Understanding colorectal cancer

What colorectal cancer is, how colon and rectal cancer differ, what the stages mean in plain words, and the tests used to plan your treatment.

6 min read · last reviewed September 30, 2026

A colorectal cancer diagnosis brings a lot of new words at once. Colon, rectum, stage, lymph nodes, biomarkers. It can feel like learning a new language while you are scared. That is completely normal.

This guide explains the basics in plain words, so you can follow what your team is saying and ask the questions that matter to you.

What is colorectal cancer?

Your colon and rectum make up the large bowel. It is the last part of your digestive system. After your small bowel takes in the nutrients from food, the colon absorbs water and salt, and the rectum holds stool until you go to the bathroom.

Colorectal cancer starts when cells in the inner lining of the colon or rectum start to grow out of control. Most colorectal cancers begin as a small growth called a polyp. Over many years, some polyps can turn into cancer. Most colorectal cancers are a type called adenocarcinoma, which means they start in the cells that make mucus.

Colon cancer and rectal cancer: what's the difference?

Colon and rectal cancers are grouped together because they are similar in many ways. But where the cancer is makes a difference to treatment.

• Colon cancer is in the long, looping first part of the large bowel. Surgery is usually the main treatment. Chemotherapy may be added after surgery.
• Rectal cancer is in the last 12 to 15 centimetres or so of the bowel, deep in the pelvis. Because space there is tight and close to other organs, rectal cancer is more often treated with radiation and chemotherapy, often before any surgery.

Your team will tell you which one you have, and exactly where it is.

How colorectal cancer is staged

Staging tells your team how big the cancer is and whether it has spread. It is one of the most important things in planning your care. Stages go from 0 to IV (4). In plain words:

• Stage 0: Abnormal cells are only in the very inner lining. This is sometimes called carcinoma in situ.
• Stage I (1): The cancer has grown into the deeper layers of the bowel wall, but not through it.
• Stage II (2): The cancer has grown through the bowel wall, and maybe into nearby tissue, but not into the lymph nodes.
• Stage III (3): The cancer has spread to nearby lymph nodes. Lymph nodes are small bean-shaped glands that help fight infection.
• Stage IV (4): The cancer has spread to other parts of the body, most often the liver or lungs. This is also called metastatic cancer.

Doctors also use a system called TNM. T is the size and depth of the tumour, N is whether it's in lymph nodes, and M is whether it has spread further. You may see these letters in your reports.

A higher stage does not mean there's nothing to be done. There are treatments for every stage, and many people with stage IV colorectal cancer are living with it and doing things they love.

Sometimes your final stage isn't known until after surgery, when the tumour and lymph nodes are checked under a microscope.

Something you can do: Ask your team, "What stage is my cancer, and what does that mean for my treatment?" Write the answer down.

Key tests you may have

You may not have every test on this list. Each one answers a different question.

Colonoscopy and biopsy. A doctor uses a thin, flexible tube with a camera to look inside your whole colon and rectum. You get medicine to help you relax. Small pieces of tissue (a biopsy) are taken to check for cancer. You will need to clean out your bowel the day before.

CT scan. A CT scan of your chest, belly and pelvis looks for any spread to the liver, lungs or lymph nodes. You may get a dye through a vein.

MRI of the pelvis. If you have rectal cancer, an MRI gives a detailed picture of how deep the tumour goes and whether it's close to nearby structures. This helps decide if you need treatment before surgery.

CEA blood test. CEA is a protein that some colorectal cancers release into the blood. It can't diagnose cancer on its own, but a level taken before treatment can be compared with later results to help track how things are going.

Blood tests. These check your blood counts, liver and kidney function. They help show whether you're ready for treatment.

Tests on the cancer itself

The tissue from your biopsy or surgery can be tested for certain features, sometimes called biomarkers. These can change which treatments will work best.

MMR or MSI testing. This checks whether the cancer cells can fix mistakes in their DNA. A small number of colorectal cancers are "mismatch repair deficient" (dMMR) or "MSI-high." These cancers often respond very well to immunotherapy. This test is now recommended for all colorectal cancers.

Lynch syndrome. A dMMR or MSI-high result can sometimes point to Lynch syndrome. This is an inherited condition that raises the chance of colorectal and some other cancers. If this is possible, your team may suggest genetic counselling. Knowing can help your family members get screened early.

KRAS, NRAS and BRAF testing. If the cancer has spread, it will often be tested for changes in these genes, and sometimes others such as HER2. The results help your team choose targeted therapy drugs that are more likely to work for you.

Something you can do: Ask, "Has my cancer been tested for MMR or MSI? Are there other tests I should have?"

Waiting for results

It can take a few weeks for all your test results to come back. Waiting is hard. Many people find it helps to keep busy, lean on friends, and keep a notebook of questions. It is okay to call your team to ask when results are expected.

Questions to ask your care team

• Is my cancer in the colon or the rectum? Where exactly?
• What stage is it, and is that final or could it change after surgery?
• Which tests do I still need, and what will each one tell us?
• Was my cancer tested for MMR or MSI, and what was the result?
• Should my family think about genetic counselling or earlier screening?
• Who can I call if I have questions while I wait for results?

This article is general information, not medical advice. Talk with your health care team about your own situation. In an emergency, call 9-1-1 or your local emergency number.

In an emergency, call 9-1-1 or your local emergency number.

General information, not medical advice. It does not replace your own care team. If you think you or someone else is having a medical emergency, call 9-1-1 or your local emergency number. If you are in crisis or thinking about suicide, call or text 9-8-8 (Canada and the US).

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