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 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. 

Where does anal cancer start?

Anal cancer can start in any part of the anus. Anal margin tumours are more common in men than women. Cancers that start higher up in the anal canal are more common in women.

Cancers of the anal margin usually look more like normal cells. Doctors call these well differentiated tumours.

There are different types of anal cancer depending on which type of cell the cancer starts in. This can affect what treatment you have.

Most anal cancers start in squamous cells that make up the lining of the anal canal. These are called squamous cell cancers.

The lining of the transitional zone is made up of both squamous and glandular cells. Glandular cells make the mucus that helps stools (faeces) pass through the anus smoothly. Rarely, anal cancer can develop from the glandular cells. This is called adenocarcinoma of the anus.

Melanoma is another rare type of anal cancer. It starts in cells in the skin called melanocytes.

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 1,400 people are diagnosed in the UK each year. That’s around 4 cases diagnosed every day in the UK.

The number of people in the UK diagnosed with anal cancer has been increasing. There is a larger increase for women than men. 

Who gets it

Anal cancer is more common in women than men. And it is more common in older people.

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 9 in 10 cases (90%) of anal cancer in the UK.