Diagnostic imaging of the colon is in the midst of a revolution. This is the result of recent developments in both CT technology allied with rapid development of dedicated software for interpretation of images. Recent large studies and NICE have confirmed CT colonoscopy to be as sensitive as optical colonoscopy for the detection of significant colon polyps and cancer.
Colorectal cancer is the third most frequently diagnosed cancer among men and women and the second leading cause of cancer death. According to current estimates, colorectal cancer will develop in about 5.9% of the population over their lifetime. Excluding deaths from lung cancer, colorectal cancer is the most common cause of cancer death for men and women combined.
The majority of colorectal cancers begin as benign lesions in the colon called adenomatous polyps. Over a period of several years, the cells inside the polyp transform into a malignant lesion known as colorectal carcinoma, more commonly known as colon cancer. The time for this transformation to occur in most cases is very long (usually over 10 years). Therefore, the detection and removal of these precursor polyps decreases the risk for lethal colorectal cancer significantly. Although polyps are the initial presentation of colorectal cancer, not all types of polyps will become malignant lesions. In fact, most polyps never become cancer. Research has shown that large polyps are at greater risk of becoming cancerous than small polyps. Studies of both traditional (optical) and CT colonoscopy have suggested that medium to large polyps (>6mm) can be expected to be found in approximately 6% of the population. With early detection of such polyps and their removal, it is thought that over 90% of colon cancers could be avoided.
CT colonoscopy (or Virtual Colonoscopy) can be carried out in approximately 20 minutes. A radiographer places a small catheter into the rectum allowing for the introduction of carbon dioxide to insufflate the colon. This tube is very thin and flexible more so than a colonoscope or sigmoidoscope and only 5-8 cm (2-3 inches) of the tube is inserted into the rectum. Once the tube is in place, the radiographer connects the tube to a device that introduces carbon dioxide into the colon. The aim of this manoeuvre is to distend the colon as much as possible to facilitate clear analysis of the images by the Radiologist. Occasionally muscle relaxants such as Buscopan are used to improve distension or to ease any undue discomfort.
Advantages
Virtual Colonoscopy allows the Consultant to inspect the entire colon. This is a great advantage over flexible sigmoidoscopy, which only views the lower half of the colon. Sigmoidoscopy for colorectal cancer screening has been likened to ‘mammography of one breast’. Virtual Colonoscopy is less invasive and does not carry the low, but real risks of serious complications (1 in 200), perforation of the colon (1 in 900) or of death, associated with conventional colonoscopy. It is well tolerated by patients, does not require sedation.
Disadvantages
As Virtual Colonoscopy is performed using a CT scanner, it exposes the individual to a small dose of radiation. Low dose protocols are used to ensure the radiation dose is far less than say a conventional abdominal CT scan or a barium enema. Whilst there has been controversy as to whether such low radiation exposures have any harmful effect - according to the Linear no Threshold theory, no effect at all, or even a beneficial effect (Hormesis theory), there is no doubt the very real benefits of early detection far outweigh even the most pessimistic theories.
Extra colonic pathology - Due to the fact that Virtual Colonoscopy visualises the entire abdomen and pelvis it is inevitable that unexpected extra colonic findings will be encountered.
The ability to see other significant pathology such as renal cancers, aortic aneurysms and more rarely ovarian cancers and lymphomas can undoubtedly result in improved outcomes from earlier diagnosis and treatment. It is estimated from published data and our own experience at Lifescan that significant findings including polyps are evident in up to 10% of patients. Any cost to the NHS following the detection of such findings is likely to be more than offset by savings derived from the early identification and treatment of life threatening conditions identified during a scan.
Other less significant findings such as renal stones or gall stones are not life threatening conditions but knowledge of their existence is clinically and personally useful. There will also be occasions when other findings of no clinical significance such as renal and hepatic cysts may be seen and as such do not require any further follow up.
Lifescan is the leading private provider of Virtual Colonoscopy in the UK and has completed over 30,000 to date.
Further reading - Colon Cancer
Virtual Colonoscopy
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