Anal cancer signs and symptoms include:

• Bleeding from the anus or rectum
• Pain in the area of the anus
• A mass or growth in the anal canal
• Anal itching

Anal cancer forms when a genetic mutation turns normal, healthy cells into abnormal cells. Healthy cells grow and multiply at a set rate, eventually dying at a set time. Abnormal cells grow and multiply out of control, and they don't die. The accumulating abnormal cells form a mass (tumor). Cancer cells invade nearby tissues and can separate from an initial tumor to spread elsewhere in the body (metastasize).

Anal cancer is closely related to a sexually transmitted infection called human papillomavirus (HPV). Evidence of HPV is detected in the majority of anal cancers. HPV is thought to be the most common cause of anal cancers.

Several factors have been found to increase the risk of anal cancer, including:

• Older age. Most cases of anal cancer occur in people age 50 and older.
• Many sexual partners. People who have many sexual partners over their lifetimes have a greater risk of anal cancer.
• Anal sex. People who engage in anal sex have an increased risk of anal cancer.
• Smoking. Smoking cigarettes may increase your risk of anal cancer.
• History of cancer. Those who have had cervical, vulvar or vaginal cancer have an increased risk of anal cancer.
• Human papillomavirus (HPV). HPV infection increases your risk of several cancers, including anal cancer and cervical cancer. HPV infection is a sexually transmitted infection that can also cause genital warts.
• Drugs or conditions that suppress your immune system. People who take drugs to suppress their immune systems (immunosuppressive drugs), including people who have received organ transplants, may have an increased risk of anal cancer. HIV — the virus that causes AIDS — suppresses the immune system and increases the risk of anal cancer.

Anal cancer rarely spreads (metastasizes) to distant parts of the body. Only a small percentage of tumors are found to have spread, but those that do are especially difficult to treat. Anal cancer that metastasizes most commonly spreads to the liver and the lungs.