Key Takeaway: Rectal cancer and anal cancer may often be confused due to the close proximity of the rectum and the anus; however, they are two distinct diseases with different risk factors and treatments.
What is the main difference between rectal and anal cancer?
Anal cancer is a squamous cancer, or a cancer of the skin-like cells of the anal canal. Anal cancer can even occur in the rectum.
Rectal cancer is an adenocarcinoma, or a cancer of the glandular cells of the colon. This cancer can extend into the anal canal, but is derived from the glandular tissue rather than the skin.
Almost 90% of anal cancer cases are related to a human papilloma virus (HPV) infection, whereas rectal cancer is not related to HPV. Anal cancer is also significantly less common than rectal cancer.
Rectum v. anus
The anus is the end of the large intestine, below the rectum, through which stool (solid waste) leaves the body. The anus is formed partly from the outer skin layers of the body and partly from the intestine. Two ring-like muscles, called sphincter muscles, open and close the anal opening and let stool pass out of the body.
What are the symptoms of rectal cancer and anal cancer?
Symptoms of rectal cancer include:
- A change in bowel habits
- Blood in the stool
- Diarrhea, constipation, or feeling that the bowel does not empty all the way
- Stools that are unusually narrow
- Rectal pain
- Frequent gas pains, bloating, fullness, or cramps
- Weight loss for no known reason
Symptoms of anal cancer include:
- Bleeding from the anus or rectum
- A lump near the anus
- Pain or pressure in the area around the anus
- Itching or discharge from the anus
- A change in bowel habits
Some of the symptoms are similar, but there are differences. Regardless, if you experience any of these symptoms frequently, consult your physician.
Are the risks the same for anal and rectal cancers?
The biggest risk for anal cancer is an HPV infection. Almost all cases of anal cancer are related to HPV. Another common risk factor is a chronically suppressed immune system or an HIV/AIDS infection. When someone is diagnosed with anal cancer, the physician will often recommend the patient be tested for HIV. Smoking also contributes to anal cancer risk.
In contrast, rectal cancer has no relationship to HPV. Risk factors for rectal cancer include:
- A family history of rectal or colon cancer
- Certain hereditary conditions like Lynch syndrome and the much more rare familiar adenomatous polyposis
- A history of inflammatory bowel disease (IBD), such as ulcerative colitis and Crohn’s disease
- A personal or family history of adenomas (polyps) in the rectum or colon
- Environmental/lifestyle factors like lack of exercise, obesity, smoking, and alcohol consumption
- A diet high in red and processed meat
- Low vitamin D levels
Are the treatments different for anal and rectal cancer?
In the case of anal cancer that has not metastasized (or spread to other organs), the standard for treatment is radiation and chemotherapy. Surgery is only done if the cancer does not completely resolve with radiation and chemotherapy. The vast majority of patients with early stage anal cancer can avoid surgery, and most patients with localized anal cancer are cured with chemotherapy and radiation alone.
Treatment for rectal cancer is different. Very early rectal cancer is treated with surgery alone. If the rectal cancer involves deeper part of the wall of the rectum or any of the lymph nodes, then radiation therapy and often chemotherapy is done prior to surgery. If you do not have chemotherapy before surgery, you will often have it after surgery. Most people with rectal cancer will need a temporary ostomy bag to allow healing from surgery, but not everybody.
About the Medical Reviewer
Dr. Schlechter is a medical oncologist who specializes in gastrointestinal cancers including colorectal cancer, anal cancer, pancreatic cancer, and neuroendocrine cancers, among others. He is a former intern, resident, chief resident and fellow at Beth Israel Deaconess Medical Center as well as a member of the faculty at Harvard Medical School. In the past he was the Director of Inpatient Hematology and Oncology at Beth Israel Deaconess Medical Center as well as the Assistant Program Director of the Internal Medicine Residency Program. His work at Dana-Farber focuses on providing excellent patient care while trying to advance the treatment of gastrointestinal cancer patients.