AJR: Manual volumetric calculation best to track polyp size at CT colonography

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Linear polyp measurement in the 3D endoluminal view appears to be the most reliable parameter for use in the decision to excise a polyp by the linear metric approach and manual volume is the most reliable measurement parameter for observing polyp growth over time, according to a study published in the December issue of the American Journal of Roentgenology.

According to Emily Bethea, MD, from the department of radiology at the Pritzker School of Medicine, University of Chicago, and colleagues, several published guidelines suggest that the size of a colonic polyp correlates with potential malignancy. According to the CT Colonography (CTC) Reporting and Data System Guidelines, when CTC is used as the initial colorectal cancer screening test, the size of the polyps detected is the most important factor in determining management recommendations.

The authors said that advanced adenoma—a lesion 10 mm in diameter or larger or a lesion that exhibits high-grade cellular dysplasia—should be the target of screening, but the authors said that debate continues about the management recommendations for intermediate size polyps—6-9 mm. To obtain meaningful screening results, it is important to not only determine the parameter that is the most accurate measurement of polyp size but also the measurement parameter that is best for tracking polyp size over time.

In this study, two observers analyzed  50 colonoscopically proven cases of polyps that measured 5 mm in diameter or larger, could be detected with automated software and could be paired with a virtual cononoscopic high-confidence match.

All of the analyses were performed using dedicated software (Virtual Colonoscopy, version 3.5, Philips Healthcare) on a CT workstation (Extended Brilliance Workspace, Philips Healthcare). The observers underwent one week of training in CTC image analysis and polyp measurement. The observers measured each polyp in supine and prone views.

They recorded measurements of 2D volume by summation of areas, 2D maximum diameter, and 3D maximum diameter and automated measurements of 3D maximum diameter and volume. The measurements were repeated for one of the observers. Intraobserver and interobserver agreement was calculated and analysis was performed to determine the measurement parameter that correlated most with summation-of-areas volume. Supine and prone measurements as a surrogate for tracking change in polyp size over time were analyzed to determine the measurement parameter with the least variation.

The authors found that manual linear polyp measurement on 3D endoluminal images correlated best with the reference standard measurements, which means it appears to be the most reliable and practical parameter in deciding to excise a polyp by the linear metric approach.

The reseachers also found that, in contrast to previous studies, manual volume, calculated by the summation of areas, was the most reliable measurement parameter for observing polyp growth over serial examinations. Because many patients with intermediate-size polyps are expected to choose surveillance as a management course, it is crucial that polyp measurements to determine change in polyp size over time must be highly reliable.

Therefore, the authors concluded the results of their study support the continued use of manual methods of polyp measurement.