Letters to the Editor

[Editors note: Arnold C. Friedman, M.D. FACR, Associate Chairman, University of Florida, Shands-Jacksonville Hospital Department of Radiology, wrote us in response to the June-July issue's overview of screening protocols. He cites references at the end of his letter.]

You wrote that "Air Contrast Barium Enema (BE) is not advised as a primary screening procedure." Not advised by whom? It is recommended by the American Cancer Society and the American Gastroenterological Association. It has a payable CPT code. Few published analyses assess all reasonable strategies for average-risk patients. A study from the US Congress Office of Technology Assessment (which assumed a 70% sensitivity for DCBE regardless of lesion size or histology) reached the following costs per life year saved: DCBE every five years ($13,495) and colonoscopy every ten years ($22,715) [1]. A decision analysis of expected life years saved (numerator) and significant complications (denominator) per 100,000 patients drew the following conclusions: DCBE every five years 12,568/345 and colonoscopy every 10 years 12,804/1520 [2].

"The National Polyp Study suggested that (A) BEs find fewer than 50% of the polyps detected by colonoscopy and that (B) specificity of BE is poor with 18% false positives."

In response to finding A, the total number of large lesions (<1 cm) in the national study was small (<25) and almost all of these were approximately 1 cm, so small overestimations of size may have skewed the data. That this is so is suggested by the fact that the sensitivity for polyps <1 cm was HIGHER in this study [3,4]. Other comparisons of colonoscopy and DCBE give far different results, e.g.: sensitivity of 81% and specificity of 96% for adenomas >1 cm [5], sensitivity of 71% and specificity of 98% for lesions larger than 7 mm, with most overlooked lesions between 7 and 10 mm (polyp surveillance population similar to National Polyp Study, but more subjects) [6]. As an aside, sometimes finding fewer small polyps than colonoscopy is good (especially if they are hyperplastic!). Ongoing detection and removal of very small polyps may actually be detrimental because costs escalate and complications increase without evidence that outcomes improve [4].

Regarding point B, another study reports a DCBE false positive rate of 10% for polyps larger than 1 cm [7]. The fact that "false-positive" DCBEs may truly be false-negative colonoscopies cannot be ignored. This may be the case in as many as 50% [4]. A false-positive DCBE diagnosis of carcinoma in experienced hands should be quite rare, but some
reports inappropriately (for screening) include significant disease such as large benign neoplasms or diverticulitis initially called cancer as false positives. For screening, the negative predictive value is more important than the false positive rate (screening mammography has an 80% false positive rate!). One study showed a DCBE negative predictive value of 98% for adenomas larger than 1 cm [8].

Some contend that supplemental flexible sigmoidoscopy is required for DCBE. I do not think so. I would only recommend flexible sigmoidoscopy if the DCBE were suboptimal in the rectosigmoid due to diverticular disease. Studies of the performance of DCBE in the rectosigmoid are not dissimilar from its overall performance [4]. Although the combination of the two should detect more lesions per patient, the consequent outcome improvement is uncertain. Justification of the supplemental procedure fsin a screening context is questionable- it is not essential to identify every lesion, the goal is to select patients with significant abnormalities [4].

An analogy might be drawn between colon cancer screening and breast cancer screening; breast MRI is clearly more sensitive than mammography, but more expensive and less available. No one is yet suggesting we replace screening mammography with screening breast MRI.

In summary, according to the 1997 American Gastro-enterological Association guidelines: "Colonoscopy and DCBE offer alternate ways of examining the entire colonic mucosa……..while colonoscopy detects more small polyps, there is little difference in the ability of these tests to detect large polyps (>1 cm in diameter) [9]."

Most cancers develop in larger polyps, with less than 1% of polyps <1 cm demonstrating malignancy [9].
DCBE is less expensive and safer than colonoscopy. Therefore, DCBE is a better screening test.

"Virtual colonoscopy fails to detect nearly 10% of polyps measuring 10 mm or less in diameter and misses about 5% greater than 10 mm." Virtual colonoscopy has "a sensitivity of 90-92% for the detection of 1 cm colonic polyps when compared to standard colonoscopy." These are pretty good statistics (although unreferenced) for a technique that is still evolving, not an indictment. Why the conclusion "Virtual colonoscopy should not be recommended as an alternative to endoscopic colonoscopy"? Surely not because of these statistics! A study of 300 patients revealed the following: "the sensitivity was 94% (64 of 68) for the detection of adenomas 10 mm or larger, 82% (72 of 88) for adenomas 5.0-9.9 mm, and 66.9% (95 of 142) for adenomas smaller than 5 mm." [10]

I think virtual colonoscopy performed at a center with experience and a track record interpreted by a radiologist with training and experience in the technique is right now sufficiently sensitive and accurate to serve as an acceptable screening test. As less taxing preparation methods, electronic stool subtraction, automated image reconstruction, and computer-aided diagnosis are developed, it will only get better.

Sincerely,
Arnold C. Friedman, M.D. FACR
Associate Chairman, Department of Radiology, University of Florida, Shands-Jacksonville Hospital

1. U.S. Congress, Office of Technology Assessment/. Cost-effectiveness of colorectal cancer screening in average-risk adults. BP-H-146 Washington, D.C. April 1995.
2. Winawer SJ, Fletcher RH, Miller L, et al. Colorectal cancer screening: clinical guidelines and rationale. Gastroenterology 1997; 154:154-160.
3. Winawer SJ, Zauber AG, O'Brien MJ, et al. The national polyp study: design, methods and characteristics of patients with newly diagnosed polyps. Cancer 1992; 70:1236-1245.
4. Glick S. Double-contrast barium enema for colorectal cancer screening: a review of the issues and a comparison with other screening alternatives. AJR 2000; 174:1529-1537.
5. Steine S, Stordahl A, Lunde OC, Loken K, Laerum E. Double-contrast barium enema versus colonoscopy in the diagnosis of neoplastic deisorders: aspects of decision-making in general practice. Fam Pract 1993; 10:288-291.
6. Williams CB, Macrae FA, Bartram CI. A perspective study of diagnostic methods in adenoma follow-up. Endoscopy 1982; 14:74-78.
7. Ott DJ, Scharling ES, Chen YM, Gelfand DW, Wu WC. Positive predictive value and post-test probability of diagnosis of colonic polyp on single- and double contrast barium enema. AJR 1989; 153:735-739.
8. Kjaergard H, Nordkild p, Hennild V et al. Follow-up study after colorectal polypectomy: the predictive value of a negative double contrast barium enema. Scand J Gastroenterol 1986; 21:353-356.
9. Winawer SJ, Fletcher RH, Miller l, et al. Colorectal cancer screening: clinical guidelines and rationale. Gastroenterology 1997; 112:594-642.
10. Yee J, Geetanjali A, Akerkar A, et al. Colorectal Neoplasia: Performance Characteristics of CT Colonography for Detection in 300 Patients. Radiology. 2001;219:685-692.

Jacksonville Medicine / November-December 2002
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