About the anus

The anus is the opening at the end of the bowel. It is made up of the last few centimetres of the bowel (anal canal) and the skin around the opening (anal margin). During a bowel motion (when you poo), the muscles of the anus (sphincters) relax to release the solid waste matter known as poo, faeces or stools.

What is anal cancer?

Anal cancer is cancer affecting the tissues of the anus. Cancer is a disease of the cells, the body’s basic building blocks. Normally cells multiply and die in an orderly way, so that each new cell replaces one lost. Sometimes cells become abnormal and keep growing. These abnormal cells may form a lump called a tumour.

If the cells in a tumour are cancerous, they can spread through the bloodstream or lymph fluid and form another tumour at a new site. This new tumour is known as secondary cancer or metastasis. 

Types of anal cancer

• Squamous cell carcinomas (SCCs) – Most anal cancers are SCCs. These start in the flat (squamous) cells that line much of the anus.
• Adenocarcinomas – These start in cells in the anal glands and are treated in a similar way to bowel cancer.
• Skin cancers – In rare cases, anal cancers can affect the skin just outside the anus. These are called perianal skin cancers. If they are not too close to the sphincter muscles, they can be treated in a similar way to SCCs on other areas of the skin.
  
  

Risk factors

About 80% of anal cancer cases are caused by infection with a very common virus called human papillomavirus (HPV). HPV can affect the surface of different areas of the body, including the anus, cervix, vagina and penis. Unless people are tested, most of them will not be aware that they have HPV as it usually doesn’t cause symptoms.

HPV is the main risk factor for anal cancer, but there are other factors that may increase the risk of getting anal cancer including:

• having a weakened immune system, for example because of human immunodeficiency virus (HIV), an organ transplant or an autoimmune disease such as coeliac disease, lupus or Graves’ disease
• being a man who has had sex with other men
• having anal warts
• having had an abnormal cervical screening test result or cancer of the cervix, vulva or vagina
• smoking tobacco
• being aged over 50.

Some people with anal cancer do not have any of these risk factors.

Symptoms

In its early stages, anal cancer often has no obvious symptoms. However, some people may experience the following:

• blood or mucus in faeces or on toilet paper
• itching, discomfort or pain around the anus, or a feeling of fullness, discomfort or pain in the rectum
• a lump near the edge of the anus
• ulcers around the anus
• difficulty controlling bowel movements.

Not everyone with these symptoms has anal cancer. Other conditions, such as piles (haemorrhoids) or tears in the anal canal (anal fissures), can also cause these changes.

Diagnosis and staging

The main tests for diagnosing anal cancer are:

• a physical examination which involves a doctor inserting a gloved finger into your anus to feel for any lumps or swelling. This is called a digital anorectal examination (DARE).
• an endoscopy with biopsy which involves a doctor inserting a narrow instrument called a sigmoidoscope or colonoscope into your anus to see the lining of the anal canal. This may be done under a general anaesthetic so that a tissue sample (biopsy) can be taken. 

If anal cancer is found, you may need one or more imaging scans to check if it has spread. These scans may include an MRI, an endorectal ultrasound, a CT scan or a PET–CT scan.