The inner lining of the anal canal is the mucosa. Most anal cancers start from cells in the mucosa. Glands and ducts tubes leading from the glands are found under the mucosa. The glands make mucus, which acts as a lubricating fluid. Anal cancers that start from cells in the glands are called adenocarcinomas. Anal cancers are often divided into 2 groups, which are sometimes treated differently:. The anal canal is surrounded by 2 sphincter muscles that keep the anus closed and prevent stool leakage.
Many types of tumors can develop in the anus. Not all of these tumors are cancers — some are benign not cancer. Polyps are small, bumpy, or mushroom-like growths that form in the mucosa or just under it. There are many kinds. Skin tags are benign growths of connective tissue that are covered by squamous cells. Skin tags are often mistaken for hemorrhoids swollen veins inside the anus or rectum , but they're not the same.
Anal warts also called condylomas are growths that form just outside the anus and in the lower anal canal below the dentate line. Sometimes they can be found just above the dentate line.
They're caused by infection with human papilloma virus HPV. People who have or had anal warts are more likely to get anal cancer. Some changes in the anal mucosa are harmless at first, but might later develop into a cancer. These are called pre-cancerous conditions. A common term for these potentially pre-cancerous conditions is dysplasia. Some warts, for example, contain areas of dysplasia that can develop into cancer.
Dysplasia in cells of the anus is also called anal intraepithelial neoplasia AIN or anal squamous intraepithelial lesions SILs. It needs to be watched closely. Some cases of high-grade AIN need to be treated. Sometimes abnormal cells on the inner surface layer of the anus look like cancer cells but have not grown into any of the deeper layers. Another name for this is Bowen disease. Some doctors see this as the earliest form of anal cancer. Others consider it the most advanced type of AIN, which is a pre-cancer see above , but not a true cancer.
Most anal cancers in the United States are squamous cell carcinomas. Nearly 9 out of 10 cases. These tumors start in the squamous cells that line most of the anal canal and the anal margin. Squamous cell carcinomas in the anal canal have grown beyond the surface and into the deeper layers of the lining as opposed to carcinoma in situ which is only in the surface cells. Cloacogenic carcinomas also called basaloid or transitional cell carcinomas are a type of squamous cell cancer.
They develop in the transitional zone, also called the cloaca. These cancers look slightly different under a microscope, but they behave and are treated like other squamous cell carcinomas of the anal canal. Squamous cell carcinomas of the anal margin perianal skin are treated much like squamous cell carcinomas of the skin elsewhere. A small number of anal cancers are known as adenocarcinomas.
These start in cells that line the upper part of the anus near the rectum. They can also start in the glands under the anal mucosa that release secretions into the anal canal. Most anal adenocarcinomas are treated the same as rectal carcinomas.
For more information, see Colorectal Cancer. Adenocarcinomas can also start in apocrine glands a type of sweat gland of the perianal skin. Basal cell carcinomas are a type of skin cancer that can develop in the perianal skin.
These tumors are much more common in areas of skin exposed to the sun, such as the face and hands, and account for very few anal cancers. They are often treated with surgery to remove the cancer. These cancers start in cells in the skin or anal lining that make the brown pigment called melanin. Only a very small portion of anal cancers are melanomas.
Melanomas are far more common on the skin in other parts of the body. If melanomas are found at an early stage before they have grown deeply into the skin or spread to lymph nodes they can be removed with surgery, and the outlook for long-term survival is very good. But because anal melanomas are hard to see, most are found at a later stage. If possible, the entire tumor is removed with surgery. If all of the tumor can be removed, a cure is possible.
If the melanoma has spread too far to be removed completely, other treatments may be given. For more on this, see Melanoma Skin Cancer. These cancers are much more common in the stomach or small intestine, but rarely they can start in the anal region. When these tumors are found at an early stage, they are removed with surgery. If they have spread beyond the anus, they can be treated with drug therapy.
The anal cancer information on our web site focuses mainly on anal squamous cell carcinoma, which is, by far, the most common type of anal cancer.