Until the development of biomarkers indicative of histology, polyp size will continue to be the most important biomarker for determining management of colonic polyps found at screening and diagnostic CT colonography (CTC), according to a review in this month’s Radiology.
Polyp size is a critical biomarker for clinical management, as larger polyps have a greater likelihood of being or of becoming an adenocarcinoma, wrote Ronald M. Summers, MD, PhD, from the imaging biomarkers and computer-aided diagnosis lab and the department of radiology and imaging sciences at the National Institutes of Health Clinical Center in Bethesda, Md.
To balance the referral rate for polypectomy against the risk of leaving potential cancers in situ, sizes of 6 and 10 mm are increasingly being discussed as critical thresholds for clinical decision making (immediate polypectomy versus polyp surveillance) and have been incorporated into the consensus CTC Reporting and Data System (C-RADS), he noted. Therefore, the precision and accuracy of polyp measurement is an important issue.
Summers summarized the types of polyps and their likelihood at different size thresholds: First, cancer is known to occur more frequently in larger polyps. Two other important polyp characteristics that vary with polyp size are the presence of high-grade dysplasia and villous features. Both high-grade dysplasia and villous features are viewed as intermediate stages in the path to malignancy and have been called “advanced histology.”
He added that hyperplastic polyps are found frequently at colonoscopy and represent a large fraction of polyps smaller than 1 cm. Also, most hyperplastic polyps are thought to have little or no malignant potential.
“Cancerous polyps tend to grow slowly, Summers said. “It is estimated that the polyp dwell time, the time needed for a small adenoma to transform into a cancer, may be on average 10 years.”
Based on a synthesis of several studies, he said that polyp size measurements determined at CTC lie intermediate to those made at pathologic evaluation and colonoscopy and may be closest to the in vivo size.
“To align clinical management decisions at CT colonography with risk-based guidelines published in the existing literature, one must adjust the size threshold above which polyps are thought to convey greater risk—that is, polyps reaching the 1 cm threshold or larger at colonoscopy are likely to measure 8 mm or larger at CT colonography,” he wrote. “However, to align polyp management decisions with sizes based on pathologic measurements, then polyps 1 cm or larger as measured at pathologic exam allow a higher threshold at colonography to be set, such as 1.2 cm. Size differences of a few millimeters can matter clinically when a polyp is misclassified into an incorrect size category.”
Since CTC measurements tend to lie intermediate to the pathologic and colonoscopic measurements and may be more reliable than both of these, Summers wrote, CTC measurements may represent an ideal compromise size.
However, he added that polyp size measurements at CTC are “subject to errors and variability on the order of 2–3 mm, large enough to affect clinical management.” Therefore, he recommended that radiologists consider these errors in their decision-making process.
Summers also suggested that best practices for polyp size measurement at CTC include the use of 3D endoluminal displays, 2D displays with a window level near ?500 HU and automated measurement software.
“An important research goal is to ascertain the size threshold that best balances the referral rate for colonoscopy versus the frequency of colorectal cancer following CTC screening; the author proposes a threshold of 8 mm,” he concluded. “Ultimately…the goal is to determine a feasible scheme for managing polyps of different measured sizes, particularly those numerous polyps measuring 6–9 mm for which the best management strategy is still evolving.”