Understanding Colorectal Cancer

What is Colorectal Cancer?

This is the first article in the Understanding Colorectal Cancer Guide. This guide was developed by the HealthTree Education Team and last updated and reviewed on April 3rd, 2026. 

Cancer begins when cells in the body start to grow out of control. Colorectal cancer starts in the colon or rectum, which are the final sections of the large intestine. 

When you have colorectal cancer, the cells lining your colon or rectum do not follow the normal cell cycle. Healthy cells grow, copy themselves, and then die. Cancer cells change, grow faster than normal, and do not die the way they should.

What is the colon and rectum?

The colon and rectum make up the large intestine. This is the last part of your digestive system. After food is digested in the stomach and small intestine, it passes into the colon.

The colon’s job is to absorb water and salts from the remaining material, turning it into stool. The rectum is the final 6 inches of the large intestine. It stores stool until it leaves the body.

Together, the colon and rectum are about 5 feet long. The colon has four sections: 

• Ascending colon (right side)
• Transverse colon (across the abdomen)
• Descending colon (left side)
• Sigmoid colon (lower left, connecting to the rectum)

How colorectal cancer starts

Most colorectal cancers begin as polyps. Polyps are small clumps of cells that grow on the inner lining of the colon or rectum. There are different types of polyps, but the most important type to know about are adenomatous polyps (adenomas). This type of polyp starts out as a harmless (benign) growth. But, over time, some adenomas can develop mutations and become cancerous.

This process usually takes 10 to 15 years, which is why regular screening is so effective at finding and removing polyps before they become cancerous.

How does colorectal cancer affect the body?

As colorectal cancer grows, it can affect many parts of the body:

How colorectal cancer affects the digestive system:

A tumor in the colon or rectum can block or narrow the bowel, causing changes in bowel habits, cramping, or bleeding. This may lead to blood in the stool, constipation, diarrhea, or a feeling that the bowel never fully empties.

How colorectal cancer affects the blood:

Bleeding from the tumor can lead to anemia, a low red blood cell count. Anemia can cause fatigue, weakness, and shortness of breath.

How colorectal cancer affects other organs:

When cancer spreads to other parts of the body, it has metastasized. When this happens, it is called metastatic or advanced colorectal cancer. Colorectal cancer most often travels to the liver, lungs, or nearby lymph nodes. This can affect how these organs work and may cause additional symptoms.

Learn more about the signs and symptoms of colorectal cancer.

Types of colorectal cancer

Most colorectal cancers, about 95%, are adenocarcinomas. Adenocarcinomas start in the gland cells that line the inside of the colon and rectum. There are also less common types:

• Carcinoid tumors, which start in hormone-producing cells of the intestine
• Gastrointestinal stromal tumors (GISTs), which start in connective tissue cells
• Lymphomas, which are cancers of the immune system that can start in the colon
• Sarcomas, which rarely start in the wall of the colon or rectum

Related cancers

Colorectal cancer belongs to a group of gastrointestinal (GI) cancers. Other GI cancers include stomach cancer, small intestine cancer, esophageal cancer, pancreatic cancer, and anal cancer. Some people with colorectal cancer have conditions like Lynch syndrome that can increase the risk of other GI and non-GI cancers.