The incidence of anal cancer in the general U.S. population is estimated to be 0.9 cases per 100,000 persons. In the gay community, this number has exploded in recent years. In men who have sex with men, the incidence is estimated to be 35 cases per 100,000 persons, and as high as 70 per 100,000 persons in HIV- positive men who have sex with men. These numbers are alarming because they are similar to the incidence of cervical cancer in the U.S. prior to the initiation of routine Pap (Papanicolau) smear screening.
In the U.S., 80% of anal cancers are of the squamous cell type, which are primarily caused by the human papilloma virus (HPV). Risk factors for developing anal cancer include anal receptive intercourse, multiple sexual partners (or partners with multiple partners), history of sexually transmitted disease, or anal condyloma (warts). Immunosuppression, such as that experienced by HIV positive individuals, also increases risk. While anal intercourse without using condoms is thought to increase risk, it is important to remember that using condoms does not completely protect against HPV transmission, which is possible with any genital-to-genital contact. Condoms do reduce the genital area exposed, and therefore likely reduce, but not eliminate, the risk of transmission.
There are over 100 strains of HPV, with 12-15 of these considered “high risk” for causing cancer. Several “low risk” strains are the cause of genital or anal warts, but will not cause cancer. Millions are infected with HPV every year, but in most cases, the person’s own immune system is able to fight off the infection. People with suppressed immune systems, such as those with HIV, are at highest risk of not clearing the infection. Of those who become chronically infected with a high risk HPV strain, only a small number will actually develop a cancer, but screening may help to lower this number even more.
Women are recommended to undergo routine screening with Pap smear testing, beginning at age 21 or within 3 years of becoming sexually active, to detect precancerous or cancerous changes to the cervix. Using a similar test, screening can be done for anal cancer or precancerous lesions that can lead to anal cancer. The precancerous lesions are known as anal high-grade squamous intraepithelial lesions (HSIL), anal intraepithelial neoplasia 2 or 3 (AIN 2 or AIN 3), or moderate or severe dysplasia. These descriptions are similar to those used to describe cervical specimens because the two areas are composed of similar types of cells and are susceptible to infection with the same types of HPV.
An anal pap smear is recommended by many experts for men who have sex with men, both HIV positive and negative. The test is typically performed with the man lying on his side, legs bent. Men are asked to not use an enema or to insert anything in the rectum for 24 hours before the exam. Lubricants should not be used before the test because they can interfere with the results. A swab (similar to a Q-Tip) is inserted a few inches into the anus and rubbed against the side of the bowel where the anus and rectum meet in order to gather cells from that area. The swab is either used to make a slide or it is put into a liquid preservative for transport to the laboratory. The pathologist examines the specimen under a microscope, looking for any abnormalities in the cells.
In addition, men may also undergo a digital rectal exam or an anoscopy, where the physician inserts a small tube in order to better visualize the anus with the naked eye. Some clinics also perform high-resolution anoscopy, which uses a microscope to examine the inside of the anus. The test is very similar to and uses the same equipment as the colposcope used in cervical exams. These tests allow the practitioner to examine for any abnormal growths or masses. The required follow-up care depends on the result of these tests.
To read more about understanding your results and follow up care, see the UCSF anal neoplasia research & treatment group website.
Blackwell, CW Anorectal carcinoma screening in gay men: Implications for nurse practitioners. The American Journal for Nurse Practitioners, Vol 12(1), 2008.
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