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We read with interest the article by Ginnerup Pedersen et al (Gut 2003;52:1744–7) investigating the frequency and diagnostic consequences of extracolonic findings at multidetector computed tomography (MDCT) colonography.
The authors noted extracolonic findings in 65% of cases, with the need for further workup in 12% and surgery in 3%. The authors concluded that the high prevalence of extracolonic findings may make MDCT colonography a problematic colorectal cancer screening tool for both ethical and economic reasons.
We would like to comment on the question raised by Ginnerup Pedersen et al—namely, whether MDCT colonography should be regarded as a colon examination or a sort of “Pandora’s box” (if used for abdominal screening).
Notably, a recent article has emphasised that one of the major potential advantages of MDCT colonography in comparison with all other existing colorectal diagnostic tests is its ability to detect disease outside the colon.1 Indeed, the possibility that extracolonic disease can be readily identified at CT colonography has been extensively investigated in the literature,2–4 with results similar to the ones presented by Ginnerup Pedersen et al.
However, there is evidence that although the vast majority of extracolonic findings are of little clinical importance, such findings may lead to unnecessary further workup, with obvious economic, medicolegal, and psychological implications.3 Therefore, the possibility of “looking” outside the colonic lumen can be seen as a “double edged sword” or “Pandora’s box”.3
In this regard, we feel that three important issues need to be emphasised. Firstly, CT colonography is usually performed with a low dose technique which exploits the high contrast that exists at the colonic mucosa-air interface.4,5 Such a low dose technique is adequate for evaluation of colorectal lesions but substantially limits the assessment of solid organs.4,5 In addition, there is a recent trend to reduce even further the radiation dose of CT colonography.5 For instance, the radiation dose in milliSieverts (mSv) is 10 mSv for standard abdominal CT,6 6 mSv in the study of Ginnerup Pedersen et al, and 1.8–2.4 mSv in our hospital.5 Clearly, the lower the radiation dose, the lower the extracolonic diagnostic ability.
Secondly, in order to reduce the cost and increase the safety of the examination, CT colonography is usually performed without administration of intravenous contrast material.3 Clearly, this reduces even further the ability of CT colonography to detect and characterise extracolonic findings.
Thirdly, and perhaps most importantly, CT colonography has recently been demonstrated to be suitable for colorectal cancer screening purposes.7 However, there is no agreement regarding the use of standard abdominal CT for general abdominal screening.8,9 At present, abdominal CT screening is not supported by scientific evidence,9 United States Food and Drug Administration approval,6 or the American College of Radiology recommendation.10
It is clear that due to the use of a low dose technique and lack of intravenous contrast administration, CT colonography has even lower diagnostic ability than standard abdominal CT for assessment of disease outside the colon. Thus it is of paramount importance that radiologists, referring physicians, and patients are aware that CT colonography is not designed for the detection of extracolonic findings and should therefore be considered primarily as a colon examination. Due to the high prevalence of extracolonic abnormalities, radiologists should be alert to appropriate additional workup for triage patients to avoid opening a potential “Pandora’s box”.