DOI:10.2214/AJR.05.1643
AJR 2007; 189:W1-W3
© American Roentgen Ray Society
Malignant Rectal Polyp Overlooked on CT Colonography Because of Retention Balloon: Opposing Crescent Appearance as Sign of Compressed Polyp
Eugene K. Choi1,2,
Seong Ho Park1,
Dae Yoon Kim1 and
Hyun Kwon Ha1
1 Department of Radiology and Research Institute of Radiology, University of
Ulsan College of Medicine, Asan Medical Center, 388-1 Poongnap-Dong,
Songpa-Gu, 138-040 Seoul, Korea.
2 Present address: Weill Medical College of Cornell University, New York,
NY.
Received September 15, 2005;
accepted after revision November 3, 2005.
Address correspondence to S. H. Park
(seongho{at}amc.seoul.kr).
WEB
This is a Web exclusive article.
Keywords: colon cancer • colon cancer screening • colonography • CT colonography • false-negative • rectal polyp
Introduction
Obscuration of rectal lesions by a rectal catheter or by its retention
balloon is a well-established diagnostic pitfall at barium enema examination
that often requires deflation of the balloon or removal of the catheter to
obtain an unobscured view [1].
Rectal lesions missed at CT colonography (CTC), on the other hand, are rare
because of the relatively small size of the rectal catheter, the proclivity of
the rectum for adequate luminal distention, and the fact that a retention
balloon is not needed in most patients
[2]. Although a retention
balloon is not necessarily required for CTC, it may be helpful for optimal
colonic distention in a select group of patients with insufficient anal
sphincteric tone. Its use, however, may pose another potential source of
false-negative results. A distended retention balloon may compress a rectal
polyp against the wall of the rectum, thereby altering the gross morphology of
the lesion and allowing the polyp to escape detection on CTC review.
To our knowledge, the occurrence of a rectal polyp being compressed by a
retention balloon or of a rectal polyp being compressed to the degree that was
seen in our patient has not been reported. We report a case whereby a 12-mm
polypoid rectal adenocarcinoma arising in a tubular adenoma was missed both
prospectively and retrospectively at CTC performed using an inflated retention
balloon. We also suggest a finding at CTC that may help correctly identify a
retention balloon–induced compressed polyp.
Case Report
A 57-year-old asymptomatic man was referred to our institution for
abdominal screening including CTC, which consists of CTC for colon cancer
screening and contrast-enhanced scanning of the abdomen for assessment of
other abdominal organs. Bowel-cleansing preparation began 1 day before the
examination: The patient ingested three doses of 200 mL of 5% barium sulfate
suspension and 20 mL of sodium amidotrizoate and meglumine amidotrizoate
(Gastrografin, Schering), each of which was taken immediately after a meal,
and 4 L of polyethylene glycol solution.
A reduced-size catheter of 6.7 mm in diameter (PROTOCO2L administration
set, E-Z-EM) was placed in the patient's rectum by a dedicated CT
technologist. After inflation of a retention cuff, it was gently pulled back
until its proximal end rested on the anal sphincter. The primary criterion for
retention balloon usage in our institution is low sphincteric tone that causes
air leakage and consequently prevents adequate distention of the colon.
Carbon dioxide infused through the rectal catheter by an automatic colon
insufflator (PROTOCO2L colon insufflator, E-Z-EM) was used to distend the
patient's colon. Supine and prone CT scans were obtained using a 16-MDCT
scanner (Somatom Sensation 16, Siemens Medical Solutions) with the following
parameter settings: beam collimation, 16 x 0.75 mm; slice thickness, 1
mm; reconstruction interval, 0.7 mm; beam pitch,1; gantry rotation time, 0.5
second; table speed, 24 mm/s; field of view, to fit; 120 kV; and 50–100
mAs. The mAs setting depended on anatomic locations—that is, an
automatic dose-reduction system (CARE Dose 4D, Siemens) was used. Scanning was
performed after IV injection of 150 mL of iopromide (Ultravist 370, Schering)
at a rate of 2.7 mL/s through a 20-gauge angiographic catheter inserted in an
antecubital vein.
The CT images were reviewed on a CTC system (syngo Colonography, Siemens)
by an experienced gastrointestinal radiologist (> 200 cases with
colonoscopic correlation). When prospective review was performed with primary
2D views, no colonic lesion was found. As per patient request, colonoscopy was
also performed the same day, and a 12-mm sessile polyp was found in the distal
rectum (Fig. 1A). However,
retrospective review of CTC with both 2D and 3D fly-through methods revealed
no apparent lesion.
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Fig. 1B —57-year-old man referred for routine colorectal cancer
screening. Repeat CT without retention balloon. Transverse image obtained
using standard colon window settings (width, 1,500 H; level, –200 H)
shows obvious 12-mm polyp (arrow) on anterior wall of distal rectum
adjacent to rectal catheter.
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Fig. 1C —57-year-old man referred for routine colorectal cancer
screening. Repeat CT without retention balloon. Three-dimensional endoluminal
CT colonography (CTC) image shows sessile polyp (arrow) adjacent to
rectal catheter (arrowheads) in rectum; polyp has same appearance
here as on colonoscopy.
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Fig. 1D —57-year-old man referred for routine colorectal cancer
screening. Initial CT with retention balloon. Transverse image obtained using
standard colon window settings (D) (width, 1,500 H; level, –200
H) and transverse image obtained using intermediate soft-tissue window
settings (E) (width, 400 H; level, 20 H) of CTC show plaque-shaped
structure (arrow, E) on anterior wall of distal rectum that is
significantly compressed sessile polyp. Interface between compressed polyp and
adjacent wall is noted as shallow notches on each side of polyp
(arrowheads, D).
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Fig. 1E —57-year-old man referred for routine colorectal cancer
screening. Initial CT with retention balloon. Transverse image obtained using
standard colon window settings (D) (width, 1,500 H; level, –200
H) and transverse image obtained using intermediate soft-tissue window
settings (E) (width, 400 H; level, 20 H) of CTC show plaque-shaped
structure (arrow, E) on anterior wall of distal rectum that is
significantly compressed sessile polyp. Interface between compressed polyp and
adjacent wall is noted as shallow notches on each side of polyp
(arrowheads, D).
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Fig. 1F —57-year-old man referred for routine colorectal cancer
screening. Initial CT with retention balloon. Three-dimensional endoluminal
view shows interface between compressed polyp and adjacent colonic wall as
pair of opposing, crescentlike depressed structures (arrowheads).
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Repeated scanning covering only the rectum with the patient in the supine
position (given the location of the polyp in the anterior wall on colonoscopy)
was performed without IV contrast material and without a retention balloon.
The results of the second scan showed a 12-mm sessile polyp protruding into
the lumen in the anterior wall of the distal rectum (Figs.
1B and
1C). A measurement obtained
from reformatted sagittal images indicated that the distance of the polyp from
the anal verge was 5 cm.
A second retrospective review of the corresponding region on the initial
scan revealed what appeared to be on 2D view a significantly compressed lesion
bordered by two shallow notches (Figs.
1D and
1E). The interface between the
compressed polyp and adjacent colonic wall is shown on 3D endoluminal view as
a pair of opposing, crescentlike depressed structures
(Fig. 1F). The polyp was
surgically removed, and subsequent pathologic analysis revealed a
well-differentiated rectal adenocarcinoma in the background of tubular
adenoma.
Discussion
CTC is gaining acceptance as a viable option for colon cancer screening. A
large study showed that the detection rate of CTC for clinically significant
colonic lesions was comparable to that of colonoscopy in an average-risk
screening population [3],
although concerns about performance raised by heterogeneity of reported
sensitivities have not yet been resolved
[4]. Inadequate bowel
preparation or distention, flat morphology of polyps, and small size of polyps
have been documented as major sources of false-negative results at CTC
[5].
Although the rectum is generally well evaluated at CTC, in one reported
case [2], obscuration of a
10-mm rectal tubulovillous adenoma was attributed to advanced placement of the
rectal catheter, resulting in a rectal lesion being missed on prospective
review. Our case clearly shows a diagnostic pitfall caused by an inflated
retention balloon that if overlooked may result, in the worst-case scenario,
in a missed diagnosis of significant rectal malignancy. We hasten to point out
that even large polyps can be flattened by a retention balloon to a degree
that allows them to escape CTC surveillance in prospective and even in
retrospective evaluations.
On the first retrospective view, we had difficulty reconciling the large
polyp viewed on colonoscopy with the subtle findings noted on 2D views and the
crescent structures on 3D views; both findings were dismissed as mere artifact
and thus were not correlated to each other. At the time, we could not have
imagined how a polyp of such considerable size could have been flattened to
the degree of virtual obliteration on both 2D and 3D endoluminal views. Only
on closer analysis of the lesion with correlation to the result of the repeat
CT on second retrospective review were we able to determine that the two
opposing crescent-shaped concave structures at 3D endoluminal view were
characteristic, probably specific, CTC features of polyps being compressed by
a retention balloon. Although experience with more cases is necessary before
making any generalizations, logic dictates that such a pattern is highly
suggestive of a rectal polyp being compressed by a retention balloon. Although
the two shallow notches bordering the lesion on the 2D view can also signal
the presence of a balloon-compressed polyp, we believe that the appearance of
the opposing crescent visualized only on the 3D view is a more recognizable
and overt representation than the subtle findings on 2D views. Therefore,
primary 3D review may be more appropriate than 2D review for evaluating the
balloon-compressed portion of the rectum.
Given that the distal rectum is a relative blind spot for colonoscopy
[6], accurate CTC examination
of the rectum is imperative. Although CTC does not necessarily require a
retention balloon, its use seems inevitable in select patients with low anal
sphincteric tone or low tolerance for the colonic insufflation. Appropriate
reassurance of and instructions to patients may help in performing CTC
successfully without a retention balloon. When circumstances require their
use, however, careful evaluation of the balloon-dilated segment using the 3D
endoluminal view for the presence of the previously described opposing,
crescentlike depressed structures may assist in detecting balloon-compressed
rectal lesions. Alternatively, scanning the rectum with the balloon deflated
in at least one position—preferably with the patient in the prone
position to allow air trapping in the rectum—may be necessary.
In conclusion, an inflated rectal balloon can compress and deform an
overlying rectal polyp that is significantly large to the point of nearly
complete concealment. In cases in which a rectal balloon is required for
adequate colonic distention, meticulous survey of the balloon-compressed
region for the characteristic sign of a polyp being compressed on 3D
endoluminal view may help prevent false-negative results.
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- Mulhall BP, Veerappan GR, Jackson JL. Meta-analysis: computed
tomographic colonography. Ann Intern Med2005; 142:635
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- Park SH, Ha HK, Kim MJ, et al. False-negative results at
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Introduction
Case Report
