A study, published in the journal Gastroenterology, showed that the risk of developing cancer as a result of being exposed to X-rays during computed tomography colonography ( also known as "virtual colonoscopy" or CT colonography ) is considerably less than 1 percent.

Researchers say the radiation risk can be further reduced by creating optimized protocols for performing this screening test.

Researchers from the Center for Radiological Research at Columbia University Medical Center estimated the radiation dose to different organs from adult CT colonography scans. Dose-, organ- and gender-specific excess relative risks for cancer incidence in Japanese A-bomb survivors were used as a basis to generate risk estimates for the U.S. population. This study found the estimated lifetime risk of cancer as a result of radiation from CT colonography in a 50-year-old subject is about 0.14 percent and 0.022 percent for a 70-year-old patient. The main organs found to be at risk for cancer are the colon, stomach and bladder.

" Our study shows that radiation risks associated with virtual colonoscopy are relatively small--much smaller, for example, than for CT-based lung cancer screening," said David J. Brenner, lead study author with the Columbia University Medical Center. " With the potential for low cancer risks and the use of non-cathartic bowel preparation, virtual colonoscopy is a very promising modality which could potentially increase patient compliance with current guidelines for colorectal cancer screening."

A future trends report published by the AGA ( American Gastroenterological Association ) provided a review of clinical trials and quantitative mathematical models to determine the potential role of CT colonography in colorectal cancer screening. Limitations of CT colonography included variation in results of clinical trials and limited data on its use in routine clinical practice.

Questions currently being addressed in clinical outcomes studies include:

- Is there a minimum polyp size detectable by CT colonography for which patients should be referred for polyp removal ?

- What is the minimum-sized lesion by which CT colonography sensitivity will be judged ?

- What polyp size, if any, would physicians and more importantly their patients, allow to remain in place and undergo surveillance rather than immediate removal ?

- How will CT colonography screening followed by colonoscopy for patients with polyps impact patients' compliance and health-care costs ?

Bowel preparation continues to be a large barrier to patient compliance for colorectal cancer screening.
In most current CT colonography protocols, patients must undergo rigorous bowel cleansing preparation similar to that of traditional colonoscopy.
However, significant progress is being made in the development of a minimal preparation or "prep-less" CT colonography test – which would potentially have a major impact on patient compliance.

Guidelines of multiple agencies and professional societies underscore the importance of screening for all individuals 50 years of age and older. Currently, there are a number of tests that may be used to screen for colorectal cancer, the second-leading cancer killer in the United States.
Approved tests include barium enema, fecal occult blood test, flexible sigmoidoscopy and colonoscopy.

Source: American Gastroenterological Association, 2005


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