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. 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.
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).