AJR 2005; 184:1581-1583
© American Roentgen Ray Society
Adenomatous Polyp Obscured by Small-Caliber Rectal Catheter at Low-Dose CT Colonography: A Rare Diagnostic Pitfall
Perry J. Pickhardt1,2 and
J. Richard Choi2,3
1 Department of Radiology, University of Wisconsin Medical School, E3/311
Clinical Science Center, 600 Highland Ave., Madison, WI 53792-3252.
2 Department of Radiology, Uniformed Services University of the Health Sciences,
Bethesda, MD.
3 Department of Radiology, Walter Reed Army Medical Center, Washington,
DC.
Received May 25, 2004;
accepted after revision August 10, 2004.
The opinions and assertions contained herein are the private views of the
authors and are not to be construed as official or as reflecting the views of
the Department of the Navy or the Department of Defense.
Address correspondence to P. J. Pickhardt.
Introduction
Obscuration of rectal lesions by the rectal catheter or its retention
balloon at air–contrast barium enema represents a known diagnostic
pitfall. By comparison, the preferred rectal catheter used for gaseous
distention of the colon at CT colonography (CTC) is smaller in caliber and
does not necessarily require a retention balloon. If a balloon is used, it is
generally much smaller than those used for barium enemas. Furthermore, a
significant rectal polyp is less likely to be missed at CTC, not only because
a less obtrusive catheter is used, but also because both supine and prone
scans are obtained and both cross-sectional and volume-rendered displays are
evaluated. We report a case of a 10-mm rectal tubulovillous adenoma that was
missed prospectively at CTC due to inopportune positioning of the rectal
catheter. This false-negative CTC finding may have been avoided by less
advanced placement of the catheter tip beyond the anal verge.
Case Report
A 66-year-old asymptomatic woman referred for routine colorectal screening
elected to participate in a clinical trial evaluating low-dose CTC, followed
by same-day optical colonoscopy (OC) for comparison. The patient underwent a
bowel-cleansing regimen beginning the day before the examinations that
consisted of oral sodium phosphate solution, 2% barium sulfate suspension, and
water-soluble iodinated contrast material (diatrizoate), as previously
described [1]. Colonic
distention was achieved by patient-controlled insufflation of room air using a
simple apparatus consisting of a small flexible rectal catheter (Junior
Flexi-Tip, EZ-EM) connected to a standard handheld air-bulb insufflator by
small-bore (7.9-mm lumen) plastic tubing
(Fig. 1). At our institutions,
a dedicated CT technologist places the rectal catheter, without the use of
tape for securing the tube.
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Fig. 1. —Apparatus for colonic distention with room air. Small
flexible catheter is connected to air-bulb insufflator by enema tubing. This
device allows safe, inexpensive, and effective colonic distention in
time-efficient manner.
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After air insufflation, supine and prone CT images were obtained on an MDCT
scanner (LightSpeed Ultra, GE Healthcare) with an 8 x 1.25 detector
configuration, 120 kVp, 16.7 mAs (effective), and 1.0-mm reconstruction
interval. The CT images were viewed on a commercial CTC system (V3D Colon,
Viatronix) by an experienced gastrointestinal radiologist (> 500 CTC cases
with OC correlation) who interpreted the study using both 2D and 3D displays
for polyp detection. The CTC software system that we use allows primary 3D
detection of polyps in most cases, with 2D evaluation providing a secondary
backup for detection of additional lesions. All polyps initially detected on
3D displays are confirmed on 2D displays, and vice versa. This approach
combines the complementary benefits of primary 2D and primary 3D evaluation,
thus optimizing polyp detection rates.
The CTC study was deemed to be of diagnostic quality both in terms of
colonic preparation and distention. No polyps measuring 5 mm or greater were
identified at prospective interpretation (we generally do not report on
diminutive lesions measuring less than 5 mm). At same-day OC, a polyp
measuring 10 mm was identified in the rectum
(Fig. 2A) that was found to be
a tubulovillous adenoma at histologic examination. No other significant
lesions were identified at OC. Retrospective review of the CTC examination
revealed that the polyp had been almost completely obscured by the overlying
small rectal catheter on both the prone
(Fig. 2B) and supine (Figs.
2C and
2D) views. Although the lesion
was more conspicuous on retrospective manual 3D navigation than on 2D
multiplanar reconstruction, it was apparently missed prospectively because the
evaluation was limited to the automated center line. The lesion was difficult
to see on the 2D axial views, even in retrospect.
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Fig. 2A. —66-year-old asymptomatic woman referred for routine
colorectal cancer screening. Low-dose CT colonography (CTC) was performed
immediately before optical colonoscopy (OC). Digital photograph from OC shows
10-mm lobulated sessile polyp within rectum. Note calibrated probe adjacent to
polyp, which permits more accurate size measurement than forceps estimation at
open biopsy. Polyp was confirmed as tubulovillous adenoma at histologic
examination.
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Fig. 2B. —66-year-old asymptomatic woman referred for routine
colorectal cancer screening. Low-dose CT colonography (CTC) was performed
immediately before optical colonoscopy (OC). Three-dimensional endoluminal
view from prone CTC data set simulating endoscopic retroflexed view of
anorectum (note arrowhead at anal verge) shows polypoid lesion that
is largely obscured by rectal catheter (arrows). Image was obtained
from retrospective manual 3D navigation to optimize polyp visualization.
Lesion was less apparent on navigation along automated center line.
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Fig. 2C. —66-year-old asymptomatic woman referred for routine
colorectal cancer screening. Low-dose CT colonography (CTC) was performed
immediately before optical colonoscopy (OC). Three-dimensional endoluminal
view from supine CTC data set shows polyp beneath catheter (arrow).
Note catheter tip (arrowhead). As in B, vantage has been
optimized by manual 3D navigation. Surface irregularity in B and
C is related to low-dose technique.
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Fig. 2D. —66-year-old asymptomatic woman referred for routine
colorectal cancer screening. Low-dose CT colonography (CTC) was performed
immediately before optical colonoscopy (OC). Two-dimensional sagittal supine
image viewed using polyp window settings shows catheter contacting polyp along
anterior rectal wall (arrow). Lesion was more subtle on 2D axial
images (not shown). Note catheter tip (arrowhead). Less advanced
placement of catheter may have allowed it to fall away from polyp with change
in positioning and permitted detection.
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Discussion
CTC has been shown to be an accurate screening tool for detection of
colorectal polyps when advanced 3D imaging is emphasized
[1]. This approach permits
effective polyp detection that combines the complementary nature of primary 2D
and primary 3D evaluation. The rectum is generally well evaluated at CTC
because of consistent proclivity of the rectum for adequate luminal
distention. In a prospective multicenter screening trial of 1,233 asymptomatic
adults, radiologists interpreting CTC did not miss any rectal adenomas
measuring 6 mm or greater, whereas gastroneterologists interpreting
prospective OC missed six rectal adenomas before the unblinding of CTC results
[2]. Interestingly, five of the
six missed rectal adenomas at OC were within 10 cm of the anal verge,
suggesting that this area is a relative blind spot for OC, despite routine use
of the retroflexed view [2]. At
air–contrast barium enema examination, rectal polyps may be obscured by
the relatively large-caliber rectal catheter and retention balloon, often
necessitating additional views with the balloon deflated or the catheter
removed [3,
4].
Because CTC uses a small rectal catheter with or without a small balloon,
includes both supine and prone images, and utilizes 2D cross-sectional and 3D
endoluminal interpretation, significant rectal polyps should rarely be missed.
Undue advancement of the rectal catheter in the present case allowed the
catheter to contact and largely obscure the rectal adenoma and perhaps
prevented it from shifting away from the polyp with the change in patient
positioning. The polyp was detectable retrospectively on the 3D endoluminal
view with manual navigation (Figs.
2B and
2C) but was more subtle on both
the 2D multiplanar reconstruction and 3D evaluation along the automated center
line. The 3D endoluminal view was somewhat degraded by the low-dose technique.
Less advanced placement of the catheter may have allowed the lesion to be more
readily detectable at CTC.
A variety of entities may give rise to polypoid lesions in the anorectal
region at CTC evaluation [5].
Causes that are specific to the anorectum include hypertrophied anal papillae,
internal hemorrhoids, anal condylomata, solitary rectal ulcer syndrome, and
the tip of the rectal catheter. Causes of polypoid lesions that are not
specific to the anorectum include true polyps (adenomatous and
nonadenomatous), retained stool, and prominent folds. If CTC is to be
performed as a front-line screening tool, it is important to note that
isolated anorectal abnormalities may be adequately evaluated with digital
rectal examination, anoscopy, or sigmoidoscopy alone and do not necessarily
require full colonoscopy.
The common pitfalls leading to false-positive results at CTC are well known
[6,
7], but it is equally important
to be cognizant of the potential causes for missing true polyps
(false-negative findings). Fortunately, the likelihood of a false-negative
result can be greatly reduced through proper state-of-the-art technique and
interpretation. For example, polyps submerged in retained fluid can be
detected with oral contrast opacification
[7]. Emphasis on the 3D
endoluminal fly-through also appears to improve sensitivity
[1] because polyp conspicuity
and the opportunity for detection are increased.
In conclusion, significant rectal polyps are rarely missed on a
good-quality CTC examination. The small-caliber rectal catheter represents a
rare but potential source of false-negative results, particularly if the tip
is advanced well beyond the anal verge.
References
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Introduction
Case Report
