Overview of Anal Cancer
Anal cancer is an uncommon, often curable cancer that produces slow-growing tumors and lesions in the anus and nearby anal anatomy. Most anal cancers are associated with human papillomavirus (HPV), which causes warts on the anus and genitals, similar to cervical and other cancers of the reproductive system.
Providers recommend annual anal Pap smears to HIV-positive men who have sex with men (MSM), and bi-annual Pap smears to HIV-negative MSM. Pap smears screen for HPV and abnormal tissue growth (dysplasia). Routine anal Pap smears may reduce the incidence and progression of anal cancer, as they have for cervical and uterine cancer. The prognosis is good if the cancer is discovered early.
Incidence and Prevalence of Anal Cancer
Anal cancer affects men and women, but it is the only cancer with a greater prevalence among men who have sex with men (MSM) than in the general population. About 35 in every 100,000 MSM develop anal cancer, compared to less than one in every 100,000 heterosexual men. The risk for anal cancer in HIV-positive men is twice as high as that for HIV-negative MSM.
The American Cancer Society estimates 2014 incidence of anal cancer in men and women in the United States to be about 7,210 new cases, with about 950 deaths resulting from the disease. Cancer of the perianal skin around the anus is more common in men, while tumors of the anal canal more often affect women.
Anal cancer accounts for only 4 percent of all cancers affecting the digestive tract.
Types of Anal Cancer
Most anal cancers are skin cancers, and the majority are squamous cell carcinomas (in situ or epidermoid), which originate in the first layer of anal tissue and may spread to deeper layers. This type is associated with HPV.
About 15 percent of anal cancers originate in the glands near the anus; this is called adenocarcinoma, or Paget's disease. The remaining anal cancers are basal cell carcinomas and malignant melanomas. Melanoma in the anus is difficult to see and is often discovered at a late stage, after the cancer has spread through layers of tissue.
Anal Cancer Risk Factors
The exact cause of anal cancer is unknown. Aside from general cancer risk factors, like smoking and alcohol consumption, certain risk factors increase a person's risk for developing anal cancer. HPV and anogenital warts are a significant risk factor. This includes past and current infections. Some strains of HPV that cause larger warts are not associated with cancer.
Most men that develop anal cancer are over 50 years old. Co-infection with HIV increases the chance of early development.
Symptoms of Anal Cancer
Many people have no visible symptoms of anal cancer. However, because the cancer is strongly associated with HPV infection, a history of anogenital warts is likely. Although most lesions are benign, any visible anal sore or bump should be examined. They may be a sign that others have developed in the anal canal.
Other signs include the following:
- Abnormal discharge from the anus
- Bleeding from the rectum and anus
- Itching of the anus
- Pain or pressure around the anus
- Sore around the anus that does not heal
All of these signs warrant an examination by a physician.
Anal Cancer Diagnosis
Anal cancer is diagnosed with an anal Pap smear, in which a cotton swab is inserted past the anus and swirled to capture a tissue sample. The tissue cells are examined under a microscope for signs of dysplasia.
An abnormal Pap smear shows signs of excessive cell growth and is followed by a colposcopy, the internal examination of specific lesions or areas of cell growth for biopsy. Acetic acid (vinegar) is introduced into the anal canal to prepare the cells before an anoscope, a plastic tube, in inserted. The provider inserts a colposcope through the anoscope to visualize the cells in the anus with magnification. The procedure is painless.
Lesions and tumors found during a colposcopy are typically biopsied. During a biopsy, a biopsy forceps is inserted in the suspect tissue to obtain a sample for close examination.
Anal cancer may be discovered during a routine digital rectal exam (DRE), in which a medical professional inserts a gloved finger past the anus to feel for abnormalities.
Anal Cancer Staging
Staging is the evaluation of the size and location of a tumor to determine a prognosis and appropriate treatment. Initially, abnormal tissue growth is classified as high- or low-grade squamous interepithelial neoplasia (LSIN or HSIN; high- or low-grade abnormal cell growth), or as cancer. This is determined by the biopsy performed during colposcopy.
If cancer is detected, its stages are described in the following way:
- Stage 0Cancer is found only in top layer of anal tissue (squamous cell carcinomas)
- Stage ICancer has metastasized (spread) beyond first layer of tissue but is smaller than 2 centimeters
- Stage IICancer is larger than 2 centimeters but confined to anal tissue
- Stage III ACancer has metastasized to nearby organs (bladder, reproductive organs) or lymph nodes
- Stage III BCancer has metastasized to organs and lymph nodes in the abdomen, groin, or rectum
- Stage IVCancer has metastasized to distant organs
The TNM system is also used to stage anal cancer. In this system, T corresponds to tumor size, N refers to whether or not the lymph nodes are affected, and M refers to whether or not the cancer has metastasized (spread) to other organs.