Colorectal cancer: Signs and Symptoms, How does colorectal cancer start,How colorectal cancer spreads, Risk factor and Causes

Colorectal cancer is any cancer that affects the colon and the rectum.The term colorectal describes this area that begins at the colon and ends at the anus.

Updated: June 16, 2022

Colorectal cancer is any cancer that affects the colon and the rectum. It is also known as bowel cancer, colon cancer, or rectal cancer. The term colorectal describes this area that begins at the colon and ends at the anus. These cancers can also be named 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. Colorectal cancer may be benign, or non-cancerous, or malignant. A malignant cancer can spread to other parts of the body and damage them.

Signs and Symptoms of Colorectal cancer:

Symptoms of colorectal cancer include:

  • changes in bowel habits
  • diarrhea or constipation
  • a feeling that the bowel does not empty properly after a bowel movement
  • blood in feces that makes stools look black
  • bright red blood coming from the rectum
  • pain and bloating in the abdomen
  • a feeling of fullness in the abdomen, even after not eating for a while.
  • fatigue or tiredness
  • unexplained weight loss
  • a lump in the abdomen or the back passage felt by your doctor
  • unexplained iron deficiency in men, or in women after menopause

Colorectal cancers can often bleed into the digestive tract. Sometimes the blood can be seen in the stool or make it look darker, but often the stool looks normal. Over time, the blood loss can build up and can lead to anemia. Sometimes a blood test showing a low red blood cell count is an indication of colorectal cancer.
Many of these symptoms are similar to that of infection, hemorrhoids, or irritable bowel syndrome. So a proper diagnosis is required for right treatment.

How does colorectal cancer start?

The colon and rectum is a part of the large intestine which is part of the digestive system, also called the gastrointestinal (GI) system. Most of the large intestine is made up of the colon which is a muscular tube about 5 feet long. The parts of the colon are named by which way the food is traveling through them.

  • The first part is called the ascending colon which starts with a pouch called the cecum, where undigested food comes from the small intestine. It extends upward on the right side of the abdomen.
  • The second part is called the transverse colon which goes across the body from the right to the left side.
  • The third part is called the descending colon because it travels down on the left side.
  • The fourth part is called the sigmoid colon because of its S shape. The sigmoid colon joins the rectum, which connects to the anus.

The ascending and transverse sections together are called the proximal colon where as the descending and sigmoid colon are called the distal colon. The colon absorbs water and salt from the remaining food matter after it goes through the small intestine. The waste matter that is left after going through the colon goes into the rectum where it is stored until it passes out of the body through the anus. A ring shaped sphincter muscles around the anus keeps stool from coming out until they relax during a bowel movement.
Most colorectal cancers start as a growth on the inner lining of the colon or rectum which are called polyps. Over time these polyps can change into cancer, but not all polyps become cancer. The chance of a polyp changing into cancer depends on the type of polyp it is. The types of polyps can be:

  • Adenomatous polyps (adenomas): These polyps sometimes change into cancer. Because of this, adenomas are called a pre-cancerous condition.
  • Hyperplastic polyps and inflammatory polyps: These polyps are more common, but in general they are not pre-cancerous.

Factors that can make a polyp more likely to increase the risk of developing colorectal cancer include:

  • If a polyp is larger than 1 cm.
  • If more than 2 polyps are found
  • If dysplasia is seen in the polyp after it is removed. Dysplasia is another pre-cancerous condition. It means there is an area in a polyp or in the lining of the colon or rectum where the cells look abnormal, but they don't look like true cancer cells.

How colorectal cancer spreads?

If cancer forms in a polyp, overtime it can grow into the wall of the colon or rectum. The wall of the colon and rectum is made up of many layers. Colorectal cancer starts in the innermost layer called the mucosa and can grow outward through some or all of the other layers. From the wall, the cancer can grow into blood vessels or lymph vessels and can travel to nearby lymph nodes or to distant parts of the body. The stage or extent of spread of a colorectal cancer depends on how deeply it grows into the wall and if it has spread outside the colon or rectum.

Risk Factors of Colorectal Cancer:

Factors for which your chance of getting a disease such as cancer is called as risk factor. Different cancers have different risk factors. Some risk factors, like smoking, can be changed. Others, like age of a person or family history, can't be changed.
But having a risk factor, or even many, does not necessarily mean that you will get the disease. And some people who get the disease may not have any known risk factors. Several risk factors are found that might increase the chance of developing colorectal polyps or colorectal cancer of an individual. Some of these can be changed where as others can not.

Colorectal cancer risk factors you can change:

Many lifestyle related factors such as diet, weight, and exercise have been linked to colorectal cancer.

  • Being overweight or obese can increase your risk of developing colon and rectal cancer in both men and women. However the risk is more in case of men.
  • If you are physical inactivity, you have a greater chance of developing colon cancer. Being more active can help lower your risk.
  • A diet that is high in red meats such as beef, pork, lamb, or liver and processed meats like hot dogs and some luncheon meats raises your colorectal cancer risk. Cooking meats at very high temperatures creates chemicals that might raise your cancer risk.
  • Smoking is a well-known cause of many cancer. People who have smoked tobacco for a long time are more likely to develop and die from colorectal cancer than non-smokers.
  • Moderate to heavy alcohol consumption can increase your risk of developing colon and rectal cancer.

Colorectal cancer risk factors you cannot change:

  • Being older increases your risk of developing colorectal cancer. Your risk of colorectal cancer goes up as you age. It is much more common after age 50.
  • If you have a history of adenomatous polyps, you are at increased risk of developing colorectal cancer. The risk is more if the polyps are large, if there are many of them, or if any of them show dysplasia.
  • If you had your first colorectal cancer when you were younger, you are more likely to develop new cancers in other parts of the colon and rectum even though it was completely removed.
  • If you have inflammatory bowel disease (IBD), including either ulcerative colitis or Crohn's disease, your risk of colorectal cancer is high. People who have had IBD for many years often develop dysplasia if left untreated which can develop into cancer over time.
  • People with a history of colorectal cancer in a first-degree relative such as parent, sibling, or child are at increased risk. Because of inherited genes, shared environmental factors, or some combination of these, cancers can run in the family.
  • Having family members who have had adenomatous polyps is also linked to a higher risk of colon cancer.

People who develop colorectal cancer have inherited gene changes that cause family cancer syndromes and can lead to them getting the disease.

Causes of Colorectal Cancer:

Several risk factors are found to associated with colorectal cancer, but it is not yet clear exactly how all of these factors might cause this cancer.
Usually a cancer is caused by changes in the DNA inside our cells. DNA is the chemical in our cells that makes up our genes, which control the way our cells function.  As our parents are the source of our DNA, we usually look like them. But DNA affects more than just appearance.
Some genes help control when our cells grow, divide into new cells, and die. The genes that help cells grow, divide, and stay alive are called oncogenes. Genes that help keep cell division under control or cause cells to die at the right time are called tumor suppressor genes. Cancers can be caused by DNA mutations that turn on oncogenes or turn off tumor suppressor genes leading to uncontrolled growth of cells.

Inherited gene mutations:

When DNA mutations are passed on in families, it is called as inherited mutations. Colorectal cancers can be caused by inherited gene mutations which are rare. Many of these DNA changes and their effects on the growth of cells are now known. For example:

  • Familial adenomatous polyposis (FAP), attenuated FAP (AFAP), and Gardner syndrome are caused by inherited changes in the APC gene. The APC gene is a tumor suppressor gene which helps keep cell growth in check. This brake on cell growth is turned off in n people with inherited changes in the APC gene, resulting in formation of hundreds of polyps in the colon. Over time, there is a chance of developing cancer in one or more of these polyps.
  • Lynch syndrome (hereditary non-polyposis colon cancer, or HNPCC) is caused by changes in genes that normally help a cell repair damaged DNA. A mutation in one of the DNA repair enzyme genes like MLH1, MSH2, MLH3, MSH6, PMS1, and PMS2, can result in DNA errors which can not be fixed. When these errors affect the growth regulating genes, it may lead to the development of cancer.
  • Peutz-Jeghers syndrome is caused by inherited changes in the STK11 (LKB1) gene, which is a tumor suppressor gene.
  • MYH-associated polyposis (MAP) is caused by mutations in the MYH gene, which is involved in how the cell checks the DNA and fixes errors when cells divide.
  • These inherited syndromes which are associated with gene mutations can be diagnosed by special genetic tests. Genetic counseling and genetic testing should be done if you have a family history of colorectal polyps or cancer or other symptoms linked to these syndromes.

Discussing genetic testing with a qualified cancer genetics professional before any genetic testing is recommended.

Acquired gene mutations:

These are the most gene mutations that lead to cancer where these DNA changes affect only cells that come from the original mutated cell. They happen during lifetime of a person and does not not passed on to their children. Certain risk factors probably play a role in causing these acquired mutations.
In most of the cases, the first mutation occurs in the APC gene which leads to an increased growth of colorectal cells because of the loss of brake on cell growth. Further mutations may then occur in other genes, which can lead the cells to grow and spread uncontrollably. Other unknown genes are probably involved as well.

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