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Colorectal cancer is a cancer that starts in the cells that line either the colon or the rectum. These cancers can also be named colon cancer or rectal cancer respectively, depending on where they start. Colon cancer and rectal cancer are often grouped together because they have many features in common.
Colorectal cancer usually starts from a polyp, a growth that originates from the lining of the colon or rectum. Polyps can be classified into benign (non-cancerous) or pre-cancerous. Benign (non-neoplastic) polyps are generally harmless and do not turn into cancer. Pre-cancerous (neoplastic) polyps, such as adenomatous polyps (adenomas), have the potential to become cancerous over time. This process can take about 10 years. Finding and removing polyps can prevent colorectal cancer. Regular colorectal cancer screening can help detect polyps before they become cancerous.
More than 90% of colorectal cancers are adenocarcinomas, which are cancers that begin in the glandular cells found in the inner lining of the colon and rectum. Cancer develops when changes occur in the DNA of cells, affecting the normal signals that control cell growth and division. These changes can be caused by environmental factors such as exposure to harmful chemicals, can happen because of errors that occur as our cells divide, or can be inherited from our parents. When your body is unable to eliminate these abnormal cells or correct DNA errors, the cells can multiply to form a tumour. If colorectal cancer cells grow through the wall of the colon or rectum, they may spread to nearby lymph nodes or other areas of the body. When colorectal cancer spreads to distant organs, it is called metastatic colorectal cancer (mCRC).
References:
Canadian Cancer Society. What is Colorectal Cancer. https://cancer.ca/en/cancer-information/cancer-types/colorectal/what-is-colorectal-cancer
Cleveland Clinic. (2024). How Long Does it Take Colon Cancer to Grow? https://health.clevelandclinic.org/how-quickly-do-colon-polyps-turn-cancerous
Fleming, M., Ravula, S., Tatishchev, S. F., & Wang, H. L. (2012). Colorectal carcinoma: Pathologic aspects. Journal of gastrointestinal oncology, 3(3), 153–173. https://doi.org/10.3978/j.issn.2078-6891.2012.030
Mayo Clinic. (2025). Colon Polyps. https://www.mayoclinic.org/diseases-conditions/colon-polyps/symptoms-causes/syc-20352875
National Cancer Institute. (2021). What is Cancer? https://www.cancer.gov/about-cancer/understanding/what-is-cancer#how-does-cancer-develop
The colon is a 1.5-metre (5 feet) long muscular tube that connects the small intestine to the rectum. The colon — which, along with the rectum, is also known as the large intestine — is a highly specialized organ that is responsible for processing digestive waste, so emptying the bowels is easy and convenient. The colon removes water from the stool (digestive waste matter) and stores the solid stool. Once or twice a day, it empties its contents into the rectum to begin the process of elimination.
The rectum is a 6-8-inch storage chamber that connects the colon to the anus. It is the rectum’s job to receive stool from the colon, to signal the brain that there is stool to be evacuated, and to hold the contents until you are ready to evacuate that stool.
During digestion, food moves through the stomach and small intestine into the colon. Digestion involves several organs working together to break down food, absorb nutrients, and remove waste. The colon absorbs water and nutrients from the food. Stool moves from the colon to the rectum before leaving the body.
Cancer begins when normal cells begin to change and grow uncontrollably, forming a mass called a tumour. A tumour can be benign (non-cancerous) or malignant (cancerous, meaning it can spread to other parts of the body). In colorectal cancer, abnormal cells typically begin in the innermost lining of the colon or rectum, known as the mucosa. Over time, these cells may grow deeper into the layers of the colorectal wall.
The colon and rectum are made up of different layers of tissues. A polyp will start in the innermost layer (mucosa) of the colorectum and then grow toward the outer layers (one layer at a time).
The depth to which a tumour grows through these tissue layers is an important factor used to determine the stage of colorectal cancer and guide treatment decisions.
If a tumour has grown through all layers of the colon or rectum, it may then spread to the nearby lymph nodes or tissue. Cancer cells may also enter blood vessels or lymphatic vessels, allowing them to travel to other areas of the body. The liver and lungs are the most common organs where colorectal cancer can spread. This is referred to as metastases.
References:
American Cancer Society. (2024). What is Colorectal Cancer? https://www.cancer.org/cancer/types/colon-rectal-cancer/about/what-is-colorectal-cancer.html
The colon is approximately 1.5 metres (5 feet) long and is made up of several sections, each with specific roles in moving and processing waste through the body. It is also known as the large intestine or large bowel. The colon’s main function is to absorb large quantities of water and nutrients from undigested food products. If too much water is reabsorbed, constipation may result. Not absorbing enough water may result in diarrhea.
The colon itself is divided into different parts. It includes the following five component parts:
• Cecum
• Ascending colon
• Transverse colon
• Descending colon
• Sigmoid colon
The transverse colon is the longest segment of your colon, accounting for almost one third of the total length. The sigmoid colon is the second longest, and due to its curved shape, this is a common site for polyps found during colonoscopies.
The rectum’s main function is to store feces, or waste material, before being expelled from the body.
Collectively, the colon and the rectum are known as the colorectum. Each part of the colorectum serves a specific function in the elimination of waste. In the digestive process, the colorectum’s main functions are to:
• Compact the chyme (liquefied food)
• Absorb excess water from the chyme
• Receive B vitamins that are produced by healthy intestinal bacteria
• Move compacted chyme and dead bacteria into the rectum and out of the body
Gut microbiome:
The gut microbiome refers to the microorganisms that naturally live in your digestive system. Most of these are beneficial and help digest food, support the immune system, and maintain the health of the lining of the colorectum.
Researchers are continuing to study how changes in the gut microbiome may influence the development of colorectal cancer. An imbalance in the gut microbiome, known as dysbiosis, may contribute to chronic inflammation, changes in immune function, and the production of substances that can damage cells in the colorectum. Over time, these changes may increase the risk of colorectal cancer in some individuals.
Studies have shown that the gut microbiome in individuals with colorectal cancer often differs from that of healthy individuals. Diet, obesity, antibiotic use, and other lifestyle factors may influence the gut microbiome and may continue to contribute to the risk of colorectal cancer.
Maintaining a healthy lifestyle, including eating a diet rich in fibre and whole grains, staying physically active, limiting processed and red meat, maintaining a healthy weight, and avoiding smoking and excessive alcohol consumption, may help support a healthy gut microbiome and reduce the risk of colorectal cancer.
References:
Petras, R. E., & Frankel, W. L. (2009). Large Intestine (Colon). Modern Surgical Pathology, 755–836. https://doi.org/10.1016/B978-1-4160-3966-2.00023-0
Shen, Y., Fan, N., Ma, S. X., Cheng, X., Yang, X., & Wang, G. (2025). Gut Microbiota Dysbiosis: Pathogenesis, Diseases, Prevention, and Therapy. MedComm, 6(5), e70168. https://doi.org/10.1002/mco2.70168
Not all colorectal cancers are hereditary, but all colorectal cancers are caused by genetic mutations.
Approximately 70-75% of colorectal cancers are sporadic: these are cancers that occur in people who do not have a family history of that cancer or an inherited change in their genetic material (DNA) that would increase their risk for that cancer.
All cancers are the result of gene mutations. The vast majority are sporadic occurrences with only some caused by hereditary genetic syndromes. Genetic conditions involve a largely unpredictable interplay of many factors and processes. Just because you hold a genetic mutation for something does not necessarily mean it will be expressed in your lifetime, but knowing your risk can save your life. Please read on to learn more about the genetics behind colorectal cancer.
To learn more about the role of genetics in colorectal cancer, check out our information booklet.
If a person has been diagnosed with colorectal cancer, one of the first things they will want to know is the stage of their cancer. The stage refers to the extent of the cancer or how far it has spread. The stage of colon or rectal cancer is important because it will determine the best therapeutic approach in the management of the disease.
The tool used to describe the stage of a patient’s disease is the TNM Staging System. Doctors use the results from diagnostic tests and scans to answer these questions:
Tumor (T): Has the tumor grown into the wall of the colon or rectum? If so, how many layers of the colorectal wall are affected?
Node (N): Has the tumour spread to the lymph nodes? If so, where and how many lymph nodes are involved?
Metastasis (M): Has the cancer spread to distant organs of the body? If so, where and how many organs are involved?
The results are combined to determine the stage of cancer for each person.
For colorectal cancer, staging often can’t be completed until after surgical resection of the primary tumour has taken place to remove the primary tumour along with surrounding tissue containing lymph nodes, and possibly lesions found on other organs.
There are five stages (Stage 0 through IV) to colorectal cancer. After each element has been determined, they are combined to form an overall stage of the cancer in roman numerals. The higher the roman numeral, the more advanced the cancer. This is generally how the cancer is referred to between doctor and patient:
As colorectal cancer progresses from Stage 0 to Stage IV, the cancer cells grow through the layers of the rectum wall and spread to lymph nodes and then to distant organs. The liver and lungs are the most common sites of spread in patients with colorectal cancer.
The size of the colorectal tumor does not appear to be important when it comes to outcomes. The aggressiveness of colorectal cancer is based entirely upon its ability to grow and invade the colorectal wall, lymphatic system, and blood vessels.
While some people start the journey due to routine recommended checkups (screening), others do so because of troubling symptoms, or an abnormal physical exam, or a new finding on a lab test such as iron deficiency anemia. Screening, such as FOBT and FIT, is intended for the “average risk Canadian” – a person who does not have symptoms from colorectal cancer and does not have a first degree relative who was diagnosed with colorectal cancer. It typically is recommended for people aged 50 and older, but the test cannot officially diagnose colorectal cancer. Instead, a diagnostic (rather than a screening) test is required such as a colonoscopy, biopsy, or imaging tests to confirm the diagnosis of colorectal cancer, as well as to define the extent of the disease.If a screening test such as FOBT or FIT has come back positive, your doctor will recommend that you undergo a diagnostic colonoscopy. Your doctor may also recommend such a test if symptoms or the results of a physical exam or blood tests suggest that colorectal cancer might be present.
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