Is there still a role for CT colonography?

Despite nearly 15 years of clinical availability, computed tomography colonography (CTC) remains an infrequently utilized method of colorectal cancer (CRC) screening. High-quality trials and meta-analyses demonstrate comparable test performance between CTC and colonoscopy to detect colorectal polyps in average-risk individuals, especially for polyps 1 cm or greater. Variation in insurance coverage, limited endorsement by gastroenterology and other medical society guidelines, along with the lack of recommendation from the Centers for Medicare and Medicaid Services have contributed to its current circumscribed role.

Today, CTC is most commonly performed following incomplete colonoscopy supplanting the role of barium enema. Additionally, patients with high procedural risk for colonoscopy may qualify for frontline CTC screening; specifically, those with advanced chronic obstructive pulmonary disease (COPD), adverse reactions to anesthesia or high bleeding risk. In this capacity, CTC may reduce the number of patients exposed to sedation and procedural risk in this high-risk cohort, although many patients with abnormal findings will still need colonoscopy for definitive diagnosis and therapy. Non-screening uses for CTC, for example as a substitute for colonoscopy in post-CRC resection surveillance, are under investigation. Availability of CTC for these indications or initial screening test for average-risk individuals varies based on local availability, expertise and insurance coverage. In addition, new tests, such as stool DNA, have entered the marketplace as potentially attractive options for patients and ordering providers interested in non-invasive screening.

CTC reliably visualizes the entire colorectum, does not require sedation and, unlike colonoscopy, poses little to no risk of perforation or bleeding.  Like any diagnostic test, CTC has potential disadvantages, several of which are unique to CTC itself. Incidental, extra-colonic findings are identified in 40 to 70 percent of cases. Many of these findings do not require follow up testing, based on predefined radiology benchmarks to guide management (C-RADS criteria).  However, as many as one-third will require further diagnostic and sometimes invasive testing with no guarantee that eventual outcomes will improve. Other disadvantages of CTC include radiation exposure and cost, especially considering the increased frequency of CTC testing as compared to colonoscopy. Interval screening after index CTC is recommended every five years as compared to 10 years with colonoscopy screening. The average radiation exposure per test has decreased with improving CT technology, however the potential risk of malignancy remains a concern. In addition, reports from structured screening programs have found as many as one-third of all patients screened initially with CTC require a follow up therapeutic colonoscopy for CTC finding. At current market pricing, CTC is not as cost-effective for population-based screening as other methods like fecal immunochemical testing.

Flat polyps, generally arising in the right colon, are implicated as a major cause of interval CRC in patients who pursue colonoscopy-based screening. This concern exists, and may be of even greater concern, for CTC given the test’s decreased sensitivity for flat polyps when compared to colonoscopy. Risk is partially mitigated by use of barium stool tagging thereby increasing CTC specificity. Like colonoscopy, the efficacy of the test is influenced by the operator as well as protocols used for pre-procedure preparation. However, metrics of test quality, such as polyp detection rates or rate of incomplete preparations limiting examinations, are not as robust as those for colonoscopy.

Since CTC has no therapeutic capacity, establishing clinically relevant cutoff values for polyps detected on examination is essential to minimize unnecessary colonoscopy utilization considering the latter’s attendant costs. The one-time sensitivity and specificity of CTC for polyps more than 1 cm is higher than stool-based testing and is similar to colonoscopy. Radiology specialty guidelines advocate that polyps less than 6 mm do not need to be reported. Proponents of this practice claim these smaller polyps do not harbor advanced histology, especially cancer, frequently enough to require routine removal. Controversy over the absolute risk of CRC from sub-centimeter polyps exists, especially in the 6 mm to 1 cm range. By extension, when to refer a patient for therapeutic colonoscopy, rather than repeat surveillance imaging, remains an unresolved issue for many patients and physicians.

Given the availability of multiple average-risk screening strategies, it is not clear where CTC should fit. Recent screening guidelines from the U.S. Preventive Services Task Force and U.S. Multi-Specialty Task Force include CTC, but not as a primary choice. It is important, however, to take patient preference and satisfaction into account as these presumably influence screening adherence over time, a key feature in longitudinal CRC screening which can span decades. For non-invasive methods, stool-based testing (blood or DNA) offers an alternative screening practice that avoids issues of harm inherent to CTC and colonoscopy. On the contrary, due to its higher specificity, CTC has fewer false-positive results than stool-based tests, thereby potentially decreasing the need for unnecessary colonoscopy. CTC has a higher one-time possibility for polyp detection compared to stool-based studies although head-to-head comparisons of the two methods are lacking.

Since issues of safety, cost, and compliance all enter into shared decision-making for CRC screening, for a small fraction of the population — particularly patients at high-procedural risk — CTC may represent the preferred modality of choice. However, absent new data or a reduction in cost, other tests will continue to dominate the average risk screening landscape.

Key takeaways:

  • CTC is included in the major CRC guidelines, but its exact role remains limited.
  • Significant barriers of use remain for CTC including issues of insurance reimbursement for using CTC as a primary CRC screening test. Variations in availability and test performance may also limit widespread adoption of CTC as a primary screening tool.
  • CTC should be considered for patients with incomplete colonoscopy, supplanting the role of barium enema. Until further data becomes available further addressing cost effectiveness, issues of safety such as radiation exposure and management of incidental extra-colonic lesions, and test performance compared to other CRC screening methods, CTC will likely continue to play a limited role in the landscape of CRC screening.

 

Dr. Weinberg has no conflicts to disclose. Dr. Gotfried has no conflicts to disclose.

References

1. Lin,  J.S., Piper,  M., Perdue,  L.A.,  et al.  Screening for colorectal cancer: A systematic review for the U.S. Preventive Services Task Force: Evidence Synthesis No. 135. Agency for Healthcare Research and Quality publication. 2016;14-05203-EF-1.

1. Plumb, A.A., Halligan, S., Nickerson, C., et al. Use of CT colonography in the English Bowel Cancer Screening Programme. Gut. 2013;63:964-973.

1. Pickhardt, P.J. Imaging and screening for colorectal cancer with CT colonography. Radiol Clin North Am. 2017;55:1183–96.

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