Colorectal cancer starts in the colon or the rectum. These cancers can also be called colon cancer or rectal cancer, depending on where they start. Colon cancer and rectal cancer are often grouped together because they have many features in common. Cancer starts when cells in the body start to grow out of control.
The colon and rectum
To understand colorectal cancer, it helps to know about the normal structure and function of the colon and rectum.
The colon and rectum make up the large intestine (or large bowel), which is part of the digestive system, also called the gastrointestinal (GI) system (see illustration below).
Most of the large intestine is made up of the colon, a muscular tube about 5 feet (1.5 meters) long. The parts of the colon are named by which way the food is traveling through them.
- The first section is called the ascending colon. It starts with a pouch called the cecum, where undigested food is comes in from the small intestine. It continues upward on the right side of the abdomen (belly).
- The second section is called the transverse colon. It goes across the body from the right to the left side.
- The third section is called the descending colon because it descends (travels down) on the left side.
- The fourth section is called the sigmoid colon because of its “S” shape. The sigmoid colon joins the rectum, which then connects to the anus.
The ascending and transverse sections together are called the proximal colon. The descending and sigmoid colon are called the distal colon.
How do the colon and rectum work?
The colon absorbs water and salt from the remaining food matter after it goes through the small intestine (small bowel). The waste matter that’s left after going through the colon goes into the rectum, the final 6 inches (15cm) of the digestive system. It’s stored there until it passes through the anus. Ring-shaped muscles (also called a sphincter) around the anus keep stool from coming out until they relax during a bowel movement.
How does colorectal cancer start?
Polyps in the colon or rectum
Most colorectal cancers start as a growth on the inner lining of the colon or rectum. These growths are called polyps.
Some types of polyps can change into cancer over time (usually many years), but not all polyps become cancer. The chance of a polyp turning into cancer depends on the type of polyp it is. There are different types of polyps.
- Adenomatous polyps (adenomas): These polyps sometimes change into cancer. Because of this, adenomas are called a pre-cancerous condition. The 3 types of adenomas are tubular, villous, and tubulovillous.
- Hyperplastic polyps and inflammatory polyps: These polyps are more common, but in general they are not pre-cancerous. Some people with large (more than 1cm) hyperplastic polyps might need colorectal cancer screening with colonoscopy more often.
- Sessile serrated polyps (SSP) and traditional serrated adenomas (TSA): These polyps are often treated like adenomas because they have a higher risk of colorectal cancer.
Other factors that can make a polyp more likely to contain cancer or increase someone’s risk of developing colorectal cancer include:
- If a polyp larger than 1 cm is found
- If more than 3 polyps are found
- If dysplasia is seen in the polyp after it’s removed. Dysplasia is another pre-cancerous condition. It means there’s an area in a polyp or in the lining of the colon or rectum where the cells look abnormal, but they haven’t become cancer.
How colorectal cancer spreads
If cancer forms in a polyp, it can grow into the wall of the colon or rectum over time. The wall of the colon and rectum is made up of many layers. Colorectal cancer starts in the innermost layer (the mucosa) and can grow outward through some or all of the other layers (see picture below).
When cancer cells are in the wall, they can then grow into blood vessels or lymph vessels (tiny channels that carry away waste and fluid). From there, they can travel to nearby lymph nodes or to distant parts of the body.
The commonest symptom of colorectal cancer is a change of bowel habits. There may be increasing constipation, or perhaps alternating bouts of constipation and diarrhoea. There may be blood or mucus in the stools. A feeling that you haven’t completely emptied your bowels is quite common if the tumour is in the rectum. This can be uncomfortable and you may constantly feel the urge to go to the toilet.
You may feel a colicky type pain, or vague discomfort in your abdomen. You may also feel generally unwell, for example listless or tired, because you have been losing blood from the bowel and may have become anaemic (lack of red blood cells).
If your symptoms have lasted six or more weeks, including bleeding from the rectum, you need to see a specialist.
Colorectal cancer diagnosis
There are a series of tests and investigations which can be done to confirm or rule out a diagnosis of colorectal cancer, and to find out whether the cancer has spread to other parts of your body.
There are several ways which the doctor can examine your bowel. Whatever examination is used your bowel will need to be prepared. It must be as empty as possible so that the doctor can get a clear view inside.
The preparation may vary but will usually include:
- Eating a low fibre (roughage) diet for a day or so before the test to clear your bowel of any residue
- Drinking clear fluids only the day before the examination
- Taking laxatives to clear your upper bowel
- An enema to make sure the lower part of the bowel is empty.
- If you have any questions or the explanation is unclear, please ask your doctor or nurse.
Sigmoidoscopy or colonoscopy
During these investigations the doctor passes a scope (a tube with a small camera on the end) into your bowel. A sigmoidoscopy looks at the lower part of your large bowel, while the colonoscopy looks further up the colon.
The doctor can see if there is part of the lining of the bowel which looks different, for example there may be a polyp (a small smooth growth) or an ulcer.
If the doctor does see something unusual, a biopsy (a sample of tissue) will be taken from this area. The tissue will be sent to the laboratory for examination under the microscope.
Before these examinations you will be given something to make you more relaxed and prevent any discomfort. If you have any questions, please ask your doctor or nurse.
This is an X-ray examination using barium which brightens the X-ray picture. The barium is given as an enema and will outline the lower part of your bowel.
The procedure lasts 15–30 minutes and you should try to hold the contents of the enema for the length of the examination. Afterwards you will be able to empty your bowels. You may be prescribed a mild laxative because barium can cause constipation. Barium can also be very difficult to flush away in the toilet. If you have any questions, please ask your doctor or nurse.
You may have other tests, which can include blood tests, abdominal ultrasound, CT scan or MRI scan.
The information from these tests is used to assess the size of the cancer and how far it has spread. This is called ‘staging’. Your doctors need to know the extent of the cancer to help them decide on the most appropriate treatment for you.
Staging of colorectal cancer is based on a physical examination, the results of tests and what the doctors find at the time of surgery.
Colorectal cancer treatment
Treatment will usually be an operation to remove the cancer and/or to relieve your symptoms. You may also have chemotherapy or radiotherapy as well as an operation.
There are several ways of treating colorectal cancer and your treatment will be planned individually for you. Don’t be concerned if you talk to other people who are receiving similar, but different, treatments.
An operation may be performed to remove the cancer and part of the bowel on either side of this. Depending on the extent of the operation the two ends of the bowel may be stitched together.
If the tumour is sited low in rectum, there may not be enough bowel left to join together. In this case you may have to have a colostomy formed. A colostomy, or stoma, is an artificial opening created when the healthy part of your bowel is brought out onto the surface of your abdomen. Your stool will be passed through this opening instead of through your rectum as before. You will need to wear an appliance (bag) to collect your stools. This type of colostomy will be permanent.
In some situations you may need to have a temporary colostomy formed to rest of the bowel while healing takes place. This is usually only for a few weeks and will be discussed with you by your doctor.
If it is possible to say before your operation that you will need to have a colostomy, a stoma care nurse will visit you. They will explain exactly what will happen and what to expect. The stoma care nurse will show you how to care for your colostomy and help you adapt to living with a stoma. They can provide support over a long period of time.
If you have any questions or there is anything you don’t understand, please speak to your doctor or nurse.
Chemotherapy (drug treatment) may be recommended for you. Chemotherapy means treatment with anti-cancer drugs, which are given to destroy or control cancer cells by damaging them so that they can’t divide and grow.
Radiotherapy uses high-energy X-rays to kill cancer cells and is given using a machine similar to an X-ray machine but slightly larger. The treatment area will include the tumour and the surrounding lymph nodes (glands) if necessary. The treatment will planned specifically for you to make sure that the cancer cells are destroyed with the least amount of damage to normal tissues.
Your bowel habits may change during radiotherapy, for example you stool may become loose or you may develop diarrhoea. If this happens, please tell your doctor, radiographer or nurse. You will be given advice on diet and medicine can be prescribed to help you. Make sure you drink plenty of fluids.
Your bladder may be included in the treatment field and, if so, you may experience some discomfort when passing urine. You may also want to pass urine more frequently. Once again, make sure you drink plenty of fluids and tell your doctor about this problem.
After treatment for colorectal cancer
Having and being treated for colorectal cancer will have an effect on your life, and there are other things which you should be aware of in addition to the side effects of treatment.
The doctor will ask you to attend hospital at regular intervals during and after your treatment. You will be given an appointment for the outpatients clinic and, as time passes, the appointments will probably become less frequent.
Each time you attend, the doctors will examine you. Blood tests or X-rays may be repeated to check your recovery and make sure the cancer hasn’t come back.
Eating and drinking: After treatment for colorectal cancer there are usually no restrictions on what you can eat and drink, including alcohol in moderation. If you would like advice about your diet, please ask to see the dietician.
Disclaimer: All contents on this site are for general information and in no circumstances information be substituted for professional advice from the relevant healthcare professional, Writer does not take responsibility of any damage done by the misuse or use of the information.
- American Cancer Society
- Harvard Medical School