Anal Cancer Screening for People at High Risk

What causes anal cancer?

Each year about 10,500 people in the United States are diagnosed with anal cancer. About 80% of anal cancers in the United States are squamous cell carcinoma. This type of anal cancer is often caused by human papilloma virus (HPV). Risk factors for anal cancer are:

• HPV infection, especially HPV-16. 
• Receiving anal intercourse.
• History of many sexual partners.
• Anal warts. Warts themselves don't turn into cancer, but people with anal warts are more likely to develop anal cancer because they are more likely to have HPV infections.
• History of cervical, vaginal, or vulvar cancer. 
• If you have a weakened immune system (immunosuppression). Especially if you have HIV (the human immunodeficiency virus) or are taking medication to suppress your immune system (such as after an organ transplant).
• Smoking. 

HPV infections and anal cancer

Only a small number of people with HPV will get cancer, but screening may help lower your risk. There are over 100 strains of HPV. HPV-16 is the type most likely to cause anal cancer. HPV-6 and HPV-11 can also cause anal or genital warts. Millions of people get some type of HPV every year. In most cases, the person’s immune system fights off the infection. People with suppressed immune systems, like those with HIV, are at the highest risk of not fighting off the infection. 

Using condoms during anal intercourse does not completely protect against getting HPV. You can get HPV with any genital-to-genital contact. Condoms lower the amount of genital area exposed and can lower the risk of transmission (getting the infection).

Who should have anal cancer screening tests?

Anal cancer screening is not recommended for the general population. However, screening tests may help find anal cancer early in people at higher risk for anal cancer. Those at high risk are:

• Men who have sex with men. 
• Women with a history of cervical or vulvar cancer. 
• Being HIV-positive. 
• Anyone who had an organ transplant. 
• Anyone with a history of anal warts.
• Women who are 45 years and older who are HPV-16 positive.

If you are at higher risk,  you may benefit from screening. There are no official guidelines for anal cancer screening, but experts agree screening people at high risk can help prevent anal cancer or find it early. An anal Pap smear is recommended for men who have sex with men, every 1-2 years for those who are HIV positive, and every 2-3 years for those who are HIV negative. Many experts recommend anal Pap testing for HIV positive women and women with a history of cervical dysplasia as well.

Tests to Screen for Anal Cancer

Digital Anal Rectal Exam

A healthcare provider inserts their gloved finger into the anus. This is done to feel the wall of the anus to feel any lumps, warts, or ulcerations.

Anal Cytology/Anal Pap Smear

This test collects cells from the anus to be looked at in a lab. The test is done by:

• Lying on your side with your legs bent. 
• A swab (similar to a Q-Tip) is put a few inches into the anus. It is rubbed against the side of the bowel where the anus and rectum meet. This gathers cells from that area. 
• The cells that are collected are sent to the lab to be looked at under a microscope. 
• The pathologist looks at the cells under a microscope for any abnormalities in the cells. Do not use an enema or put anything in your rectum for 24 hours before your exam. Lubricants should not be used before the test because they can affect the results.
• The swab must be done before a digital rectal exam.

Anoscopy or High Resolution Anoscopy

A small plastic tube is put into the anus to see the inside of the anus better. In high resolution anoscopy, a special microscope called a colposcope is used to look at the anus through the plastic tube. If an abnormal area is seen, a biopsy (tissue sample) can be taken with these tools.

What can screening tests find?

Anal cytology can find abnormal lesions or pre-cancerous lesions in the anus. Results are often reported as:

• High-grade squamous intraepithelial lesions or HSIL: These are moderate to severe abnormalities (dysplasia). These can develop into cancer over time. 
• Low-grade squamous intraepithelial lesions or LSIL: These are mild abnormalities (dysplasia). These don’t usually turn into cancer. 
• Squamous cell cancer: Both HSIL and LSIL describe abnormal areas on the top of the skin in the anus, with HSIL being more abnormal. Once the abnormality spreads below the top layer, it is considered anal cancer.
• Atypical squamous cells of undetermined significance, or ASCUS: These cells look abnormal, but are not always pre-cancerous. They can be caused by things like infection or inflammation.

What happens after screening?

The next steps after screening depend on the result of the tests and your HIV status. For some abnormalities, follow up will be repeat cytology testing a few months later. Some people with HSIL results will need treatment of the abnormal areas. Treatments can include topical medications (put on the surface of the skin), cryotherapy (freezing the area), laser therapy, and surgery.

While only a small number of people with abnormal cytology will develop anal cancer, providers have no way of knowing who will progress to cancer. Close monitoring helps to detect cancers early when it is most treatable.

Resources for More Information

UCSF Anal Dysplasia Clinic website

Anal Cancer, HIV and Gay/Bisexual Men - National LGBT Cancer Network