Anal cancer is a malignant tumour that starts in the cells of the anal canal or anus. Malignant means that it can spread, or metastasize, to other parts of the body.
The anal canal and anus are part of the digestive system. The anus is the opening to the outside of the body at the lower end of the anal canal. The anal canal is the hollow tube that connects the rectum, which is the lower end of the large intestine, to the anus. The anal canal is surrounded by muscles that relax to let stool pass out of the body through the anus.
Cells in the anal canal or anus sometimes change and no longer grow or behave normally. These changes may lead to non-cancerous, or benign, conditions such as polyps or hemorrhoids.
Changes to cells of the anus can also cause precancerous conditions. This means that the cells are not yet cancer but there is a higher chance these abnormal changes will become cancer. The most common precancerous condition of the anus is anal intraepithelial neoplasia (AIN).
In some cases, changes to cells in the anal canal or anus can cause cancer. Most often, anal cancer starts in flat, thin cells called squamous cells. These cells cover the anus and the inside of the anal canal. This type of cancer is called squamous cell carcinoma (SCC) of the anal canal or anus. Cancer can also start in glandular cells, which make mucus. These cells are in the lining of the anal canal. This type of cancer is called adenocarcinoma of the anus.
Rare types of anal cancer can also develop. These include basal cell carcinoma (BCC), melanoma and gastrointestinal stromal tumour (GIST).