INTRODUCTION

Recommendations to patients that polyps smaller than 1 cm, detected by computed tomographic colonography (CTC), lack importance and can be ignored for long intervals may subject patients to colorectal cancer risk. Many patients will be lost to follow-up. Following small polyps by repeated CTC will subject patients to radiation and involve high costs. There is insufficient information on the natural history of small polyps to construct ideal recommendations, but a reasonable compromise would be to offer colonoscopy to healthy patients who have polyps 6 mm or larger or who have three or more polyps of any size on CTC. Patients with only one or two polyps smaller than 5 mm could be referred for colonoscopy or considered normal.

WHY SHOULD ALL POLYPS BE REMOVED?

There is nothing radical about the recommendation that patients with polyps smaller than 1 cm detected by a colorectal cancer screening test should be offered colonoscopy and polypectomy. Rather it is radical to suggest that none of these patients need to be offered colonoscopic polypectomy. I recall very little discussion of whether healthy patients with polyps smaller than 1 cm detected by barium enema should be referred for polypectomy and it is common practice to remove such polyps. Further, the idea that tubular adenomas smaller than 1 cm (¹) with only low-grade dysplasia detected by flexible sigmoidoscopy may be ignored never really took hold and most patients with adenomas of any size detected by flexible sigmoidoscopy are still referred for colonoscopy (²). Since 95% of all colon polyps are smaller than 1 cm in size and since all polyps seen during colonoscopy are removed, it is a major paradigm shift to institute a policy of leaving most colorectal neoplasms in place. Such a policy would be truly radical in view of the evidence that the reduction in colorectal cancer incidence via endoscopy appears entirely dependent on polypectomy (^3,4,5,6).

Next, a decision to not perform colonoscopy in patients with positive screening tests eliminates the effect of colonoscopy on diagnosing synchronous lesions missed by the screening test. Patients with a polyp detected by barium enema often have additional polyps, some of which may be very significant lesions, detected by colonoscopy. Likewise, some advanced adenomas and cancers detected by colonoscopy in patients with positive fecal occult blood tests are not the cause of the bleeding (⁷). Therefore, if we avoid diagnostic colonoscopy in the majority of patients with positive screening tests we will lose this benefit of colonoscopy. This argument may take on increased relevance if the role of flat lesions in the polyp cancer sequence is indeed important (^8,9,10,11), since CTC's sensitivity to flat lesions is low (¹²).

COST-EFFECTIVENESS OF CTC?

There is no doubt that this discussion is driven in large part (particularly when made by radiologists and likely less so when made by Dr. Ransohoff) by the recognition that the cutoff size for referring patients with polyps detected by CTC must be fairly high or CTC is rendered cost-ineffective. Thus, at a cutoff size of 1 cm, about 10% of the screened population will be referred for polypectomy, whereas at 6 mm, 30% or more will be referred (¹³). For an expensive diagnosis-only test the "cutoff" may well determine the cost-effectiveness of the test. If all patients with polyps at CTC are referred for colonoscopy, then CTC is not viable from a cost perspective as after only one or two rounds of screening the great majority of the population will have been referred for colonoscopy. Much of the reason the cost discussion rarely came up with barium enema is that barium enema is relatively inexpensive, and thus at any level of referral for polypectomy, cost was not a major issue. However, it is problematic to permit any discussion regarding the optimal cutoff size for CTC to be driven primarily by notions of which size will make CTC cost-effective.

It should be clear that we are not only discussing what size polyp should be referred for polypectomy but also what size polyp will be considered normal and treated the same as a normal or negative examination. Thus, the most common scenario I have seen with CTC and small polyps that are not referred for colonoscopy is the recommendation that CTC be repeated to monitor the polyp, and some have suggested that the test be repeated within 1–3 yr. This raises very significant issues regarding cost as well as cancer risk and radiation risk (¹⁴). Decision analysis experts should consider whether the practice of monitoring small lesions is cost-effective and at what frequency it would remain cost-effective. It seems disingenuous to suggest that a polyp does not need removal and then proceed to monitor it closely. Rather it seems appropriate that polyps which are not going to be removed should not be monitored, i.e., the patient's next exam will be at the normally recommended interval of 10 yr. It should be recalled that CTC must be performed at 10-yr intervals to be cost-effective (^15,16). In addition, planning repeat CTC to follow small polyps exposes patients to the risk of being lost to follow-up.

ARE ALL SMALL POLYPS EQUAL?

I was asked to argue that all patients with small polyps detected by CTC should be referred for colonoscopy and polypectomy. Admittedly, to take this tack is to essentially argue that CTC is useless. As noted earlier, an expensive diagnosis-only test that results in nearly all patients with polyps detected being eventually referred for colonoscopy and polypectomy is a test that is so cost-ineffective that its use makes no sense. Is there some compromise position that provides a high level of safety for patients and still maintains CTC as a potentially cost-effective screening strategy? To optimally design such a position we would need a clear understanding of the natural history of small polyps, and such an understanding does not exist. CTC provides information on polyp size, number, and shape but not on polyp histology. A reasonable compromise position might be estimated from what is known about the associations of polyp size with histology and the association of the number of polyps with clinical outcome. For example, polyps smaller than 5 mm have a very low risk of invasive cancer or high-grade dysplasia, and villous elements are uncommon (^17,18,19,20). Thus, it may be appropriate to leave them in place for a while. For polyps measuring 6–9 mm the risk of invasive cancer has been estimated to be as high as 0.9%, the risk of high-grade dysplasia about 4%, and 10% or more will have villous elements (^17,18,19,20). Some will consider those numbers to be low, but many patients and primary care physicians will consider them sufficiently high that they prefer to have such lesions removed. Indeed, the argument that we do not know the natural history of these lesions and therefore do not need to remove them (²¹) can be easily reversed: since we are not sure that such lesions will remain benign over a 10-yr (or even 5-yr) interval, we should remove them based on what we do know regarding their histology.

Size is not the only factor associated with cancer risk in adenomas or with the concomitant or subsequent occurrence of other advanced adenomas (²²). As noted above, high-grade dysplasia and villous elements are associated with cancer risk. None of these histologic risk factors in colon polyps can be identified on CTC studies. However, the presence of three or more adenomas has been the most consistent predictor of subsequent advanced adenomas in post-polypectomy studies (²²). Thus, it seems appropriate to recommend that three or more polyps of any size detected by CTC should also constitute a reason to refer for colonoscopy and polypectomy. The argument here is that since these multiple polyps might be adenomas, the determination by polypectomy that they are adenomas would indicate the need for closer follow-up.

A REASONABLE COMPROMISE

As a compromise position then, it seems reasonable to suggest that colonoscopy and polypectomy should be offered at least to healthy patients with polyps 6 mm or larger or who have three or more polyps of any size. This is not to say that some patients with one or two polyps smaller than 5 mm should not undergo colonoscopy. Indeed, it would seem better to remove these polyps and plan follow-up based on histology and number of polyps than to follow them by repeated CTC. In general, monitoring patients with polyps smaller than 1 cm by repeated CTC will subject them to colorectal cancer risk, radiation risk, and very high costs.

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