After image acquisition and transfer of datasets, studies should be read and reported in a timely fashion. The following discussion will address aspects of image interpretation including who should read the datasets, how much training is required, how many readers are required and when and where studies should be read.
The primary aim of CT colonography is accurate identification of significant colorectal polyps and cancers in a minimally invasive manner. For CT colonography to be a safe, accurate and attractive alternative to colonoscopy, radiologists reading these studies must confidently recognise polyps and cancers, identify pitfalls and therefore reduce the number of false positive findings, and report significant extracolonic findings in a consistent and reliable manner. It is increasingly clear that to achieve this, radiologists must have specific CT colonography training. The effect of training and experience on reader performance has been the subject of a number of studies to date and has been a topic of intense discussion at many scientific meetings including the annual International Symposia on Virtual Colonoscopy in Boston. Training and its relationship to an individual's ability to report accurately CT colonography studies is a complex issue and is currently the subject of investigation of an ESGAR-funded research study. It appears that radiologists with a specific interest in CT colonog-raphy who have read many hundreds of cases perform better than abdominal radiologists who have been trained on 50 cases alone, who, in turn, perform better than those with little or no specific CT colonography training. This is as one might expect -however what is not clear is just how steep the learning curve is and when, or if, one reaches a plateau in reader performance.
Current recommendations are that radiologists should be specifically trained in a supervised manner on cases that have either endoscopic verification or have been read by an 'expert' reader. The datasets should include an appropriate mix of normal studies, cancers of various morphology, polyps (pedun-culated and flat), and extracolonic findings as well as studies limited by underdistension and poor bowel preparation. Emphasis should also be placed on familiarity with CT colonography software applications and recognition of the various pathologies in both 2D and 3D formats. While supervised training on 50 proven cases has been regarded as a minimum initial requirement, it would be wrong to assume that this is adequate for every radiologist or that it provides a level of 'expertise' as performance clearly improves with increasing experience.
Differences in reader experience has been identified as one of the factors contributing to the wide range of reported accuracies of CT colonography. In recent studies the reported sensitivities for detection of polyps >1 cm varied from 52 to 92% (JohnsoN et al. 2003a; Cotton et al. 2004; Pickhardt et al. 2003). Although there were some differences in the study populations and the methods used for bowel preparation, image acquisition, and interpretation, it is widely believed that differences in performance were due, at least in part, to variability in reader experience. Adequate and widespread access to reader training will be required before acceptance of CT colonography as a screening tool.
Clearly, if a radiology department is to offer a CT colonography service, at least one 'experienced' radiologist will be required to read studies, and likely more depending on workload. Experience as a general radiologist does not automatically qualify one to reading datasets as even the most experienced radiologists can miss large lesions on CT colonography (Halligan et al. 2004). Experience as an abdominal radiologist confers some advantage compared with a general radiologist but it by no means qualifies as adequate training.
Further evidence that reader experience impacts on the diagnostic performance of CT colonography comes from the ACRIN 1 trial (American college of Radiology Imaging Network). This study examined the ability of radiologists of various experience to detect clinically important neoplasia (lesions >1 cm). The results suggest that readers could achieve high accuracies only with extensive experience (JohnsoN et al. 2003b). Reader inexperience not only impacted on the ability of the reader to detect polyps but also increased inter- and intraobserver errors with regard to polyp size measurement. In another study Belloni et al. examined the performance of novice readers after every 25 patients for almost 100 CT colonography examinations. They found that the sensitivity achieved by readers for polyps of all sizes increased from 32% after the first 25 cases to 92% for the final 25 cases (Spinzi et al. 2001). Although controversy remains as to what qualifies as 'adequate experience' a minimum of 40-50 proven datasets was proposed based on a questionnaire sent to 18 international experts and presented by Dr J.A. Soto at the RSNA in 2004.
Even with suitable training errors of judgment will continue to be made by even the most experienced of radiologists. Potential 'pitfalls' leading to both false positive and negative results must be highlighted. These pitfalls typically relate to retained stool, complex fold and polyp morphology, and the relationship of polyps to folds and flexures. A number of publications have addressed these pitfalls (Fenlon 2002; Macari and Megibow 2001; Gleucker et al. 2004), and training courses should include examples. Training courses should also include formal lectures on image acquisition parameters, non-interpretive matters and a review of the data supporting virtual colongraphy for screening.
Training courses are available that meet these criteria in many centres in both North America and Europe. These courses are typically held over a two-day period. Although there is currently no obligation on radiologists to receive specific training prior to reporting CT colonography studies it may become mandatory in the future if screening with CT colonography becomes a reality. Ferrucci has compared such a colonic screening program with the template that already exists for mammographic screening (Ferruci 2000). He correctly predicts that in the setting of a colon screening program there will be a demand by certain third parties such as insurance companies or the American college of Radiologists that radiologists reporting CT colonography studies reach certain levels of competence and maintain those standards. The impact of such a step is to be welcomed as it would establish prerequisites that every reporting radiologist should meet in terms of their training and level of experience.
In our practice and most others, radiologists perform the study and interpret the scans. However, some believe that CT colonography studies could be performed and read by trained radiographers in the same way as barium enema examinations are provided in some institutions. The data supporting the use of non-radiologists is limited - at the 5th International Symposium on Virtual Colonoscopy in Boston 2004 results were presented from a mul-ticentre trial, suggesting that there is no significant difference in reader performance or the time taken to read studies between radiology trainees and radiographic technicians. However in a recent consensus on training from 18 leading radiologists in the field of CT colonography, 78% of those questioned felt that radiologists alone should report datasets and, currently, it is the view held by ourselves.
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