ABOUT BOWEL CANCER

Anal cancer

Anal cancer affects the anus, which is the final 4 cm of the large bowel that opens to facilitate defaecation. While abnormal cells in the anus can sometimes be harmless, they may eventually progress into cancer. However, the outlook for patients with anal cancer is often better than for other types of bowel cancer, especially if it is identified at an early stage.

How common is anal cancer?

Anal cancer is a rare disease, accounting for less than 2.5% of all cancers diagnosed in the gastrointestinal tract.

In New Zealand, around 250 new cases of anal cancer are identified each year, more often in women than in men.

Age is a significant risk factor for anal cancer. In 2015, there were no cases reported in New Zealand for people under the age of 29 years old. However, rates increase notably with age, with the highest figures observed in those aged 70+ years.

How common is anal cancer?

Annual incidence rates of anal cancer in New Zealand per 100,000 population by age group†

* No cases were registered for this age group

† Estimates of incidence derived from the national cancer registry of New Zealand 2015

Types of anal cancer

Anal cancers are defined their location and type of cell they arise from:

  • Squamous cell carcinomas are the most common type of anal cancer (about 75% of cases) and are derived from the cells lining either the anal canal or margin (the edge of the anus partially visible outside the body).
  • Cloacogenic (basaloid) carcinomas are a subtype of squamous cell carcinoma that develop in the transitional zone (cloaca) and involve a distinct type of cell that differs from squamous cell carcinomas in the anal canal or margin.
  • Adenocarcinomas that are derived from cells in the glands surrounding the anal area are less common (15% of anal cancers). This type of cancer, also known as Paget’s disease, can also affect the vulva, breasts, and other areas of the body.

An important distinction to make is that in addition to ‘anal cancers’, there are other types of malignancies that occur in the vicinity of the anus, which often require their own specialised treatment approaches compared with those used for anal cancers. These include:

  • Skin cancers, including basal cell carcinomas, mucosal melanomas, and malignant melanomas
  • Lymphomas
  • Gastrointestinal stromal tumours
  • Karposi’s sarcoma

Risk factors for anal cancer

The exact cause of anal cancer is unknown, but a range of factors are thought to contribute to the risk of developing the disease:

  • Persistent human papilloma virus (HPV) infection, the virus responsible for genital and anal warts, is thought to have a role in almost 90% of anal cancers
  • Smoking tobacco
  • A history of cervical or vaginal cancer, or abnormal cells of the cervix, which may also be linked with HPV or smoking
  • Having another condition, or receiving treatment, that suppresses your immune system (e.g. human immunodeficiency virus [HIV] infection, organ transplantation)
  • Receptive anal intercourse (anal sex)
  • Anal fistulas (abnormal openings between the end of the bowel and the skin around the anus)
  • Frequent anal redness, swelling, and soreness
  • Old age

Tests used to detect and diagnose anal cancer

  • Medical history and physical examination – Your doctor may review your full medical history for risk factors for cancer and perform a physical examination checking for signs of cancer. If anal cancer is suspected, more invasive tests are required to confirm the diagnosis.
  • A digital rectal examination involves a doctor inserting a lubricated, gloved finger into the lower part of the rectum to feel for lumps or anything else that seems unusual.
  • An anoscopy or proctoscopy involves examining the anus and rectum using a short, lighted, lubricated tube that allows the doctor to observe the lining of the anus and permit biopsies to be taken, if required.
  • A biopsy is when a tissue sample is taken from a suspect area of tissue and the cells in the sample reviewed for signs of cancer by a pathologist using a microscope.
  • An endo-anal or endorectal ultrasound involves an ultrasound transducer (probe) being inserted into the anus or rectum to visualise the internal structure of the anus and identify any anomalies.
  • Blood tests may be used to detect reduced red blood cell counts and analyse kidney/liver function, which can be a sign of cancer.

Treating anal cancer

The preferred method of treatment for anal cancer depends on the characteristics of the tumour and the individual. Your doctor will discuss treatment options with you to help decide the best option, which could include:

  • Surgery, where the tumour is removed from the anus along with some of the surrounding healthy tissue in an attempt to ensure that no cancerous cells are left behind (local resection). In more severe cases where it is necessary to remove the anus, the rectum, and part of the colon, patients may require a colostomy bag following surgery.
  • Radiation therapy uses high-energy X-rays or other types of radiation to kill cancer cells. This can be performed externally using a machine outside the body that sends a targeted beam of radiation toward the tumour or internally by injecting a radioactive substance directly into or near the cancer depending on the characteristics of the cancer being treated.
  • Chemotherapy drugs either kills cancer cells or stops them from dividing. The optimal choice of chemotherapy depends on the characteristics of the cancer.

Depending on the circumstances, anal cancer may be treated using one or a combination of any of these treatment options. For more details on how bowel cancers that have spread to other organs (metastasised), see the metastatic bowel cancer page.

Prognosis following a diagnosis of anal cancer

For patients with anal cancer, the outcome of treatment depends on the characteristics of the tumour and the patient themselves. However, while anal cancer is a serious disease, treatment can be effective, especially if anal cancer is diagnosed early.

Roughly half of all anal cancers are diagnosed before spreading beyond the anus. These patients have an approximately 80% chance of survival for at least 5 years after being diagnosed. Five-year survival decreases to 61% if anal cancer is diagnosed after spreading to nearby lymph nodes and 30% if the cancer has spread to other organs by the time of diagnosis.

Remember, survival statistics represent the average of all patients with a condition, and do not reflect what will happen in individual cases. Multiple factors determine patient outcomes, so a detailed discussion with your doctor is important for setting expectations regarding your treatment.

HIV and anal cancer

HIV can increase the risk of anal cancer. Treatment for anal cancer can also further damage the already weakened immune systems of patients with HIV. 

For this reason, patients who have anal cancer and HIV are usually treated with lower doses of chemotherapy and radiation than patients who do not have HIV.