Colorectal cancer starts in the colon or the rectum. These cancers are also known as colon cancer or rectal cancer, depending on where they start. Colon cancer and rectal cancer are typically grouped together since they share many features.
In this article, we’ll talk about how colorectal cancer starts, how long it takes to develop, its different stages, and more.
Colorectal Cancer: How Does It Start?
Most colorectal cancers start as a growth on the inner lining of the colon or rectum – these growths are known as polyps.
Some types of polyps can turn into cancer over time (typically many years); however, not all polyps turn cancerous. The chance of a polyp turning cancerous depends on its type. The different types of polyps include:
- Adenomatous Polyps. Also known as adenomas, these polyps sometimes turn cancerous. Because of this, adenomas are also known as a pre-cancerous condition. The three types of adenomas as villous, tubular, and tubulovillous.
- Traditional Serrated Adenomas (TSA) and Sessile Serrated Polyps (SSP). These polyps are typically treated like adenomas since they have a higher risk of colorectal cancer.
- Inflammatory Polyps and Inflammatory Polyps. These polyps are more common, but they are generally not pre-cancerous. Some people with large hyperplastic polyps (more than 1cm) might need colorectal cancer screening more often.
Other factors that can make a polyp more likely to contain cancer or increase someone’s risk of developing colon cancer include:
- If a polyp is larger than 1cm
- If more than 3 polyps are found
- If dysplasia (a pre-cancerous condition) is seen in the polyp after it’s removed
How Long Does Colon Cancer Take to Develop?
As mentioned above, colon cancers develop from precancerous polyps that grow larger and ultimately transform into cancer. It is believed to take around 10 years for a small precancerous polyp to grow into cancer.
So, if appropriate colorectal screening is performed, most of these polyps can be removed before they turn into cancer, which can effectively prevent the development of colon 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 are composed of many layers – colorectal cancer starts in the innermost layer 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. From there, they can then travel to nearby lymph nodes or to distant parts of the body. The stage – or the extent of the spread – of colorectal cancer depends on if it has spread outside the colon or rectum and how deeply it grows into the wall.
How Colon Cancer is Staged
If you have been diagnosed with colon cancer, one of the first things that your doctor will do is to determine the stage of your cancer. The ‘stage’ refers to the extent of cancer or how far it has spread. Staging colon cancer is crucial to identify the best treatment approach.
Colon cancer is normally staged based on a system established by the American Joint Committee on Cancer known as the TNM staging system[1].
The TNM staging system considers the following factors when determining colon cancer stages:
- Primary Tumor. A primary tumor refers to how big the original tumor is and whether the cancer has grown into the wall of the colon, or if it has spread to nearby areas.
- Regional Lymph Nodes. Regional lymph nodes refer to whether cancer cells have spread to nearby lymph nodes.
- Distant Metastases. Distant metastases refer to whether cancer has spread from the colon to other parts of the body, such as the liver or the lungs.
Cancer Stage Classifications
Within each category, colon cancer is further classified and assigned a letter or number to indicate the extent of the disease. These assignments are based on the structure of the colon, and how far cancer has grown through the layers of the colon wall.
Colon cancer stages are as follows:
- Stage 0. Stage 0 is the earliest form of colon cancer. At this stage, it has now grown beyond the innermost layer of the colon.
- Stage 1. Stage 1 indicates that cancer has grown from the inner layer of the colon to the next layer of the colon known as the submucosa. At this stage, cancer has not spread to the lymph nodes.
- Stage 2. Stage 2 indicates that cancer has grown beyond the mucosa and the submucosa of the colon. Stage 2 colon cancer is further classified as stage 2A, 2B, or 2C.
- Stage 2A. At this stage, cancer has not spread to the lymph nodes or nearby tissue. However, it has reached the outer layers of the colon but has not completely grown through.
- Stage 2B. At this stage, cancer has not spread to the lymph nodes but has already grown through the outer layer of the colon and to the visceral peritoneum – a membrane that holds the abdominal organs in place.
- Stage 2C. At this stage, the cancer is not found in nearby lymph nodes, but in addition to growing through the outer layer of the colon, it has grown to nearby structures or organs.
- Stage 3. Stage 3 colon cancer is further classified as 3A, 3B, and 3C.
- Stage 3A. At this stage, the tumor has grown to or through the muscular layers of the colon and is already in nearby lymph nodes. It has not spread to distant organs.
- Stage 3B. At this stage, the tumor has grown through the outmost layers of the colon and infiltrates the visceral peritoneum or invades other organs and is found in one to three lymph nodes. Or the tumor is not through the outer layers of the colon wall but is found in four or more nearby lymph nodes.
- Stage 3C. The tumor has grown beyond the muscular layers and the cancer is already found in four or more nearby lymph nodes, but not distant sites.
- Stage 4. Stage 4 colon cancer is further classified into two categories, 4A and 4B.
- Stage 4A. At this stage, cancer has spread to one distant site, such as the liver or the lungs.
- Stage 4B. This is the most advanced stage of colon cancer. At this stage, cancer has spread to two or more distant sites, such as the liver or the lungs.
Low-Grade vs. High-Grade Colon Cancer
Aside from staging, colon cancer is further classified as either low- or high-grade.
When a pathologist inspects cancer cells under a microscope, they assign a number from 1 to 4 depending on how much the cells look like healthy cells. The higher the grade assigned, the more abnormal the cells look.
Although it can vary, low-grade cancers tend to grow slower compared to high-grade cancers. Also, the prognosis is considered better for people with low-grade colon cancer.
The Bottom Line
Colon cancers develop from precancerous polyps that grow larger and ultimately transform into cancer. It is believed to take around 10 years for a small precancerous polyp to grow into cancer.
If appropriate colorectal screening is performed, most of these polyps can be removed before they turn into cancer, which can effectively prevent the development of colon cancer.
Disclaimer: The information on this site is not intended to be a substitute for professional medical advice, diagnosis, or treatment. All information contained on this website is for general information purposes only.
