What is Colorectal Cancer?

What is Colorectal Cancer?



Key statistics and facts of colorectal cancer

  • After skin cancers, colorectal cancer is the third most common cancer in both men and women in the United States, according to American Cancer Society data.
  • The American Cancer Society’s estimates for the number of cases of colorectal cancer in the United States for 2018 are:
    • 97,220 cases of colon cancer
    • 43,030 cases of rectal cancer
  • The lifetime risk of developing colorectal cancer is slightly lower in women than in men. In men, it is about 4.5%, and in women it is about 4.2%.
  • Colorectal cancer is estimated to cause about 51,000 deaths in 2018. It is the third leading cause of cancer-related deaths in men and women in the United States.
  • The death rate from colorectal cancer has been dropping for the past several decades in both genders. One of the reasons for this is that colorectal cancers are now found more often through advanced screening techniques, when the polyps can be removed before they actually convert into cancers or when the cancer is easier to treat. Treatment has also improved over the last several decades.
  • According to estimates collected by American Cancer Society, there are now more than 1 million survivors of colorectal cancer in the United States.
  • The statistics available for bowel cancer (colorectal cancer) in the United Kingdom (UK) are:
    • New cases of bowel cancer in 2015: 41,804
    • Deaths from bowel cancer in 2014: 15903
    • Survival rate for colorectal cancer for 10 or more years (2010-11) in England and Wales: 57%
    • Preventable cases of bowel cancer in the UK: 54%

The colon and rectum

Our intestine is part of the digestive system. It is made up of:

  • a small intestine, and
  • a large intestine

The colon and rectum are parts of the large intestine. These are parts of our digestive system and are together known as large bowel.

The colon is a U-shaped cavity made of muscle and located right below the stomach. It absorbs large quantities of water, salts, and nutrients from undigested remaining food material as they pass through it. The waste material that remains after being absorbed in the colon goes to the rectum. The rectum is located at the end of the colon and stores faeces and waste material before they are passed out of the body through the anus. Rectum is the last 6 inches chamber of the digestive system.

At the end of the rectum are located sphincter muscles. These sphincter muscles prevent the rectum from emptying out the waste accidentally. When a person is ready for bowel movement, the sphincter muscles relax and, with straining, the waste is pushed out.

The colon and rectum together are about 2 metres (6 feet) in length. Other organs such as the pancreas, spleen, liver, bladder and human reproductive organs are located around the colon.

Overview and definition of colorectal cancer

Colorectal cancer, also known as bowel cancer, colon cancer or rectal cancer, depending on where they start from, is a cancer of the colon and the rectum. It starts in the colon or the rectum. Colon cancer and rectal cancer are often grouped together because many features are common in them.

According to World Health Organization and CDC, it is the second most common cancer worldwide, after lung cancer. According to American Cancer Society, about 1 in 20 people in the US will develop colorectal cancer during their lifetime.

The risk of developing colorectal cancer is slightly higher in men than in women. Most people diagnosed with it are above the age of 60.

More: Graphics, images, and photographs for colorectal cancer (bowel cancer)

How Does Colorectal Cancer Start? Where does colon cancer start?

Any cancer starts when the growth of cells in our body becomes out of control. Technically, cells in almost any part of our body can become cancer and can divide in an uncontrolled way, and can also spread to other areas of the body.

Mostly, colorectal cancers start as a growth called a polyp on the inner lining of the colon or the rectum. Not all polyps though are cancerous, but some polyps can change into cancer over a period of several years. There are two main types of polyps:

  • Adenomatous polyps (adenomas): These polyps can convert into cancers. They are also called pre-cancerous conditions.
  • Hyperplastic polyps and inflammatory polyps: These polyps are often not pre-cancerous. These are more common than adenomatous polyps.

A polyp can grow into the wall of the colon or rectum if a cancer is formed in it. Finding and removing the polyps can help prevent the colorectal cancer.

Polyps may be small and may show no or little symptoms. Therefore, doctors recommend regular screening tests to help diagnose them and prevent their spread before they become colon cancer. To know about signs and symptoms of colorectal cancer, read this article.

The size of these polyps is linked to the development of cancer. Polyps less than 1 centimetre in size have a slightly lesser chance of becoming cancer, but those polyps that are larger than 2 centimetres have significantly higher chance of converting into cancer. Mostly, the colorectal cancers form from polyps in glandular tissues of intestinal linings.

The wall of the colon and rectum is made up of several layers. Colorectal cancer begins in the innermost layer, called mucosa, and can grow outward through other layers. Cancer cells can also grow into blood vessels or lymph vessels from where they can travel to surrounding lymph nodes or to various organs of the body. Based on how deep and far the cancer grows indicates the stage of the colorectal cancer.

If the colorectal cancer is diagnosed early and treated properly, when the tumor is still localized, there are greater chances of getting the cure – with about 5 year survival rates of about 90%. However, if the tumor continues to grow, the colorectal cancer can spread through the bowel wall to nearby lymph nodes, tissues, and organs, and into the bloodstream. In such conditions, successful treatment is difficult. You can read about tests and diagnosis of colorectal cancer here.

How does colorectal cancer spread to other parts of the body?

If the cancer develops in a polyp, there are high chances that it can grow into the wall of the colon or rectum. Colorectal cancer starts in the innermost layer of the colon called mucosa and advances outward through outer layers of the colon.

Cancer cells can also grow into blood vessels or lymph vessels from where they can travel to nearby lymph nodes or organs or to the distant parts of the body.

The stage of a colorectal cancer defines how aggressively the cancer grows into the wall and if it has spread outside of the original site that is the colon or rectum. When a doctor diagnoses colorectal cancer in a patient, the next step is to assign a stage to the cancer. Treatment of colorectal cancer depends on the stage of the cancer. Read about treatment of colorectal cancer.

Types of bowel cancer (colorectal cancer)

Colorectal cancer or bowel cancer can start in the large bowel (colon) or the rectum, as discussed above. It is classified into different types based on the type of cell in which the cancer starts.

The various types of colorectal cancer are:


Adenocarcinomas originate in the gland cells in the lining of the colon wall (bowel wall). These gland cells produce mucus, which makes it easier for the poo (faeces or stool) to pass through the colon. It is the most common type of bowel cancer.

Adenocarcinoma can be of two types:

  • mucinous tumours: Mucinous cancer is a distinct form of colorectal cancer and is found in about 10–15% of patients with CRC.
  • signet ring tumours: One of the histologic sub-types of colorectal cancer is signet-ring cell carcinoma (SRCC). Primary SRCC diagnosed at an early stage is very rare and most of the cases are detected at an advanced stage. Overall prognosis of SRCC is therefore poor.

Other rare types of colorectal cancer

Squamous cell tumours

Squamous cells are skin cells that make up the lining of the colon and the gland cells. These tumors originate from the squamous cells.

Carcinoid tumours

Carcinoid is a type of slow growing tumour called a neuroendocrine tumour. They grow in hormone-producing tissue, mostly in the digestive system.


Sarcomas are generally cancers of the cells of bone or muscle. Most sarcomas in the colon are called leiomyosarcomas. They start in smooth muscle.


Lymphomas are cancers of the lymphatic system.

More: Anti–Vascular Endothelial Growth Factor Therapy (Anti VEGF Therapy) for Eye Disorders and Cancers

What is metastatic colorectal cancer?

One major reason why cancer is considered so serious is its ability to spread beyond the original site in the body. Colorectal cancer cells can spread locally by moving into nearby normal tissue of the digestive system. Colorectal cancer can also spread regionally, to nearby lymph nodes, tissues, or organs. It can spread to distant parts of the body too. Technically speaking, it can spread to any part of the body.

When colorectal cancer spreads to other body parts (nearby or distant), it is called metastatic colorectal cancer. Sometimes, it is also called stage IV (four) colorectal cancer. This process by which cancer cells spread to other body parts is called metastasis.

It is found that metastatic cancer cells have features to that of the primary cancer and not like the cells of the region where the cancer is spread to. This characteristic of the metastatic cancer cells is used by doctor to understand if a cancer that has spread from another part of the body is a different cancer or a metastatic cancer originated from the primary site.

Any type of metastatic cancer has the same name as the primary cancer. For example, breast cancer that reaches to the lung is called metastatic breast cancer, and not lung cancer. Colorectal cancer that reaches to genital organs is called stage 4 colorectal cancer or metastatic colorectal cancer. It is treated as the stage 4 colorectal cancer is treated, and not as genital organ cancer.

Patterns of metastasis in colon and rectal cancer: Where does colon cancer usually metastasize to?

Research epidemiology of metastatic colon and rectal cancer is quite challenging, because cancer registries and documentation often fail to record metastatic sites. Some research has been done on select group of population to identify metastatic spread in colon and rectal cancers and accordingly metastatic sites were identified. It was found that rectal cancer more frequently spread into thoracic organs and the nervous system and less frequently within the peritoneum.

Thoracic metastases are almost as common as liver metastases in cases of rectal cancer with a low stage at diagnosis. Metastatic patterns were found significantly different between colon and rectal cancers.

A research found that with metastatic colorectal cancer, the most common sites of metastasis were the liver, thorax , and peritoneum. In colon cancer, the third most common site was the peritoneum and in rectal cancer, it was bone. Nervous system metastases were also found in some cases of colon cancer and rectal cancer.

Approximately 20% of patients with colorectal cancer (CRC) already have metastases at the time of diagnosis. This figure has been almost same over the last two decades. In some studies, there is found a decreased cancer-specific mortality from CRC in screening groups. One of the reasons for this is that recent progress in metastasis research has greatly enhanced our understanding of metastasis on the cellular and molecular level.