What is anal cancer?

Anal cancer is a malignant tumour that starts in the cells around the anus. Malignant means that it can invade, or grow into, and destroy nearby tissue. It can also spread, or metastasize, to other parts of the body.

The anus is part of the digestive system. Stool leaves the body through the anus. It is the opening at the end of the large intestine after the rectum.

Diagram of the digestive system
Diagram of the digestive system

The area around the anus is called the anal region. It includes the anal canal, which is the short tube just above the anus. The anal region also includes the perianal skin, which is the skin around the anus.

Diagram of the anus and surrounding area
Diagram of the anus and surrounding area

Cells in the anal canal or perianal skin sometimes change and no longer grow or behave normally. These changes may lead to non-cancerous, or benign, conditions such as anal warts, polyps or skin tags.

Changes to cells in the anal region can also cause a precancerous condition called anal intraepithelial neoplasia (AIN). This means that the cells are not yet cancer, but there is a chance that they may become cancer if they aren’t treated.

In some cases, changes to cells in the anal canal or perianal skin can cause cancer. Anal cancer usually starts in round, flat cells called squamous cells. These cells are inside the anal canal and make up the perianal skin. This type of cancer is called squamous cell carcinoma of the anus.

Rare types of anal cancer can also develop. These include adenocarcinoma, basal cell carcinoma, melanoma and gastrointestinal stromal tumour (GIST).

Expert review and references

  • Czito BG, Ahmed S, Kalady M, Eng C . Cancer of the Anal Region. DeVita VT Jr, Lawrence TS, Rosenberg SA. Cancer: Principles and Practice of Oncology. 10th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2015: 61:842-856.
  • Eng C, Ajani J . Anal canal cancer. Kelsen DP, Daly JM, Kern SE, Levin B, Tepper JE, Van Cutsem E (eds). Principles and Practice of Gastrointestinal Oncology. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 2008: 47:641-652.
  • Martini FH, Timmons MJ, Tallitsch RB. Human Anatomy. 7th ed. San Francisco: Pearson Benjamin Cummings; 2012.
  • Meyer JJ, Willett CG, Czito BG . Anal cancer. Jankowiski J, Hawk E (eds.). Handbook of Gastrointestinal Cancer. Wiley-Blackwell; 2013: 6:137-160.
  • Reidy-Lagunes D, Goodman K, Saltz LB . Unusual tumors of the colon, rectum, and anus. Raghavan D, Blanke CD, Honson DH et al (eds.). Textbook of Uncommon Cancer. 4th ed. Wiley Blackwell; 2012: 32:453-463.

The anus

The anus is the opening at the end of the large intestine. Learn about the anus, including its structure and function.

Cancerous tumours of the anus

Cancerous anal tumours are cancerous growths that can spread to other parts of the body. Learn about squamous cell carcinoma and other malignant tumours.

Precancerous conditions of the anus

Learn about a precancerous condition of the anus, anal intraepithelial neoplasia, or AIN, which may become anal cancer if not treated.

Non-cancerous tumours of the anus

Learn about non-cancerous tumours of the anus, such as anal warts, which do not spread to other parts of the body and are not usually life-threatening.