It is well established that screening for colorectal cancer is of paramount importance in the race to detect a malignancy while there is time for effective treatment. What is less clear is whether novel techniques can be used to supplement or replace colonoscopy. Investigators have questioned this in a prospective study comparing colonoscopy with CT colonography in the general population. The study showed that each technique has its advantages and that both could be used to form part of an effective national cancer prevention strategy.

Colorectal cancer has a high incidence and the efficacy of therapy is dependent on when the cancer is detected. In the Netherlands, if a colorectal cancer is detected and treated at stage I there is a 5-year overall survival rate of 94%; if we compare this with the 8% overall survival rate for a cancer that presents as stage IV disease then the importance of early detection becomes immediately apparent.

Although colonoscopy is the optimal screening tool in terms of sensitivity and the ability to treat areas of concern at the time of screening, it is an unpleasant experience and many people do not take up the offer of an appointment for this reason. Investigators led by Evelien Dekker decided to determine if screening using a CT scan to perform a 'virtual colonoscopy' or colonography would have higher uptake and an acceptable sensitivity profile.

The researchers invited members of the general population to either a colonoscopy or a CT colonography appointment using a 2:1 randomization pattern. Of the 5,924 individuals invited to undergo a colonoscopy, 1,276 underwent the procedure; this compares with 982 people who had a colonography out of 2,920 invitees. Therefore, significantly more people accepted the invitation for screening using the CT scan than the traditional colonoscopy (34% versus 22%).

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Getting people to attend the appointment is obviously only part of the story, the screening modality also needs to be highly sensitive for the screening program to have an impact on incidence and mortality. By this measure the colonoscopy outperformed the colonography. In the colonoscopy group, 111 participants were diagnosed with advanced neoplasia (8.7 per 100 individuals), and in the colonography group 60 were diagnosed (6.1 per 100 participants. In the population who underwent screening this difference was significant, but if we consider the whole invited population there was no significant difference in cancer diagnosis between the two groups (1.9 and 2.1 in 100 invitees for colonoscopy and colonography).

In an independent commentary published to support the original study, Perry Pickhardt states: “the issue with screening for colorectal cancer is not related to the test efficacy per se, but rather to the willingness of patient participation. By offering the additional option of CT colonography for screening, overall patient outcomes will be positively affected.” It is hard to disagree with this assessment of the situation; there is little point having a very effective method for population screening if only a few members of the population take part in the program.

It seems that CT colonography is a useful addition to a colorectal cancer screening strategy, but Dekker and her coauthors offer a note of caution: “we should keep in mind that the detection rates for advanced neoplasia might be different in subsequent screening rounds... Additionally, other factors such as cost-effectiveness ... should be taken into account.” The authors also point out that both colonoscopy and colonography have a lower take up rate than other screening methods, including the fecal occult blood test, something that should likely be considered in future studies.