What is colorectal cancer?
In colorectal cancer, a malignant (cancerous) tumour begins to grow in the cells of the colon or rectum. A cancerous tumour is a mass of cells that is growing abnormally and out of control, and is able to invade other tissues. Colon cancer and rectal cancer are similar and are outlined together below.
The normal digestive system
Food that has been chewed and swallowed first goes to the stomach, where it is further digested. It then travels to the small intestine, where it is broken down more and nutrients are absorbed. The small intestine is around 6 metres long. After the small intestine, what is left of the food travels to the colon, or large intestine. The colon, around 1.5 metres long, absorbs more nutrients, and also absorbs water. It also holds the remaining waste (stool). The stool travels to the rectum, which is around 15 centimetres long, to be passed out of the body through the anus.
Colorectal cancer usually starts as a polyp, which is a growth that begins from the cells in the colon or rectum’s inner lining and starts to grow towards the centre. These polyps are mostly non-cancerous, except for a type called adenomas that can become cancerous. To prevent polyps from turning into cancer, they should be removed early (as soon as possible after they are detected).
Most (> 95%) of colorectal cancers are a type called adenocarcinomas. This type of cancer starts in gland cells, which produce and secrete specific fluids, and are found in the inner lining of the colon and rectum. Other types of colorectal cancers are rare and will not be discussed in this overview.
There are several factors that are connected with your chance of getting colorectal cancer. It is important to note that they do not determine for certain whether or not someone will get colorectal cancer.
- Risk of colorectal cancer increases with age.
- Family history. If relatives have had colorectal cancer, including familial adenomatous polyposis (FAP) or hereditary non-polyposis colon cancer (HNPCC or Lynch syndrome), your risk increases.
- Your medical history. If you have had colorectal cancer or some kinds of polyps in the past, your risk for colorectal cancer increases. Risk also increases if you have a history of ulcerative colitis and Crohn’s disease. Having Type 2 diabetes is also a risk factor.
- Being African-American or Ashkenazi-Jewish increases the risk of colorectal cancer.
- Eating a lot of red meats (e.g., lamb, liver, beef) and/or processed meats (e.g., lunch meat, deli meats, hot dogs) and/or meats cooked at very high temperatures (e.g., grilled, fried, broiled) increases the risk of colorectal cancer.
- Too little exercise increases the risk of colorectal cancer.
- Being overweight or obese increases the risk of colorectal cancer.
- Substance use. Smoking and alcohol consumption are risk factors for colorectal cancer.
Screening for colorectal cancer
Colorectal cancer can be prevented by looking for polyps and removing them before they turn cancerous. This process is called screening. Screening also allows for the early detection of cancer, which improves the effectiveness and predicted outcome of treatments. Screening tests include:
- Fecal occult blood test (FOBT) or fecal immunochemical test (FIT). In these tests, stool samples are collected and checked for the presence of blood which may indicate cancer.
- A long, thin and flexible tube with a light is inserted into the rectum and lower colon to look for polyps and tumours.
- A longer tube than is used for sigmoidoscopies is used to scan the rectum and all of the colon for polyps and tumours.
- Double contrast barium enema. X-rays are used to look for polyps and tumours in the rectum and colon.
- Computed tomography (CT) colonography (also known as a virtual colonoscopy). A special type of X-ray scan is used to look for polyps and tumours in the rectum and colon.
Symptoms of colorectal cancer
Early colon cancer doesn’t normally cause any symptoms. With cancer progression, some symptoms may include:
- Abnormal bowel movements (e.g., constipation, diarrhea, narrow stool) that occurs over longer than a few days.
- Constant feeling of needing to have a bowel movement, even immediately after a bowel movement.
- Stomach pain
- Stomach cramping
- Unexplained weight loss
- Low red blood cell count (anemia)
- Bleeding from the rectum, stools that appear dark, stools that contain blood.
To determine if cancer is present after a screening test shows something of concern, or you have symptoms of colorectal cancer, diagnostic tests (in addition to a review of your medical history and your family’s medical history, and a physical exam by your doctor) include:
- Blood tests. Blood tests can provide signs of colorectal cancer. A low red blood cell count may be caused by bleeding from a tumour in the colon or rectum. Blood tests can also be used to check the function of your liver, which can be affected if cancer has spread to it.
- A long, thin and flexible tube is used to scan the rectum and all of the colon for polyps and tumours. Abnormal growths can be removed during a colonoscopy to be tested for cancer cells (called a biopsy).
Laboratory examination of biopsy tissue
Examining the tissue from a biopsy under a microscope to find cancer cells is the only way to obtain a diagnosis of colorectal cancer. Laboratory tests on the biopsy tissue can also give other information about the cancer, such as how it can be best treated based on genetic changes in the cancer cells.
Imaging the inside of the body with the tests described below can be used to identify potential areas with cancer, see whether and where the cancer has spread, and to check if treatment is killing the cancer.
- CT scan. Many X-rays are taken around the body that are computationally combined to create a detailed image of the inside of the body. A CT scan can show the spread of the cancer. Special dyes are sometimes used to improve the image. CT scans are also used to guide biopsy needles.
- Sound waves are used to image the inside of the body. A special wand-like device is placed on the skin of the abdomen after a special gel has been spread on the skin of the abdomen. The wand can also be placed in the rectum to check for tumours both in the rectum and in nearby tissues, or it can be placed specifically over the liver to check if tumours are present there.
- Magnetic resonance imaging (MRI) scans. Cross-sectional images of the body are created using radio waves and strong magnets. The patient must lie in a narrow tube for this imaging test. MRIs are used to check for spread of the cancer and to locate cancer in the liver.
- Chest X-ray. This is used to check if there is cancer in the lungs.
- Positron emission tomography (PET) scan. A radioactive sugar is given intravenously and spreads throughout the body. Since cancer cells use a lot of sugar, the radioactivity will be strongest near tumours. A special imaging device is used to determine these “hot spots.” PET is useful for determining where exactly a cancer has spread. It can be combined with CT.
- X-ray is used to inspect blood vessels. This test involves inserting a thin tube into a blood vessel and moving it to the area of interest. Dye is pushed through the tube so that X-ray images can be created. Angiography is sometimes used to find blood vessels near tumours in the liver, which can help to guide surgeons removing the tumour.
Staging indicates the spread of the cancer, which is important for how it will be treated and the treatment outcomes. Although there are different systems for staging cancer, all of them are based on the growth of the cancer through the colon or rectum layers from where it starts in the inner layer, as well as on its spread to other organs in the body. Roman numerals I to IV are used to label stages, with lower numbers indicating less spread and higher numbers indicating cancers that are more advanced.
Grade indicates how similar a cancer is to normal tissue in the colon or rectum when inspected using a microscope. A lower grade means the cancer is more like normal tissue, while a higher grade means the cancer looks less normal. Grade is also important for treatment decisions and outcomes.
This is the main type of treatment used for colorectal cancer in early stages. A section of the colon or rectum that contains the tumour is removed and the remaining ends are re-attached. If the remaining ends cannot be re-attached because there is too little tissue on one side, the other side is then attached to the abdomen to allow stool to empty into a bag that is outside of the body.
If the cancer has spread from the colon or rectum to the bladder, uterus, or prostate, then these organs may need to be removed as well.
If the cancer is only located on the end of a polyp, then only the polyp is removed during colonoscopy. If the cancer is also in the stalk of the polyp, surgery may be needed to remove a section of the colon or rectum.
In radiation therapy, X-rays or high-energy particles are used to kill cancer cells, either after or before a surgery. Before surgery, it can be used to shrink a tumour. After surgery, it can be used to kill any remaining cancer cells. Chemotherapy is often combined with radiation before surgery for rectal cancer to make the radiation therapy more effective. Radiation is used to help people who are not healthy enough for surgery, or to relieve symptoms for those with advanced colorectal cancer.
There are several types of radiation therapy:
- External beam radiation. Radiation is targeted to the area with cancer from outside the body using a machine. This type of radiation is the most commonly used for colorectal cancer.
- Radioactive pellets in a small container can be placed beside or in a tumour.
- Endocavitary radiation therapy. A small device is placed into the rectum to administer radiation.
Cancer-killing drugs are injected, or administered orally in pill or liquid form. The drugs are able to reach throughout the body by traveling in the bloodstream, and can act on cancer that has spread to multiple organs. Because the drugs also kill normal cells, side effects include hair loss, fatigue and nausea.
Chemotherapy can also be given before or after surgery, and/or in combination with radiation therapy. To specifically target the part of the body where there is a tumour, chemotherapy drugs can also be injected into an artery that directly leads to the targeted region (called regional chemotherapy).
By elucidating the gene changes that are involved in colorectal cancer, drugs have been developed that can directly target these changes. Because of their specific action, side effects are usually less severe. These drugs include Avastin®, Cyramza® and Zaltrap®, which target the vascular endothelial growth factor (VEGF), a protein that helps blood vessels to form in tumours. Other drugs (e.g., Erbitux® and Vectibix®) target the epidermal growth factor receptor (EGFR) that helps cancer cells grow.
Close follow-up is required after treatment to ensure continuing health. This may involve physical exams, blood test, or imaging tests. Side effects from treatment, and whether cancer has returned or spread is monitored in follow-up appointments.
Some cancer is not cured by many different treatments and becomes resistant to treatment. At this point, treatment is given to maintain comfort and relieve symptoms (e.g., pain and nausea), and is called palliative treatment.
Marc Bardou, Alan N. Barkun, Myriam Martel. “Obsesity and colorectal cancer” Gut. 2013;62(6):933-947. doi:10.1136/gutjnl-2013-304701.
Doris S. M. Chan, Rosa Lau, Dagfinn Aune et al. “Red and Processed Meat and Colorectal Cancer Incidence: Meta-Analysis of Prospective Studies” PLoS One. 2011;6(6):e20456. doi: 10.1371/journal.pone.0020456.
Carol DeSantis, Chun Chieh Lin, Angela B. Mariotto et al. “Cancer Treatment and Survivorship Statistics, 2014” CA Cancer J Clin. 2014;64(4):252-271. doi: 10.3322/caac.21235.
Amir Qaseem, Thomas D. Denberg, Robert H. Hopkins Jr., et al. “Screening for Colorectal Cancer: A Guidance Statement from the American College of Physicians” Ann Intern Med. 2012;156(5):378-386. doi: 10.7326/0003-4819-156-5-201203060-00010.
Rebecca Siegel, Carol DeSantis, Ahmedin Jemal. “Colorectal Cancer Statistics, 2014” CA Cancer J Clin. 2014;64(2):104-117. doi: 10.3322/caac.21220.