has become more common over the last 20 years in the UK. There is a larger increase for women than men. 

Who gets it

Anal cancer is more common in women than men. Your risk of developing anal cancer increases as you get older. The risk is greatest in those aged 90 years and older.

Many anal cancers are linked to lifestyle or other risk factors. The main risk factor for anal cancer is human papilloma virus (HPV) infection. HPV is linked to around 90 out of 100 cases (90%) of anal cancer in the UK.