What Is Colorectal Cancer?
Colorectal cancer is a cancer that starts in the colon or the rectum. 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. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?
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 changing into cancer depends on the type of polyp it is. The 2 main types of polyps are:
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.
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 2 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 don't look like true cancer cells.
For more details on the types of polyps and conditions that can lead to colorectal cancer, see Understanding Your Pathology Report: Colon Polyps.
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.
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 stage (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. For more on staging, see Colorectal Cancer Stages.
Where does colorectal cancer grow?
To understand colorectal cancer, it helps to understand the parts that make up 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 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 extends 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 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.
What do the colon and rectum do?
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 of the digestive system. It's stored there until it passes out of the body through the anus. Ring-shaped sphincter (SFINK-ter) muscles around the anus keeps stool from coming out until they relax during a bowel movement.
Types of cancer in the colon and rectum
Adenocarcinomas make up about 96% of colorectal cancers. These cancers start in cells that make mucus to lubricate the inside of the colon and rectum. When doctors talk about colorectal cancer, they're almost always talking about this type. Some sub-types of adenocarcinoma, such as signet ring and mucinous, may have a worse prognosis (outlook).
Other, much less common types of tumors can start in the colon and rectum, too. These include:
Carcinoid tumors. These start from special hormone-making cells in the intestine. They're covered in Gastrointestinal Carcinoid Tumors.
Gastrointestinal stromal tumors (GISTs) start from special cells in the wall of the colon called the interstitial cells of Cajal. Some are not cancer (benign). These tumors can be found anywhere in the digestive tract, but are not common in the colon. They're discussed in Gastrointestinal Stromal Tumor (GIST).
Lymphomas are cancers of immune system cells. They mostly start in lymph nodes, but they can also start in the colon, rectum, or other organs. Information on lymphomas of the digestive system can be found in Non-Hodgkin Lymphoma.
Sarcomas can start in blood vessels, muscle layers, or other connective tissues in the wall of the colon and rectum. Sarcomas of the colon or rectum are rare. They're discussed in Soft Tissue Sarcoma.
Colon cancer is the third leading cause of cancer-related deaths in the United States in both men and women. Most cancers in the colon develop from polyps, which are growths that form within the inner lining of the colon. While most polyps do not actually turn into cancer, the ones that are most likely to are called adenomatous polyps or adenomas. Large polyps (greater than one centimeter), polyps that contain abnormal cells (called dysplastic polyps), and having two or more polyps within the colon also increases the likelihood for colon cancer.
In terms of risk factors, a person's chance of developing colon cancer increases as he or she gets older, especially after the age of 50. Furthermore, having type 2 diabetes or inflammatory bowel disease (for example, ulcerative colitis), or a family history of colon cancer also increases a person's risk for developing the disease, as do some modifiable risk factors like being overweight and eating a diet rich in red and processed meats.In the end, knowing the causes and risk factors for colon cancer can help you understand the importance of routine screening for colon cancer, as well as learn if you are one of the people who should begin screening at an earlier age.
Common Risk Factors
There are a number of factors that increase a person's risk for developing colon cancer, some within a person's control (considered modifiable) and some not, like age, ethnicity, and race, or genetics.
Age is the number one risk factor for colon cancer. According to the Centers for Disease Control and Prevention (CDC), more than 90 percent of colon cancer cases occur in people who are 50 years of age or older.
That said, young adults can get colon cancer too. In fact, the incidence of colon cancer in young people aged 15 to 39 is increasing, and experts are not sure why. Moreover, contrary to popular thought, most colon cancers in young people are not linked to genetic syndromes, but occur sporadically.
Ethnicity and Race
Ethnicity is also a well-known factor associated with cancer risk. African Americans are more likely to develop and die from colon cancer than Caucasians. Another high-risk group for getting colon cancer are people of Jewish eastern European descent.
Being Overweight or Obese
The link between colon cancer and obesity is strong. All told, people who are obese are over 30 percent more likely to develop this type of cancer than people of normal weight. The good news is that in your journey to lose weight, regular physical activity can actually protect you from developing colon cancer.
Type 2 Diabetes
Research has consistently shown a link between type 2 diabetes and the development of colon cancer, and this link exists independent of diet or being overweight.
Personal History of Colonic Polyps
The term colon polyp refers to an abnormal growth in the lining of the colon. Most commonly, cancers of the colon develop from adenomatous polyps, with adenocarcinoma being the most prevalent type of colorectal tumor. Adenomatous polyps can be villous (frond or leaf-like), raised, or flat.
Virtually all colon cancers develop from adenomatous polyps; having one or more adenomatous polyps increases your risk of developing colon cancer. This risk is even higher the larger the polyp, the more polyps you have, and whether the polyp show dysplasia, meaning it contains some abnormal-looking cells.
The upside is that when these polyps are found and removed via colonoscopy, they no longer have the opportunity to transform from precancerous to cancerous.
Inflammatory bowel disease (IBD) is characterized by conditions such as ulcerative colitis and Crohn's disease. Both are associated with the development of colon cancer, and disease duration is a major risk factor for identifying who (with IBD) is most at risk.
For example, while the results of different studies vary slightly, according to one large study, the risk of colon cancer for people with ulcerative colitis was found to be 0.7 percent at 10 years, 7.9 percent at 20 years, and 33.2 percent at 30 years.
In addition to disease duration, people with more extensive colitis (colon inflammation) are at a higher risk. More specifically, people whose entire colon is diseased (called pan-colitis) have the highest risk of developing colon cancer.
It's important to note that IBD should not be confused with irritable bowel syndrome (IBS), which does not increase a person's risk of developing colon cancer.
Radiation to the abdomen, pelvis, or spine as a child increases the risk of developing colon cancer. This is why the Children's Oncology Group recommends colonoscopy every five years (starting at age 35 or 10 years after the treatment, whichever is later) for people who received a significant amount of abdominal pelvic and/or spinal radiation in their younger years.
Research also suggests that men who have received radiation therapy to treat prostate and testicular cancer have a higher rate of colon and rectal cancer (the rectum is the digestive tube located between the colon and anus).
Research has shown that one in four cases of colon cancer has some sort of genetic link. So if you have a first-degree family member (brother, sister, father, mother, child) with colon cancer or polyps, your risk of developing colon cancer is increased.
Having a diet free from red meat can help women reduce the risk of a type of colon cancer, suggests new research.
"The impact of different types of red meat and dietary patterns on cancer locations is one of the biggest challenges in the study of diet and colorectal cancer," said lead author of the study Diego Rada Fernandez de Jauregui from the University of Leeds in Britain.
"Our research is one of the few studies looking at this relationship and while further analysis in a larger study is needed, it could provide valuable information for those with a family history of colorectal cancer and those working on prevention," he said.
The findings, showed that those regularly eating red meat compared to a red meat-free diet had higher rates of distal colon cancer -- cancer found on the descending section of the colon, where faeces is stored.
The study used data from the United Kingdom Women's Cohort Study. This cohort included a total of 32,147 women from England, Wales and Scotland.
"Our study not only helps shed light on how meat consumption may affect the sections of the colorectum differently, it emphasises the importance of reliable dietary reporting from large groups of people," said study co-author Janet Cade, Professor at the University of Leeds.
Previous studies have blamed several factors for the link between red meat and colon cancer, including on chemicals produced when meat is cooked at high temperatures and on preservatives such as nitrates used in processed meats.
The study was published in the International Journal of Cancer.
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