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.
The colon and rectum are parts of the large intestine and the digestive system. The colon absorbs water and nutrients and passes waste (stool) to the rectum.
Colon and rectal cancers are grouped together as colorectal cancer because these organs are made of the same tissues and there isn’t a clear boundary between them.
How does colorectal cancer start?
Let’s start with some of the basics and go from there.
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 taking many years), but it is important to note that not all polyps become cancerous.
The chance of a polyp changing into cancer depends on the type of polyp it is. There are 2 main types of polyps:
- Adenomatous polyps (adenomas): These polyps sometimes change into cancer. Because, adenomas are often a pre-cancerous condition that later turn into cancer.
- Hyperplastic polyps and inflammatory polyps: These polyps are the more common type of polyp. Generally speaking, they are not pre-cancerous. They are usually found in people with an inflammatory bowel disease (IBD) such as ulcerative colitis.
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, or;
- If dysplasia is seen in the polyp after it is removed. Note: 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 does colorectal cancer spread?
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 called the mucosa, and can grow outward through some or all of the other layers of the colon or rectum.
When cancer cells are found in the wall of the colon or rectum they can then spread into the blood vessels or lymph nodes. From there, the cancer cells can travel to nearby or to distant parts of the body.
The stage(s) of cancer refer to the spread or extent of a cancer within your body. In the case of colorectal cancers it depends on how deeply it has grown into the wall of the colon or rectum, and if it has spread outside the colon or rectum. If this happens you will hear the doctor use the term “metastasize”.
The different types of cancer found 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 are almost always talking about this type of cancer. Some sub-types of adenocarcinoma, such as signet ring and mucinous, may have a worse prognosis.
Other, less common types of tumors can start in the colon and rectum, too. These include:
- Squamous cell carcinoma, which is treated like anal cancer
- Neuroendocrine tumor (NET)
- Soft tissue sarcoma, such as leiomyosarcoma
- Gastrointestinal stromal tumor (GIST)
- Non-Hodgkin lymphoma, such as MALT lymphoma
Non-cancerous tumors of the colon or rectum
A non-cancerous or benign tumor of the colon or rectum is a growth that does not spread. Besides the polyps described earlier, some non- cancerous forms may also include:
- Hamartoma, also called a hamartomata’s polyp, can form in many parts of the body, including the colon or rectum.
- Lipoma starts in fat cells and can form anywhere in the body where there are fat cells, including the colon and rectum.
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