Colorectal cancer is the abnormal and uncontrolled growth of cells in the colon or rectum. Depending on where the cancer starts, it can be called colon cancer or rectal cancer. However, these cancers share many common features. There are 5 parts to the large intestine – the ascending, transverse, descending and sigmoid colon, and the rectum. Colorectal cancer can develop in any of these sections.
Colorectal cancers typically begin as polyps. These are growths on the inner lining of the large intestine. These polyps begin as being non-cancerous, but some may develop into cancer over many years, whereas others may not.
There are 2 main types of polyps – adenomatous polyps (adenomas) and hyperplastic/inflammatory polyps. Adenomatous polyps can sometimes change into cancer, and therefore having an adematous polyp is considered a pre-cancerous condition.
Hyperplastic/inflammatory polyps are more common than adenomatous polyps, but they are generally not pre-cancerous.
In Singapore, colorectal cancer is the most common cancer in men (1 in 6 of male cancer patients), and is the 2nd leading cause of cancer deaths in men. In women, it is the 2nd most common cancer (1 in 7 of female cancer patients), and is the 3rd leading cause of cancer deaths in women.
Adenocarcinomas make up the majority of colorectal cancers. This type of cancer begins in the cells that produce mucus in the large intestine. There are other subtypes of cancers under this type of cancer, and they may result in different prognoses.
Other less common types of colorectal cancers include:
- Carcinoid tumours
- Gastrointestinal stromal tumours
If cells in the polyp become cancerous, it can invade into the wall of the colon or rectum over time. This wall is comprised of multiple layers, and the cancer spreads from the innermost layer towards the outer layers.
The wall of the large intestine also contains blood vessels or lymph vessels. If the cancer reaches the blood or lymphatic supply, it may be able to travel to the nearby lymph nodes, or to other parts of the body.