Colon Cancer vs. Anal Cancer: How They Differ

Colon cancer and anal cancer are two types of cancer that can occur in the gastrointestinal tract, a 25-foot-long series of organs that start with the mouth and end with the anus. Between food digestion and waste excretion, the colon and anus play an important role in what amounts to your body’s own plumbing system.

But beyond the close proximity, the colon and anus don’t have much in common when it comes to cancer. To coincide with National Colorectal Cancer Awareness Month in March, this blog will help explain what makes colon cancer so much different than anal cancer, ranging from how each originates to treatment modalities. 

Is anal cancer the same as colon cancer?

Cancerous cells can form at any point along the GI tract. Some of the most common types of gastrointestinal cancers include esophageal cancer, stomach cancer, colon cancer, pancreatic cancer and liver cancer.

While it is true that the colon, rectum and anus are all part of the large intestine, there are several major differences between colorectal cancers – an umbrella term to describe cancers of the colon and rectum – and anal cancer.

In general, colon cancer and anal cancer have the following distinctions:

• Location of origin
• Patient demographics
• Cause of cancer
• Prevalence
• Method of treatment

What is colon cancer?

The colon is a 5-foot-long organ that helps remove water and nutrients from the food you eat. The leftover waste solidifies into stool, which is stored in the rectum before it leaves the body through the anus.

In certain people, abnormal cells begin to form in the colon, leading to colon cancer. Most cancers of the colon and rectum are called adenocarcinomas and start in mucosal cells found in glandular tissue. By comparison, anal cancer begins in epithelial, or skin, tissues.

Colon cancer is much more common than anal cancer. In fact, colorectal cancer is the fourth most common type of cancer in terms of estimated new cases (149,500) behind breast cancer (284,200), prostate cancer (248,530) and lung cancer (235,760).

What is anal cancer?

The final stage of the digestion process occurs in the anus when stool moves from the rectum through the upper part of the anus (called the squamocolumnar junction) to the anal canal and finally to the anal verge before leaving the body. The anal canal includes sphincter muslces that tighten to prevent stool from involuntarily exiting the anus.

Anal cancer typically occurs either in the anal canal or the anal verge. The anal canal and anal verge both contain squamous cells. About 90 percent of anal cancer cases are squanmous cell cancer. In more rare cases, you may develop adenocarcinomas, a type of anal cancer that starts in the cells that line the upper part of the anus.

Anal cancer is uncommon with fewer than 10,000 new cases expected in 2022, according to the American Cancer Society. Further, anal cancer accounts for only 1 to 2 percent of all intestine cancers. The average person has about a 1 in 500 chance of developing anal cancer in their lifetime compared to 1 in 23 for colon cancer.

Anal cancer is about twice as common in women than men, while rectal cancer is more common in men. In general, anal cancer is found in people older than 35 years old with the average age in the 60s. 

What causes colon cancer and anal cancer?

While the specific cause of colon cancer and anal cancer is unknown, there is a distinct difference in the risk factors that increase the likelihood of developing these types of cancers.

The presence of the human papilloma virus (HPV) is the most common risk factor in developing anal cancer. There are more than 150 types of HPV, and certain types of HPV, such as HPV-16, make proteins that eliminate tumor-suppressing proteins. In other words, the harmful proteins in HPV deactivate the good proteins in healthy cells that are responsible for regulating cell growth. As a result, cells in and around the anus grow out of control and mutate into cancer.

Conversely, age, family history and a personal history of pre-existing gastrointestinal diseases (Crohn’s disease or ulcerative colitis) heighten your risk of developing colon cancer. In almost all colorectal cancer cases, abnormal growths called polyps are present in the colon or rectum before mutating into cancerous cells. This growth process occurs slowly, making colorectal cancers highly treatable.

Due to the location of the anus and colon (the anus is at the bottom of the large intestine while the colon is at the top of the large intestine), the two types of cancer tend to produce different symptoms. For example, anal cancer can cause pain in the anus while colon cancer can lead to pain in the pelvis, changes in bowel habits, or no symptoms at all (asymptomatic colon cancer can be found on a colonoscopy).

Anal cancer symptoms

• Anal bleeding
• Anal pain
• Growth on or near the anus
• Itching around the anus
• Change in bowel habits
• Discharge from the anus

Colon cancer symptoms

• Pelvic or abdominal pain
• Change in bowel habits
• Blood in stool
• Unexplained weight loss
• Fatigue or feelings of lethargy
• Nausea or vomiting

Treating anal cancer and colon cancer

Surgical intervention is usually what distinguishes anal and colon cancer. In most instances, surgery is a mainstay of treatment for colorectal cancer. By comparison, surgery is rarely used to treat anal cancer. Instead, chemotherapy and radiation are the most effective forms of treatment. 

Only certain cases require surgery, such as small tumors that haven’t spread or more advanced stages that don’t respond to radiation and chemotherapy. For small tumors, a colorectal surgeon can perform a local resection in which the tumor is removed from the anus. 

For more complex cases, an abdominoperineal resection involves an incision in the abdomen to remove the anus, rectum and the sigmoid colon, the bottom part of the colon that connects to the rectum. With this surgery, you can no longer remove stool from your body. Your surgeon will divert the remainder of the colon to your abdomen to create a stoma. This procedure, called a colostomy, allows stool to collect in a bag on the surface of your skin.

People with colorectal cancer typically undergo surgery as the most common form of treatment. Most tumors require a colon resection in which part of the colon is removed and reconnected through a procedure called an anastomosis. If an anastomosis isn’t possible, a colostomy is needed to create a stoma at the abdomen so stool can exit the body.

Surgery for rectal cancer uses similar approaches. For small tumors, a transanal endoscopic microsurgery (TEMS) involves a small scope to remove cancerous growths that couldn’t be taken out during a standard colonoscopy. TEMS has a short recovery time since there is no incision involved.

Low anterior resection involves the removal of parts or the entire rectum affected by cancer cells. Either an anastomosis or a colostomy is performed even though the anus remains. This decision is up to your surgeon.

More advanced rectal cancers may require an abdominoperineal resection in which the anus and parts of the colon are removed. A colostomy is performed in the abdomen so waste can leave the body.

If you are experiencing pain in your anus, rectum or colon or have bleeding in these areas, contact your gastroenterologist. They can then refer you to a colorectal surgeon to examine you further.