Anal cancer

The anus 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 progress into cancer. Anal cancer is highly treatable if detected early, and the outlook for patients with anal cancer is often better than for other types of bowel cancer. Precancerous and cancerous changes in the anal canal are similar to cervical cancer.

Anal cancer is rare and accounts for less than 0.2%* of all cancers diagnosed in New Zealand. Around 72* new cases of anal cancer are detected each year in New Zealand and age is a significant risk factor in its development. In 2015, no cases were reported in New Zealand for people under 29 years and the highest rates were seen in the 70+ age group. After age 50, anal cancer is slightly more common in women.

*2020 statistics from International Agency for Research on Cancer

Anal cancers are defined by their location, and the type of cell from which they arise.

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

Other types of malignancies can occur within the vicinity of the anus. These often require their own specialised treatment approaches which may differ to those used for anal cancers. These cancers include:

• skin cancers including basal cell carcinomas, mucosal melanomas, and malignant melanomas
• lymphomas
• gastrointestinal stromal tumours
• Karposi’s sarcoma

The exact cause of anal cancer is unknown however, there are several factors that may increase the risk of developing the disease, including:

• 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

The following tests and examinations are commonly used to detect and diagnose anal cancer:

• Medical history and physical examination: your doctor may review your full medical history for cancer risk factors and perform a physical examination to check for signs of cancer. More invasive tests are required to confirm the diagnosis, if anal cancer is suspected.
• A digital rectal examination: your doctor will insert 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: this procedure examines the anus and rectum using a short, lighted, lubricated tube, allowing the doctor to observe the lining of the anus and to take biopsies if needed.

The method of treatment for anal cancer depends on the characteristics of the tumour and the individual. Anal cancer may be treated using one, or a combination, of the following treatment options. Your doctor will discuss treatments with you to determine what is best for your circumstances. Treatment options may include:

• Surgery: 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, it may be necessary to remove the anus, the rectum, and part of the colon and patients may require a colostomy bag post-surgery.
• Radiation therapy: high-energy X-rays or other types of radiation are used to kill cancer cells. Radiation therapy can be performed in two ways depending on the characteristics of the cancer:

The outcome of treatment for anal cancer depends on the characteristics of the tumour and the patient themselves. Anal cancer is a serious disease but treatment can be very effective, especially if the cancer is detected early.

About half of all anal cancers are diagnosed before spreading beyond the anus. The five-year survival rate for these patients is approximately 80%. The five-year rate decreases to about 61% if anal cancer is diagnosed after spreading to nearby lymph nodes and 30% if the cancer has spread to other organs prior to diagnosis.

HIV can increase the risk of developing anal cancer. Treatment for anal cancer can further damage the weakened immune systems of HIV patients. For this reason, HIV patients with anal cancer are usually treated with lower doses of chemotherapy and radiotherapy.