Anal cancer is a rare tumor and constitutes 2 to 4% of all large intestine cancers. It can be either anal squamous cell cancer, Cloacogenic cancer, ano-rectal or anal adeno-carcinoma, or anal Melanoma.
The number of cases has increased in the past few years. This directly correlates to an increase in the number of HIV positive patients in whom this cancer is commonly found. For example - San Francisco in Unites States has a higher incidence of anal cancers due to a larger number of homo-sexual population.
Anal canal Anatomy:
The anus or anal canal is the lowermost part of the digestive tract, after the rectum, through which feces exits the body. It is an inch and a half long. The muscles surrounding the anus form ring like structures called the internal and external sphincters. These sphincters control the passage of stool. The inner lining of the canal is divided into two parts by a line called the dentate line. Different types of cells line the two parts. Anal cancers can be divided into four types depending on the type of cells they arise from:
- Squamous cell cancer (the most common anal cancer)
- Cloacogenic cancer (arises from above the dentate line)
- Rare tumors like adenocarcinoma (arises from local glands)
Like other cancers, anal cancer spreads locally along the anus and adjacent organs like the bladder and vagina. It also spreads to small bean shaped masses called lymph nodes and to distant organs like the liver.
Causes and Risk Factors for Anal
A virus called human papilloma virus
(HPV) causes anal cancer. It spreads through sexual contact. People at
increased risk of developing anal cancer
- HIV patients – HPV infection is more common in HIV patients, thus predisposing them to anal cancer. The reduced immunity in HIV patients also favors the virus growth
- People with multiple sexual partners
- People older than 50 years of age
- People who practice anal intercourse including homosexual males
- Cigarette smokers (2 to 5 fold increase in risk)
- People with genital or anal warts and women with cervical cancer – HPV causes these conditions, hence these patients run at a risk for anal cancer
- People with decreased immunity such those taking medicines after organ transplants to prevent rejection (risk can increase by 200 fold in transplant patients)
- Patient’s with Crohn’s disease also have been reported to have a higher risk for anal cancer