Colonic polyps and colorectal cancer
Colonic polyps are growths occurring in the large bowel. They can take many forms – pedunculated (stalked), sessile (rounded), flat, or even depressed. There are many different types of polyps and their nature varies greatly with their histological characteristics (i.e. what sort of cells make up the polyp, as seen under a microscope). Some types of polyps are premalignant (have the potential to turn into cancer). These polyps are called “adenomas”. Some types of adenomas have a greater risk of turning cancerous than others.
Polyps are usually asymptomatic and can exist for years or decades before turning cancerous. As such, there is a clearly demonstrated benefit for screening for colorectal cancer in its early or premalignant stage. Even if colorectal cancer is found, if it is still in an early stage then it can be surgically removed and cured.
There are many ways to screen for colonic polyps and colorectal cancer, including:
- Testing for faecal occult blood – This technique may miss many polyps, and is better for screening for colorectal cancer. Must be followed up with a more specific investigation if positive e.g. colonoscopy.
- Colonoscopy – When the correct quality assurance procedures are followed, this technique has the highest yield for detecting polyps, especially flat or depressed polyps, which may be missed by CT colonography. Removal of the polyps can be performed simultaneously.
- Double contrast barium enema – This technique is seldom used anymore as its sensitivity for detecting polyps and colorectal cancer is significantly lower than colonoscopy. Usually reserved for situations where colonoscopy is not possible.
- CT colonography – A CT scanning technique whereby a “virtual” colonoscopy can be performed - a 3D reconstruction of the colon is created. Smaller polyps and flat lesions are more likely to be missed.
- Colon capsule endoscopy – A new technology which allows visualization of the colon mucosa using a video capsule that is ingested. Guidelines on its use have not yet been established, but it is a less invasive alternative to colonoscopy and with time may replace it.
Polyps – The management of polyps depends on the technique used to detect them. If colonoscopy is performed, then polyps are usually removed during the procedure. If a polyp is very large, it may not be removed immediately and may require surgery. Biopsies are taken if the polyp is not removed, and if they do not show any signs of cancer, it may be possible to remove the polyp during a repeat colonoscopy - even if the polyp is very large. This is referred to as Endoscopic Mucosal Resection (EMR). If alternative screening measures detect one or more polyps, then there are guidelines that advise whether colonoscopy should be performed, based on the size and number of the detected polyps.
Colorectal cancer – Once colorectal cancer is detected (it must be confirmed histologically i.e. using endoscopy) then the next step is to stage the disease. This means that radiological examination (usually CT scan) will be performed to determine if and how far the cancer has spread. Early cancer can be cured by surgically resecting it. Advanced cancer will require chemotherapy or radiotherapy.