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 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 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 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:
Resection can cause anal stenosis (narrowing) of the anal canal and anus due to scar tissue. Surgery is sometimes required to widen the anus and restore proper function.
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.
The Canadian Cancer Society recommends a higher fiber and lower fat diet to help lower the risk of cancer, especially colon and rectal cancers.
Study subjects who ate two or more servings of fish weekly had a much lower risk for esophageal, stomach, colon, rectum, and pancreatic cancers than those who avoided fish. In fact, the rates of these types of cancer were 30-50% lower among fish eaters. High fish consumption was also associated with lower risks for cancers of the larynx (30% lower risk), endometrial cancer (20% lower risk), and ovarian cancer (30% lower risk).