What is anal cancer?

Anal cancer starts in the cells of the anus or the tissues around the anus. A cancerous (malignant) tumour is a group of cancer cells that can grow into nearby tissue and destroy it. A tumour can also spread (metastasize) to other parts of the body.

The anus is part of the digestive system. It is the opening at the end of the large intestine after the rectum. Digestive waste (stool, or poop) leaves the body through the anus.

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 that connects the anus to the rectum. 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 (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) or squamous cell intraepithelial lesions (SILs). These changes to cells are not yet cancer, but there is a chance that they may become cancer if they aren't treated.

But in some cases, changes to cells in the anal canal or perianal skin can cause cancer. Most often anal cancer starts in round, flat cells called squamous cells. These cells are inside the anal canal and they make up the perianal skin. This type of anal 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).

The anus

The anus is the opening at the end of the digestive system. The waste from digestion (poop) leaves the body through the anus.

Cancerous tumours of the anus

Squamous cell carcinoma is the most common type of anal cancer. Adenocarcinoma is the second most common type.

Precancerous conditions of the anus

Precancerous conditions of the anus are changes to cells that make them more likely to develop into cancer. Precancerous conditions of the anus include anal intraepithelial neoplasia (AIN) and squamous cell intraepithelial lesions (SILs).

Non-cancerous tumours of the anus

Anal warts, polyps and skin tags are the most common non-cancerous conditions of the anus.

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