CT colonography has been developed as a less invasive alternative to colonoscopy to decrease perforation-related morbidity and mortality and potentially to increase population adherence with colorectal cancer screening. However, we demonstrate that primary screening with CT colonography is more expensive and leads to more overall deaths than colonoscopy. Our conclusions are sensitive to CT colonography's test performance characteristics, the malignancy risk of missed adenomas, the perforation risk and perforation-related mortality estimates for colonoscopy, the procedural costs, and differences in screening adherence.
It is difficult to know what the test performance characteristics of CT colonography will ultimately be in routine clinical practice. However, even if they rival those of colonoscopy, we have shown that the cost of a CT colonography strategy prohibits recommending it as a primary method for colorectal cancer screening. Even in the best-case scenario, the cost per life-year gained was unattractive by conventional standards.34,35
It should be noted that Pickhardt and colleagues11
used a primary 3-dimensional approach for the detection of polyps rather than a primary 2-dimensional approach with 3-D reconstruction for problem-solving. Although this enhanced technology may increase polyp detection rates, it may also raise the cost of CT. The higher performance of this technology also needs to be confirmed by others.
Our results remained robust when plausible changes in the malignant potential of adenomas missed with CT colonography were considered. Although the incremental cost per life-year gained was $42 900 when the risk of a missed 6–9 mm adenoma was 0, this is unlikely to be realistic. In fact, the real malignant risk of missed adenomas may be higher than we modelled, given that the natural-history studies cited contained a mixture of hyperplastic (no malignant potential) and adenomatous polyps. Furthermore, the risk of a missed adenoma 10 mm or more in size may be much higher than the 1.5% risk suggested by Stryker and coworkers.19
In the 3-year surveillance arm (n
= 428) of the National Polyp Study, 2 cancers within large adenomas and 12 additional advanced nonmalignant adenomas of 10 mm or more were found at 3 years of follow-up.18
If we assume that these 14 large adenomas were present and missed at the baseline exam, which is a reasonable assumption, the 3-year malignant risk of a missed large adenoma may be as high as 2 in 14, or 14%.
Although a 1-in-1000 risk of perforation from diagnostic colonoscopy is generally accepted,36
the risk may be significantly lower, especially when performed by trained gastroenterologists in an otherwise healthy outpatient screening population.27,37,38
In a large prospective study involving healthy outpatients,37
the perforation rate was 0.005% for diagnostic and 0.06% for therapeutic colonoscopy. In contrast, the risk of complications may be higher in centres with less experienced endoscopists.37
We demonstrated that substantially increasing the risk of perforation or the risk of death from perforation resulted in a reasonable incremental cost-effectiveness ratio for CT colonography. Therefore, the choice of screening strategy in a given institution may depend on a balance between the availability and local quality of CT colonography and the experience and complication rates of available endoscopists and surgeons.
Including an estimate of indirect costs from predicted lost wages was shown to be important in our sensitivity analysis. Further research is needed to determine the actual indirect costs associated with colorectal cancer screening.
Lastly, using CT colonography to screen for colorectal cancer offers the potential to enhance screening adherence. Data in this area are lacking, however. Some studies have reported increased patient satisfaction with CT colonography over colonoscopy11,39
but also increased overall discomfort with CT colonography.11,40
A recent community-based study41
in which CT colonography was offered to nearly 1500 asymptomatic screening candidates had a participation rate of only 28%. Therefore, even if CT colonography can increase participation for colorectal cancer screening, the overall impact might be small. Furthermore, we have shown that the cost per life-year gained remains excessive even with a large increase in screening adherence.
Our study has limitations. First, the 3-year period may be considered by some to be a limitation of our study design; but we feel, as outlined in the methods section, that key data are unavailable to permit a longer time horizon without increasing the degree of speculation and amplification of errors over time. Furthermore, the short-term cost of adopting CT colonography is an important consideration for health policy decision-makers. Second, we assumed that all cancers arising from missed adenomas would be early-stage and identified and promptly treated at the 3-year mark. This represents an ideal scenario for the strategy with the higher miss rate (CT colonography). Many early cancers are asymptomatic and can progress in stage before being diagnosed, if the next screening is delayed. Third, only polyps larger than 5 mm were considered clinically significant. However, it is possible that, in practice, polyps 5 mm or smaller identified by means of CT colonography would not be ignored, leading to a colonoscopy referral or a repeat CT colonography within a shorter time interval. Both would increase the cost of a CT colonography strategy. Finally, we did not incorporate a measure of quality of life (e.g., health utility, a measure of overall quality of life ranging from 0 to 1) into the model, because the data are either limited or do not yet exist. Their incorporation would be highly unlikely to change our overall conclusions, however.
In conclusion, CT colonography does not appear to be cost-effective for primary colorectal cancer screening in Canada. Although perforation-related mortality can be reduced, this is counterbalanced by excess cancer-related deaths from missed adenomas. Even if the test performance characteristics of CT colonography could eventually rival those of colonoscopy, the current cost of a screening strategy involving CT colonography far exceeds what most would consider good value for health care money. CT colonography has a potential role in centres where the risks of colonoscopy are high or in patient populations with high operative mortality. Finally, given that the results of our analysis appeared sensitive to the inclusion of indirect cost estimates, further research into determining the actual indirect costs associated with colorectal cancer screening is warranted.