Colorectal cancer is a collective term used to describe cancer that starts in the colon, rectum, or both. These cancers can also be named colon cancer or rectal cancer, depending on where they start. 

Colon cancer begins in the large intestine (colon), which is the final part of the gastrointestinal (GI) tract (digestive system). Rectal cancer begins in the rectum — the last several inches of the large intestine. The rectum starts at the end of the final segment of your colon and ends when it reaches the short, narrow passage leading to the anus. 

The most common type of tumor that affects the colon and rectum is adenocarcinoma. However, there are some other, rarer types of colorectal cancer worth noting. 

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Adenocarcinomas account for 90 to 95 percent of all colorectal cancers. Colorectal adenocarcinomas grow in the epithelial cells, which line the inside surface of both the colon and the rectum. Sometimes, they spread to other areas of the body. 

Adenocarcinomas often start out as small polyps, or growths on the inside of the colon. Most polyps are benign (non-cancerous) but some occasionally become cancerous. They can often be found during a screening colonoscopy and removed.

Adenomatous polyps (adenomas) — one of the various types of colorectal polyps — are of more concern because they can become cancerous. The likelihood that adenomas will progress to cancer is dependent on their size in that the risk increases as polyps grow larger. 

Related reading: What are Colon Polyps and Are They Cancerous?

GI Carcinoid Tumors

Gastrointestinal carcinoid tumors are among a group of cancers called neuroendocrine tumors (NETs). This is because they develop in nerve cells called neuroendocrine cells. Most colorectal neuroendocrine tumors are slow-growing and usually occur in the gastrointestinal system (primarily in the small intestine and rectum). Carcinoid tumors account for 1% of all colorectal cancers. 

It’s important to understand that colon or rectal cancer will only be treated as such if it actually originated in the colon or rectum. If your colorectal cancer is a result of another type of cancer spreading (metastasizing) to the colon or rectum, it is not considered a true colorectal cancer and will be treated as the original type of cancer, not colorectal cancer. 

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More Rare Types of Colorectal Cancer

Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors (GISTs) are a rare type of tumor that forms in the lining of the gastrointestinal (GI) tract. Some gastrointestinal stromal tumors grow slowly over time and may never cause a problem for a patient, while others can grow and spread very quickly. They are most common in the stomach and small intestine but they can also develop in the colon. 

Lymphoma in the Intestines

Lymphoma is a cancer of the immune system that primarily starts in the lymph nodes. Although it’s a blood cancer, lymphoma can develop in other areas of the body. When it occurs in the colon and rectum, it is referred to as gastrointestinal, or colorectal, lymphoma. Colorectal lymphoma is very rare, accounting for a small percentage of all colorectal cancers. 


Leiomyosarcoma is a rare cancer that grows in smooth muscle tissue. Because smooth muscle is present throughout the body, leiomyosarcoma can develop anywhere, including the colon and rectum. This is a very rare type of colorectal cancer that accounts for about 0.1 percent of all colorectal cases.

Squamous Cell Carcinoma

Squamous cell carcinoma (SCC) is more commonly associated with skin cancer, however, it can occur in the colon and rectum. With that said, primary squamous cell carcinomas of the colon and rectum are extremely rare, accounting for less than 1% of all colorectal cancer diagnoses. 


Melanomas typically affect the skin, but in rare cases, they can develop in areas like the colon or rectum. They may also spread to the GI tract. Melanomas account for a very small percentage (about 1-3%) of all digestive system cancers.  

Hereditary Colorectal Cancer

Most colorectal cancer diagnoses are in people who have no family history of colorectal cancer. Still, about 1 in 3 people will develop colon or rectal cancer because of genetic mutations that they inherited, according to the American Cancer Society. This rare type of hereditary colorectal cancer is called familial adenomatous polyposis. 

Learn more about genetics and colorectal cancer. 

The GI cancer experts at RMCC understand that information about the types of colorectal cancer can be a lot to process. Know that we are here to answer any questions you may have about colorectal cancer in general or your individual situation.

Learn more about colorectal cancer treatments by stage.

Find a Colorectal Cancer Specialist