Abdom Imaging 20:381-386 (1995) Abdominal Imaging

�9 Springer-Verlag New York Inc. 1995

The q u e r y c o r n e r

I. I. Rabi, the Nobel prize-winning physicist, has told of an early influence upon his sense of inquiry: On returning from grade school each day, his mother would ask, not "Did you learn anything today?," but rather "Did you ask a good question today?"

Readers are urged to contribute questions intended to elicit a focus of illumination from an authority. They should often be directed toward "How?" or "Why?", bridging the field of imaging with normal and pathologic anatomy, physiology, biochem- istry, and other clinical disciplines, and may be accompanied, if necessary, by a single illustration and up to three references. If authors wish to have their questions published anonymously, this should be indicated when the question is submitted. The selection of questions published remains an editorial decision. Items published in The Query Comer will be covered in MEDLINE/Index Medicus.

Screening for colorectal cancer and suspected lower gastrointestinal bleeding

A Two sets of questions are posed:

Implications of Right- and Left-sided Colonic Neoplasms

In the controversy over the uses of colon- oscopy and double contrast barium enemas (DCBE), the plethora of statistics presented seems to raise a few highly salient inquiries: Screening for colorectal cancer: Screening with flexible sigmoidoscopy seems to gainsay the re- ality that up to 40% of colorectal carcinomas oc- cur proximal to the splenic flexure [1, 2]. I f the discovery on sigmoidoscopy to only 35 or 60 cm of an " index polyp" iS presently considered an indication for complete colonoscopic examina- tion, what assurance does the absence of an "in- dex po lyp" provide regarding the remainder of the colon? Is further study recommended? Suspected lower gastrointestinal bleeding: The success rate for total colonoscopy with cecal in- tubation and visualization is currently above 90% [3-6] providing not only the identification but also treatment approaches to right colonic cancers, pol- yps, and angiodysplasias. On the other hand, about 10% of attempted colonoscopies are unsuccessful for a variety of reasons [3-5]. What is the role of DCBE in these circum- stances?

The Editor-in-Chief

The implications of left-sided adenomas have recently been discussed by Atkin and colleagues [7]. As Table 1 shows, only one-third of subjects with adenomas prox- imal to the splenic flexure have index adenomas distally [8-12].

In the UK, about 65% of colorectal cancers lie within reach of the flexible sigmoidoscope and the fig- ure in the US may be nearer 50%. Calculations show that flexible sigmoidoscopy used once only between the ages of 55 and 65, might prevent 5,628 of 17,781 co- lorectal cancers in the UK or 5,628 of the 12,447 sub- jects screened--representing 31% and 45% respec- tively. These calculations were based on 64% of cancers being distal, and a 70% compliance. Is this a reasonable proposi t ion?--Namely, to consider a screening pro- gram that uses a tool that is unable to reach more than perhaps 50% of tumors, with a maximum goal of pre- venting only 30% of tumors. Some who favor colon- oscopy or barium enema may argue that the tool se- lected for screening must be at least capable of detecting tumors in any segment of the bowel. To suggest flexible sigmoidoscopy as a screening tool may seem at first glance like recommending screening mammography on only one breast, but the issue is more complex. The risks of two-sided mammography are no more than one- sided, and the potential yield is doubled. Flexible sig- moidoscopy is safe and can detect the 50% of lesions on the left, and lead to the detection of 30% of those on

382 The query comer

Table 1. Relation of proximal to distal adenomas in subjects undergoing screening colonoscopy. (Reprinted from [7] with permission.)

Number (%) of subjects Distal adenomas Total subjects with proximal adenomas

Present 97 32 (31) Absent 407 71 (69) Total 504 103

the right, for a total of 67% detection. Completing ex- amination of the colon with colonoscopy quadruples the cost, and introduces a mortality of 1 in 5,000 to the screening procedure. Furthermore, screening tools with far less sensitivity and specificity than flexible sig- moidoscopy have been strongly advocated. For exam- ple, fecal occult blood testing (FOBT) is positive in 2 - 6% of subjects, and only 5 - 1 0 % of those who test positive have colorectal cancer. Worse, of those who have colorectal cancer, 30 -50% will test negative to FOBT, and 75% of those with large adenomas will test negative. And yet, FOBT is accepted as an acceptable screening method. There is general acceptance of poor specificity and sensitivity if the test is both cheap and safe. Flexible sigmoidoscopy occupies an intermediate position symptoms. The patients with a high expectation of disease can reasonably be subjected to more expen- sive and dangerous tests with greater sensitivity and specificity than will be acceptable for screening. There is reasonable evidence for effectiveness of flexible sig- moidoscopy in screening for colorectal cancer in a low risk population, and further research on this is amply justified. It is much harder to justify colonoscopy as a screening tool for low risk people. Barium enema is again in an intermediate position, but radiologists have not yet carried out any large scale studies to confirm that barium enema can achieve in a screening context the excellent results that academic centers have reported over the years in clinical practice.

I f the discovery of an index polyp on flexible sig- moidoscopy is considered an indication for complete colonoscopy, what assurance does the absence of an in- dex polyp provide? Limited assurance indeed, as shown in the data given above; but is the discovery of a distal polyp necessarily an indication for complete colonos- copy? What is the significance of a polyp in the distal bowel?

This has also been addressed on both sides of the Atlantic [11, 12]. In the St. Marks study, subjects with small ( < 1 cm) tubular adenomas were not at increased risk of developing colon cancer in the long term (Table 2). Subjects with either large or villous adenomas were at a 4-fold risk: 90% of the colon cancers that developed during follow-up were in this group.

Table 2. Risk of colon cancer after removal of adenomas via the rigid sigrnoidoscope. (Reprinted from [7] with permission.)

Colon cancer

Rectosigmoid Number of Number of Relative risk adenoma type patients (%) patients (%) (95% cl)

Small tubular adenomas < 1 cm 776 (48) 4 (11) 0.5 (0.1-1.3)

Tubulo villous, villous, or > 1 cm 842 (52) 31 (89) 3.6 (2.4-5.0)

Total 1618 35 2.1 (1.5-3.0)

Thus a case can be made that tiny adenomas are of no clinical significance as they are not associated with any increased risk of cancer development. Moreover, small adenomas are in some circumstances reversible lesions, and are not associated with the changes in RAS oncogenes that are seen in advanced adenomas and can- cers. The role for radiologists (and colonoscopists) is not the detection of 3 - 5 mm polyps, which is an ex- pensive exercise in futility, but rather the detection of significant adenomas of 6 - 7 mm and larger. Thus the finding of a 3-mm polyp in the rectosigmoid is not a good sole indication for colonoscopy, and barely war- rants the risk of a major complication.

Is a further study indicated when flexible sig- moidoscopy (or colonoscopy or barium enema) is to- tally negative? It depends on the clinical situation and specifically on estimating the risk of disease being present. Colonoscopy at its best misses about 10% of significant polyps. Barium enema at its best misses about 1 0 - 2 0 % of significant polyps. The figure for flexible s igmoidoscopy is unknown but it is probably 10% with good bowel preparation, and higher when preparation fails. Thus, in a patient with rectal bleed- ing and no hemorrhoids, or iron deficiency anemia without rectal bleeding, further examination is clearly indicated when one of the above investigations is neg- ative. The same may be true in a subject at very high risk for inherited colon cancer who is being screened, although in this case a repeat examination may be deferred for 2 years since the risk is long term rather than immediate. Indeed, i f the pre-test chance of find- ing an adenoma is 50% or more, then colonoscopy should probably be the initial diagnostic test. In a sub- ject at low risk, a negative colonoscopy or bar ium enema may be taken at face value.

Implications of the Success Rate and Completeness of Right Colon Examination by Radiology and Endoscopy

Colonoscopy is unsuccessful in 10% of cases. Is there a role for bar ium enema in such patients? The failure

The query comer 383

ra te o f c o l o n o s c o p y is ha rd to ascer ta in . In the bes t hands it m a y be under 5%, but 10% is a c o m m o n l y exp re s sed figure. H o w e v e r mos t repor t s are o f co lon- o scop ie s k n o w n to be i n c o m p l e t e b y the examine r , and s tudies o f p a t h o l o g i c a l ma te r i a l are few. Colon- o s c o p y m a y be i n c o m p l e t e when the e x a m i n e r be- l i eves i t to be comple te , in one s tudy in 6% of al l examina t i ons , and the uppe r marg in o f a t u m o r m a y be m i s t a k e n for the end of the colon. A recen t r epor t b a s e d on ana lys i s o f al l co lo rec t a l cancers in 1990 in one r eg ion r e v e a l e d that when b a r i u m e n e m a was the first e x a m i n a t i o n the cancer was o v e r l o o k e d in 24% of cases , and that when c o l o n o s c o p y was the first ex- amina t i on the cancer was o v e r l o o k e d in 15% of cases. These ho r r i fy ing f igures ind ica te that the miss rates p u b l i s h e d by m a j o r cen te rs ca re fu l ly r e v i e w i n g the i r w o r k m a y bea r l i t t le r e l a t ion to the rea l i t i es o f com- m u n i t y m e d i c a l p rac t ice , and that there are subs tan t ia l p r o b l e m s wi th the qua l i ty o f c o l o n o s c o p y and b a r i u m enemas . P r o b l e m s wi th qua l i ty o f b a r i u m studies are we l l k n o w n to those who conduc t pee r r ev i ew audits , but the l eve l o f qua l i ty of c o l o n o s c o p y is less appa ren t as there are no c o m p l e t e v i sua l records , and a pauc i ty o f p a t h o l o g y - b a s e d rev iews . It is t rue that in te res ted cen te rs can de tec t co lo rec t a l cancer on b a r i u m e n e m a in excess o f 95% o f cases , and the same is a lmos t ce r t a in ly t rue for c o l o n o s c o p y , but there m a y be a l a rge r gap b e t w e e n the o p t i m u m and the c o m m o n re- a l i ty than has been gene ra l l y apprec ia t ed . Fo r rad i - o log i s t s , the mos t impor t an t so lu t ion is doub le r ead- ing o f f i lms as the vas t ma jo r i t y o f errors are pe rcep t ive . The techniques for h igh qua l i ty D C B E s have been e s t ab l i shed for a lmos t 40 years , but there is s t i l l a need for r e f r e she r courses , at a t ime when in te res t m a y be wan ing in r e s i d e n c y p r o g r a m s in f avor o f n e w e r t echno log ies .

Whe the r there is a role for ba r ium enema when co- lonoscopy fails thus depends on the s tandards of ba r ium enema practice. W h e n it is high, then the answer is that every fai led co lonoscopy should be fo l lowed by DCBE. Usual ly this can be per formed after the pat ient has res ted for an hour, but i f the patient is very deeply se- dated or i f sessi le po lyps have been removed , the radi- o logical examinat ion can be deferred until the fo l lowing morning. In those patients with negat ive co lonoscopy and ba r ium enema, angiography and/or nuclear medi- cine studies wil l somet imes be helpful in reveal ing an- g iodysplas ia , M e c k e l ' s d iver t icula or small bowel tu- mors, and examinat ion of the upper GI tract should not be omit ted as duodenal ulcers and even esophagi t is may rarely present with rectal b leeding, though more often with anemia.

Gi les S t e v e n s o n , M .D .

McMas te r Univers i ty Medica l Center Hamil ton , Ontario, Canada

References

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2. Kelvin FM, Maglinte DDT, Stephens BA. Colorectal carcinoma detected initially with barium enema examination: site distribu- tion and implications. Radiology 1988;169:649-651

3. Rex DK, Weddle RA, Lehman GA, et al. Flexible sigmoidoscopy plus air contrast barium enema versus colonoscopy for suspected lower gastrointestinal bleeding. Gastroenterology 1990;98:855-61

4. Waye JD, Boshkoff E, Total colonoscopy: is it always possible? Gastrointest Endosc 1991;37:152-4

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6. Marshall JB, Barthel JS. The frequency of total colonoscopy and terminal ileal intubation in the 1990s. Gastrointest Endosc 1993;39:518-520

7. Atkin WS, Cuzick J, Northover JMA, Whynes DK. Prevention of colorectal cancer by once only sigmoidoscopy. Lancet 1993;341:736-40

8. Lieberman DA, Smith FW. Frequency of isolated proximal co- lonic polyps among patients referred for colonoscopy. Arch Int Med 1988;148:473-475

9. Foutch PG, Mai H, Pardy K, et al. Flexible sigmoidoscopy may be ineffective for secondary prevention of colorectal cancer in asymptomatic middle aged men. Dig Dis Sci 1991 ;36:924-928

10. Johnson DA, Gurney MS, Volpe Rj et al. A prospective study of the prevalence of colonic neoplasms in asymptomatic patients with age related risk. Am J Gastroenterol 1990;85:969-974

11. Rex DK, Lehman GA, Hawes Rh et al. Screening colonoscopy in asymptomatic average risk persons with negative fecal occult blood tests. Gastroenterology 1991;100:64-67

12. Atkin WS, Morson BC, Cuzick J. Long term risk of colorectal cancer after excision of rectosigmoid adenomas. N Engl J Med 1992;326:658 -662

13. American Society of Gastrointestinal Endoscopy. The role of co- lonoscopy in the management of patients with colonic polyps. Gastrointest Endosc 1988;34:6S-7S

Complications of barium enema study following colonoscopic biopsy

Q Is there significant r isk in per forming a bar ium enema study after a co lonoscopic bi- opsy? Wha t interval of t ime provides a safe- guard? Is there a difference be tween single and double contrast enemas?

The Editor-in-Chief

The radiographic examinat ion (double contrast ba r ium enema [DCBE]) can immedia te ly fo l low if the p roc tos igmoidoscopy or co lonoscopy was per formed