Anatomy of the anus
The anus is the lower most part of the large bowel that opens outside of the body. Stool and body waste passes out through this opening.
The cells that line the anus are made up of cells that are flattened and appear like skin cells. These are called squamous cells.
The area at the junction of the anus and rectum is called the transitional zone. Here the lining of the anus changes and is made up of both squamous and glandular cells. Glandular cells are those that secrete various fluids including mucus. This helps in smooth passage of the stools.
This line separating the anus and the rectum is called the dentate line. There are two sphincters or gates that regulate the opening of the anus.
Types of anal cancer
One of the commonest types of anal cancer is squamous cell cancer (80% of all anal cancers). There are several different types of squamous cell anal cancer. They are sometimes called epidermoid cancers.
A rarer type is adenocarcinoma that affects the glandular cells in the transitional zone. Other types of anal cancer include basal cell carcinoma and melanoma.
Sometimes cells on the surface layer of the anus look like cancer cells but have not grown into the deeper layers. This is known as carcinoma in situ. It may also be called Bowen disease. This may be a form of pre-cancer that raises the risk of true anal cancer.
Causes and risk factors of anal cancer
Anal cancer is rare. Women are more commonly affected than men. Some of the risk factors include Human papilloma virus (HPV) infections. There are over 100 different types of HPV. Some of these may spread via sexual contact. Nearly 80% of those diagnosed with anal cancer have evidence of HPV infection.
Other risk factors include aging, presence of sexually transmitted disease, smoking, immune-deficiency etc.
Symptoms of anal cancer
The symptoms of anal cancer are similar to other anal disorders like piles (haemorrhoids) or anal fissure. Some individuals (nearly 20% with the cancer) may not manifest with any symptoms.
Some of the symptoms include:-
- Rectal bleeding
- Discharge of mucus from anus
- Pain around the anus
- Sensation of a lump in the anus and feeling of incomplete evacuation after emptying the bowel
- Small lumps around the anus that appear as piles
- Difficulty in controlling bowels or fecal incontinence
- Lump(s) in the groin indicating swollen lymph nodes.
- Anaemia and its symptoms
Screening and diagnosis
Screening refers to testing large susceptible but healthy populations to detect early stages of a disease before there are any symptoms. The test needs to be accurate, reliable and must not give false positive results. At the moment there are no such suitable screening tests for anal cancer.
Epidemiology of anal cancers
Anal cancers are not very common but females are more prone to this cancer than males. Anal cancers are much less common than cancer of the colon or rectum.
According to the American Cancer Society estimates in 2013 there will be around 7,060 new cases (4,430 in women and 2,630 in men) of anal cancers and there will be around 880 deaths (550 in women and 330 in men) due to anal cancers.
According to Cancer Research UK estimates around 1,100 people are diagnosed each year in the UK and the rates are increasing over the past decade.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)