Abstract IA15: CT colonography in colorectal cancer screening

Abstract

CT colonography (CTC) is an imaging-based colorectal cancer screening test. Due to its underlying computed tomographic nature, it is one of the structural screening exams which allows for detection of both colorectal polyps and masses. Like colonoscopy, this exam holds the possibility of cancer prevention from polyp detection in addition to early cancer detection. Unlike colonoscopy, CTC has an excellent safety profile without a real risk for perforation. However, concerns have been raised for CTC regarding radiation dose and extracolonic findings, despite the published literature which suggest otherwise. CTC represents a low dose CT that is undertaken with a specialized protocol. The patient undergoes bowel preparation with cathartic and oral contrast tagging agents prior to the exam. The cathartic agents purge the colon of bulk material and the tagging agents tag any residual material for easy differentiation from potential soft tissue polyps. The colon is distended with carbon dioxide for image acquisition. This is a continuous low pressure administration (ie, CTC holds many positive attributes as a screening exam and occupies an optimal middle ground between stool studies and colonoscopy in regards to safety and effectiveness. Like stool studies, it is a very safe exam. The perforation rate is near nonexistent (0.009%)(1) and CTC does not require sedation (and thus precludes the risks related to sedation). Like colonoscopy, CTC is much more effective that stool-based screening because of the opportunity to primarily prevent cancer through the detection of polyp precursors in addition to early cancer detection. CTC accomplishes this in a more efficient manner than colonoscopy by removing only the polyps most likely to progress to cancer, stratified by size.(2) Polyps not meeting criteria for removal subsequently declare an increased risk for cancer development by demonstrating interval growth at future screening. This polypectomy strategy has been shown to be very effective at markedly decreasing the amount of pseudo-disease that is removed at colonoscopy.(3) Radiation exposure and extracolonic findings are two reported liabilities for this screening method. However, the existing evidence would suggest that neither substantially holds a negative impact for CTC. Regarding radiation, the dose levels for CTC is a fraction of CT and typically is less than 5 mSv (mean of 3 mSv). At these levels, there is no empiric data supporting future cancer induction. The dose exposures are similar to other well studied groups including airline pilots and nuclear reactor safety workers, who do not demonstrate increased rates of cancers.(4,5) Indeed, future risk is a theoretical one based on a mathematical model (LNT) which many physicists believe is flawed. The official Health Physics Society statement states:(6) “. . . below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero. . .” and “. . .because of statistical uncertainties in biological response at or near background levels, the LNT hypothesis cannot provide reliable projections of future cancer incidence from low-level radiation exposures. . .” This has led the society to conclude “. . . The Health Physics Society advises against estimating health risks to people from exposures to ionizing radiation that are near or less than natural background levels because statistical uncertainties at these low levels are great. . .” Extracolonic findings are a second issue said to hold negative consequences. Because of the underlying CT nature, incidental findings outside of the colon can be seen. People have suggested that too many work-ups for these findings would result- many of which would ultimately be benign. Such work-ups increase cost and anxiety with the potential for complications. There is now a large body of evidence over the past decade of CTC use which shows that work-up rates are reasonable (less than 10%) and are balanced by the beneficial findings of unsuspected important diagnoses such as asymptomatic aortic aneurysms and extracolonic cancers. The lecture will cover this evidence. Serrated polyps can be detected at CT colonography. This relatively recently recognized cancer precursor typically presents as a subtle flat polyp which is often right-sided. Although people have theorized that the characteristics of these lesions (i.e., flat nature) would preclude CTC detection, the clinical experience over the past 6-8 years has shown otherwise. It is now known that CTC is able to detect these lesions because of the tagging agents utilized in the bowel preparation which adhere to the surface of flat serrated polyps. These lesions then present as a flat plaque of barium contrast which has soft tissue undermining the contrast tag.(7) This lecture should provide an overview of CT colonography for the participant, covering some of the major issues regarding this colorectal cancer screening test. With the recent approval by USPSTF which now aligns with the approval from the American Cancer Society, CTC use should hopefully improve upon the current screening rates for colorectal cancer.