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.
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.
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.
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:-
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.
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.
Risk factors are those factors that when present raise the chance of a person getting that disease. However presence of a risk factor does not always mean a person will surely get the disease.
Different cancers have different risk factors. While some, like smoking and its risk of causing lung cancers, are avoidable lifestyle risk factors, some are unavoidable such as genetic predisposition to getting a cancer.
Squamous cell cancers are the commonest type of anal cancers. Most squamous cell anal cancers again have been found to be associated with HPV infection in the past.
HPV infections are also linked to cervical cancer. It is seen that women with a history of cervical cancer have an increased risk of anal cancer.
There are over 100 types of HPV. These are called papilloma viruses because some of them cause papillomas or warts. These viruses may cause warts in the genital and anal areas called condyloma acuminatum.
The 2 types of HPV that cause most cases of anal and genital warts are HPV 6 and HPV 11. They are called low-risk types of HPV because they tend to cause warts but not cancer.
The subtypes most likely to cause anal cancers include HPV-16, HPV 18, HPV 31, HPV 33, and HPV 45. These subtypes also cause cancers of the cervix, vagina, and vulva in women and cancer of the penis in men, and throat cancer in both women and men.
HPV is transmitted from one person to another during skin-to-skin contact especially during sexual intercourse that includes vaginal, anal intercourse and oral sex.
The risk rises with multiple sexual partners, ever having anal sex and no use of condoms. HPV can also spread via hand to genitals contact. Condoms can provide some protection against HPV, but they do not completely prevent infection.
Those who have had other cancers such as those of cervix, vagina, or vulva are at an increased risk of anal cancers. This is likely because these cancers are also caused by infection with HPV.
Those who are infected with human immunodeficiency virus (HIV) are much more likely to get anal cancer than those not infected with this virus.
Having multiple sex partners increases the risk of infection with HIV and HPV and in turn raises the risk of anal cancer. Anal intercourse especially the receiver also raises the risk of anal cancers in both men and women especially in those below 30 years of age.
Smoking also increases the risk of anal cancer. Quitting smoking reduces the risk.
Those with a reduced immunity such as those who have HIV infection, are on long term corticosteroids, after an organ transplant etc. are at a greater risk of anal cancers.
Anal cancer is more common in African-Americans than in whites. Among African Americans it is more common in men than in women. Overall the risk in women is slightly greater than in men.
When anal cancer is suspected (due to symptoms of the condition) diagnosis is made with the help of physical, pathological and laboratory tests along with imaging studies. Diagnosis of anal cancer includes:-
A low grade cancer means that the cancer cells look very much like normal cells and are thus slow growing and less aggressive. High grade cancer cells are very different from normal cells and are thus aggressive and fast growing. These are more likely to spread.
The stage of a cancer describes its size and whether it has spread beyond its original site. This helps in deciding on the most appropriate treatment and predicts the outcome or prognosis.
Stage 1 cancer only affects the anus and is smaller than 2cm (¾in) in size. It has not begun to spread into the sphincter muscle.
Stage 2 cancer is bigger than 2cm (¾in) in size, but has not yet spread into nearby lymph nodes or to other parts of the body.
In Stage 3 cancer there is spread to the lymph nodes near the rectum, or to nearby organs such as the bladder or vagina or to lymph nodes in the groin and pelvis.
In Stage 4 anal cancer, the cancer has spread to lymph nodes in the abdomen or to other parts of the body, such as the liver.
Anal cancer is usually treated by a multidisciplinary team. This includes:
The main type of treatment for anal cancer is a combination of both radiation therapy and chemotherapy. The two treatments are normally given at the same time. This is called chemoradiation. Combined treatment is usually very successful. For small tumors surgery may be useful.
For radiation therapy high-energy x-rays are used to destroy the cancer cells. These beams are directed at the cancer from outside the body using a large X ray like machine.
For this the patient needs to visit the facility frequently. The treatment is often given for a few minutes each weekday for 4-6 weeks. This is called external radiotherapy.
Common side effects of radiation therapy include:
For chemotherapy, anti cancer drugs are used to kill the cancer cells. Drugs commonly used to treat anal cancer include mitomycin, capecitabine and fluorouracil (5FU).
Chemotherapy can cause several side effects including:-
Surgery is useful for small anal cancers. Types of surgery include local resection of the tumor. The operation only removes the area of the anus containing the cancer cells. The anal sphincter is spared and the bowel function remains normal.
A more extensive surgery is the abdominoperineal resection where both the anus and rectum are removed. The operation requires a permanent colostomy that helps drain the feces from an artificial opening.