Out of all cases of colorectal cancer, some 70% of malignancies occur in the colon (the large intestine) and 30% appear in the rectum. Anal cancer is uncommon, accounting for only 4% of all cancers affecting the digestive tract. It is an often-curable cancer that produces slow-growing tumors and lesions in the anus and nearby anal anatomy.
Anal cancers are skin cancers. The majority of anal cancers 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% 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 carcinoma and malignant melanoma. 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.
The American Cancer Society estimated in 2001 that there would be 3500 new cases of anal cancer among men and women, and 500 deaths resulting from it. Cancer of the perianal skin around the anus is more common in men, while tumors of the anal canal more often affect women.
The exact cause of anal cancer is unknown. Aside from general cancer risk factors such as 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 – both past and current infections. Some strains of HPV that cause larger warts are not associated with cancer.
Other contributory risk factors include:
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.
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:
Anal cancer is diagnosed by means of 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), is inserted. A colposcope is then inserted through the anoscope to visualize the cells in the anus with magnification. The procedure is painless.
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.
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 intraepithelial 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:
Treatment depends on the type of tumor and its stage of development. There are three types of conventional treatment:
A local surgical resection (removal) of the tumor and a small amount of surrounding tissue may suffice in cases of Stage 0 cancer, where the cancer has not metastasized and does not affect the sphincters. In later stages, where cancer has spread, a surgeon may perform an abdominoperineal resection to remove affected tissue, lymph nodes, or sections of or organs in the abdomen. This procedure requires a colostomy, wherein the anus is removed and waste is diverted from the colon through a surgically created opening, through the abdominal wall, and to an external bag for disposal. Abdominoperineal resection is used less frequently because radiation and chemotherapy are effective and do not require a colostomy.
Radiation, the shrinkage of cancerous tumors with energy waves (e.g., X-rays), is performed with external radiation or internal radioactive implants (radioisotopes). Internal radiation using implants (brachytherapy) involves placing plastic-covered radioactive seeds (pellets) inside the anus, near the cancerous tissue, to shrink tissue. The seeds stay in for the appropriate length of time, perhaps permanently, and require fewer trips to the physician's office. Radiation may be combined with chemotherapy. Radiation causes side-effects including loss of control of the sphincter muscle (incontinence), temporary browning of the skin, and fatigue.
Chemotherapy involves drug treatment to kill cancer cells. Drugs used to treat anal cancer include 5-fluorouracil (5-FU), mitomycin, and cisplatin, which are administered orally or intravenously. Chemotherapy is a systemic treatment; the drugs enter and travel throughout the body to kill cancer cells wherever they are. All of these drugs (antineoplastic agents) inhibit the normal production and use of deoxyribonucleic acid (DNA), which is needed for cell growth and division. Arrested cell growth results in tumor shrinkage.
Chemotherapy drugs cause acute side-effects, including nausea, fatigue, vomiting, fever, diarrhea, and sensitivity to sunlight. Certain drugs are associated with specific side effects. 5-fluorouracil may cause low white blood cell count, ulcers, and visual problems. Mitomycin is associated with bone marrow, kidney, mucous membrane, and pulmonary toxicity, as well as kidney failure. Cisplatin may cause hearing problems, serious disorientation, and anaphylactic (allergic shock) reactions, including respiratory distress and swelling.
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.
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