DOI:10.2214/AJR.06.0436
AJR 2007; 188:953-959
© American Roentgen Ray Society
Flat Colorectal Neoplasms: Definition, Importance, and Visualization on CT Colonography
Seong Ho Park1,
Seung Soo Lee1,
Eugene K. Choi2,
So Yeon Kim1,
Suk-Kyun Yang3,
Jin Ho Kim3 and
Hyun Kwon Ha1
1 Department of Radiology and Research Institute of Radiology, University of
Ulsan College of Medicine, Asan Medical Center, 388-1 Pungnap-2dong,
Songpa-gu, 138-736, Seoul, Korea.
2 Weill Medical College of Cornell University, New York, NY 10021.
3 Department of Internal Medicine, University of Ulsan College of Medicine, Asan
Medical Center, Seoul, Korea.
Received March 27, 2006;
accepted after revision July 31, 2006.
Address correspondence to S. H. Park
(seongho{at}amc.seoul.kr).
Abstract
OBJECTIVE. We discuss the definition of flat colorectal neoplasms,
their clinical importance, CT colonography (CTC) findings, techniques for
better visualization on CTC, and diagnostic pitfalls of such lesions.
CONCLUSION. Flat lesions appear on CTC as plaque-shaped mucosal
elevations with or without a central depression, thickened haustral folds, and
nodular mucosal surfaces. The sensitivity and optimal techniques of CTC for
the detection of flat lesions have not yet been established. Three-dimensional
endoluminal fly-through may be helpful for lesion detection. Fecal tagging
helps in the distinction of true flat lesions from feces. IV contrast
enhancement and the review with intermediate soft-tissue window settings,
although not routinely used for CTC, may also help lesion visualization.
Keywords: colon • colonography • colonoscopy • colorectal cancer • CT colonography • oncologic imaging • virtual colonoscopy
Introduction
Alarge study has shown that the sensitivity of CT colonography (CTC)
for the detection of clinically significant colorectal lesions was comparable
to that of colonoscopy in an average-risk screening population
[1]. Despite some controversial
results [2,
3], CTC is gaining acceptance
as a viable option for colon cancer screening. One of the multitude of factors
that impair lesion detection on CTC is lesions of flat or nonpolypoid
morphology [4]. Although flat
colorectal lesions were once thought to be rare, studies of western
populations have shown that approximately 40% of adenomatous lesions detected
at colonoscopy were flat
[5-7].
One of the reasons for the difficulty in diagnosing flat lesions at CTC may
be an unfamiliarity with the CTC appearances of such lesions. Awareness of
flat colorectal lesions and knowledge about their appearances at CTC will help
in the detection of such lesions and will serve to enhance the efficacy of
screening CTC. Toward this end, the purpose of this essay is to present a
pictorial review of the definition of flat colorectal neoplasms, their
clinical importance, CTC findings, techniques for better visualization at CTC,
and diagnostic pitfalls of such lesions.
Definition
"Flat" or "nonpolypoid" colorectal lesion has been
colloquially used to describe superficially elevated lesions. However, flat
adenomatous lesions are categorized into slightly elevated, completely flat,
and slightly depressed lesions and other variations, such as "slight
elevation with depression" and "depressed with slightly elevated
border" [8]. A lesion
with a height that is no more than twice the height of the adjacent normal
mucosa is the most widely accepted histologic definition.
Commonly used endoscopic definitions include mucosal elevation with a flat
or slightly rounded surface and a height of less than half the greatest
diameter of the lesion. However, the endoscopic definition may be too crude to
characterize the flatness of a lesion because lesions of various heights will
be grouped into the same "flat" category on the basis of their
widths [4]. A lesion of 2 mm or
less in height with respect to the adjacent normal mucosa was a suggested
definition of flat lesions for reporting purposes at CTC
[9]. Large superficially
elevated nonpolypoid lesions are often labeled as "carpet lesions"
in the United States and as "laterally spreading tumors" in Japan
[8] (Figs.
1A,
1B,
1C and
2A,
2B,
2C).
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Fig. 1A —54-year-old man with approximately 4-cm flat villotubular adenoma
with intramucosal adenocarcinoma with irregular lesion margin in cecum.
Three-dimensional endoluminal CT colonography image shows irregular mucosal
nodularity (arrows) in cecal tip.
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Fig. 1B —54-year-old man with approximately 4-cm flat villotubular adenoma
with intramucosal adenocarcinoma with irregular lesion margin in cecum.
Two-dimensional transverse image obtained using wide window settings (width,
1,500 H; level, -400 H) shows flat elevation with nodular surfaces
(arrowheads).
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Fig. 1C —54-year-old man with approximately 4-cm flat villotubular adenoma
with intramucosal adenocarcinoma with irregular lesion margin in cecum.
Colonoscopy shows irregular mucosal nodularity in area of cecum corresponding
to that shown in A and B.
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Fig. 2A —53-year-old woman with approximately 5-cm flat tubular adenoma with
intramucosal adenocarcinoma with smooth, well-circumscribed, peripheral rim in
sigmoid colon. Three-dimensional endoluminal CT colonography image shows round
flat lesion (arrowheads) in sigmoid colon. Lesion shows nodular
surfaces and is well circumscribed by polypoid rim in periphery.
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Fig. 2B —53-year-old woman with approximately 5-cm flat tubular adenoma with
intramucosal adenocarcinoma with smooth, well-circumscribed, peripheral rim in
sigmoid colon. Two-dimensional multiplanar reformatted image obtained using
wide window settings (width, 1,500 H; level, -400 H) shows flat elevation with
nodular surfaces (arrowheads).
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Fig. 2C —53-year-old woman with approximately 5-cm flat tubular adenoma with
intramucosal adenocarcinoma with smooth, well-circumscribed, peripheral rim in
sigmoid colon. Colonoscopy shows corresponding round flat lesion with nodular
surfaces and well-circumscribed margin.
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Clinical Importance
It is widely accepted that the majority of colorectal cancers develop
slowly through polypoid growth. However, recent studies have shown that flat
colorectal lesions also contribute to the development of colorectal cancers
[8]. The malignant risk of flat
adenomatous lesions compared with that of polypoid lesions is controversial.
The results of a few large prospective epidemiologic studies from the United
Kingdom have shown that flat lesions have an increased risk of harboring
high-grade dysplasia and of progressing to invasive carcinoma than polypoid
lesions [5,
6]. In another study, one from
the United States, however, flat adenomas were found to be no more likely to
exhibit high-grade dysplasia than polypoid adenomas when data were adjusted
for lesion size, villous component, and lesion location and when data were
corrected for correlation of risk in an individual patient
[10]. They were also found not
to be more strongly associated with advanced adenomas at subsequent
surveillance colonoscopy than polypoid adenomas
[10].
Detection with CTC
Flat lesions appear as nodular mucosal surfaces (Figs.
1A,
1B,
1C and
2A,
2B,
2C), plaque-shaped mucosal
elevations (Figs. 3A,
3B,
3C,
3D,
4A,
4B,
4C,
5A,
5B,
5C,
6A,
6B,
6C,
6D), and thickened haustral
folds (Figs. 7A,
7B,
7C,
7D and
8A,
8B,
8C,
8D) at CTC. Flat lesions that
involve a significant portion of colonic surface are often referred to as
"carpet lesions" (Figs.
1A,
1B,
1C and
2A,
2B,
2C). A slight elevation with a
centrally depressed area, which may be clearly depicted on CTC (Figs.
3A,
3B,
3C,
3D and
4A,
4B,
4C), is well known to
represent a high probability of harboring invasive carcinoma
[8].
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Fig. 3A —67-year-old man with 10-mm flat adenocarcinoma with submucosal
extension in sigmoid colon. Three-dimensional endoluminal CT colonography
image shows slightly elevated lesion with central depression (arrows)
in sigmoid colon.
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Fig. 3B —67-year-old man with 10-mm flat adenocarcinoma with submucosal
extension in sigmoid colon. Two-dimensional transverse image obtained using
wide window settings (width, 1,500 H; level, -400 H) shows slightly elevated
lesion (arrowheads). Central depression, albeit subtle, is noted.
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Fig. 3D —67-year-old man with 10-mm flat adenocarcinoma with submucosal
extension in sigmoid colon. Colonoscopy with chromoscopic examination (i.e.,
mucosal spraying of methylene blue dye) shows surface topography of lesion
more clearly as dye pools in mucosal grooves, crevices, and depressions.
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Fig. 4A —76-year-old woman with 18-mm flat adenocarcinoma that extends to
submucosa in rectum. Three-dimensional endoluminal CT colonography image
depicts slightly elevated lesion with centrally depressed area
(arrows) on haustral fold in rectum. Rectal tube
(arrowheads) is seen adjacent to lesion.
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Fig. 4B —76-year-old woman with 18-mm flat adenocarcinoma that extends to
submucosa in rectum. Two-dimensional multiplanar reformatted image obtained
using wide window settings (width, 1,500 H; level, -400 H) shows thickening of
haustral fold (arrow). However, overall morphology of lesion is not
apparent on this image.
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Fig. 4C —76-year-old woman with 18-mm flat adenocarcinoma that extends to
submucosa in rectum. Colonoscopy shows slightly elevated lesion that is
plaque-shaped and has centrally depressed area (arrowheads) on
haustral fold.
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Fig. 5A —51-year-old woman with 13-mm flat adenocarcinoma with submucosal
extension in rectosigmoid junction. Three-dimensional endoluminal CT
colonography (CTC) image shows plaquelike, flat lesion with lobulated margin
(arrows) in rectosigmoid junction.
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Fig. 5B —51-year-old woman with 13-mm flat adenocarcinoma with submucosal
extension in rectosigmoid junction. Two-dimensional transverse
contrast-enhanced CTC images obtained with wide window (B: width, 1,500
H; level, -400 H) and soft-tissue window (C: width, 400 H; level, 20 H)
settings show slightly elevated lesion (arrowheads). On soft-tissue
window image (C), lesion presents as enhancing focal thickening of
colonic wall that can be distinguished from adjacent colonic wall that is
barely perceptible after distention.
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Fig. 5C —51-year-old woman with 13-mm flat adenocarcinoma with submucosal
extension in rectosigmoid junction. Two-dimensional transverse
contrast-enhanced CTC images obtained with wide window (B: width, 1,500
H; level, -400 H) and soft-tissue window (C: width, 400 H; level, 20 H)
settings show slightly elevated lesion (arrowheads). On soft-tissue
window image (C), lesion presents as enhancing focal thickening of
colonic wall that can be distinguished from adjacent colonic wall that is
barely perceptible after distention.
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Fig. 6B —52-year-old man with 8-mm flat tubular adenoma in sigmoid colon.
Two-dimensional transverse CTC images obtained with wide window (B:
width, 1,500 H; level, -400 H) and soft-tissue window (C: width, 400 H;
level, 20 H) settings show slightly elevated lesion (arrowheads). On
soft-tissue window image (C), lesion presents as focal thickening of
colonic wall that can be is distinguished from adjacent colonic wall that is
barely perceptible after distention.
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Fig. 6C —52-year-old man with 8-mm flat tubular adenoma in sigmoid colon.
Two-dimensional transverse CTC images obtained with wide window (B:
width, 1,500 H; level, -400 H) and soft-tissue window (C: width, 400 H;
level, 20 H) settings show slightly elevated lesion (arrowheads). On
soft-tissue window image (C), lesion presents as focal thickening of
colonic wall that can be is distinguished from adjacent colonic wall that is
barely perceptible after distention.
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Fig. 7A —69-year-old man with 2.5-cm flat adenocarcinoma with focal extension
to proper muscle in ascending colon. Three-dimensional endoluminal CT
colonography images show lesion that presents as smooth thickening of haustral
fold (arrows) in ascending colon.
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Fig. 7B —69-year-old man with 2.5-cm flat adenocarcinoma with focal extension
to proper muscle in ascending colon. Three-dimensional endoluminal CT
colonography images show lesion that presents as smooth thickening of haustral
fold (arrows) in ascending colon.
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Fig. 7C —69-year-old man with 2.5-cm flat adenocarcinoma with focal extension
to proper muscle in ascending colon. Two-dimensional transverse image obtained
using wide window settings (width, 1,500 H; level, -400 H) also shows smooth
thickening of haustral fold (arrows).
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Fig. 7D —69-year-old man with 2.5-cm flat adenocarcinoma with focal extension
to proper muscle in ascending colon. Colonoscopy shows thickened haustral fold
(arrowheads) that corresponds to findings in A-C.
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Fig. 8A —76-year-old man with 15-mm flat tubular adenoma with high-grade
dysplasia in descending colon. Three-dimensional endoluminal CT colonography
image shows lesion that presents as nodular thickening of haustral fold
(arrows) in descending colon.
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Fig. 8B —76-year-old man with 15-mm flat tubular adenoma with high-grade
dysplasia in descending colon. Two-dimensional transverse image obtained using
wide window settings (width, 1,500 H; level, -400 H) shows thickened haustral
fold (arrowheads).
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Fig. 8C —76-year-old man with 15-mm flat tubular adenoma with high-grade
dysplasia in descending colon. Colonoscopy with mucosal spraying of methylene
blue dye shows thickened fold with surface lobulations (arrowheads,
C) in descending colon, which is in contrast to sharp and smooth
appearance of normal haustral folds (arrowheads, D) in
adjacent area of descending colon.
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Fig. 8D —76-year-old man with 15-mm flat tubular adenoma with high-grade
dysplasia in descending colon. Colonoscopy with mucosal spraying of methylene
blue dye shows thickened fold with surface lobulations (arrowheads,
C) in descending colon, which is in contrast to sharp and smooth
appearance of normal haustral folds (arrowheads, D) in
adjacent area of descending colon.
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Because flat lesions are difficult to detect with colonoscopy, often
requiring chromoscopic examination (i.e., mucosal spraying of an agent, such
as indigo-carmine stain or methylene blue dye, for enhanced visualization of
the surface topography of the lesion and surrounding mucosa) (Fig.
3A,
3B,
3C,
3D), they are also difficult
to detect using CTC and can be a significant source of false-negative results
[4].
The paucity of data regarding the sensitivity of MDCT colonography in the
detection of flat lesions is made even more problematic by the variability of
its results [11,
12]. In a small study that
used a 16-MDCT scanner, no fecal tagging, and primary 2D review, fewer than
50% of the flat lesions were visualized even when the lesions that were missed
because of apparent technical failures, such as excessive luminal fluid, poor
bowel preparation, or poor bowel distention, were excluded
[11]. In that study
[11], lesions that were 1 mm
or less in height could not be found both prospectively and retrospectively.
On the contrary, a large study that included an asymptomatic average-risk
population and used 4- and 8-MDCT scanners, fecal and fluid tagging, and
primary 3D review showed that the sensitivity of CTC for the detection of flat
lesions was similar to that for polypoid lesions (i.e., 82.8% sensitivity for
flat adenomas 
6 mm in diameter)
[12].
To our knowledge, there is no clear consensus about which CTC techniques
are optimal to visualize flat lesions. The colon should be well distended
because lesions that present as thickened haustral folds are extremely
difficult to see unless the colon is optimally distended. IV contrast
enhancement may be helpful for lesion detection by allowing confident
distinction of enhancing true lesions from feces
[4,
11] (Fig.
5A,
5B,
5C), although IV contrast
enhancement is not routinely used for screening CTC.
Intermediate soft-tissue window settings have been suggested to be more
sensitive for the detection of flat lesions than the wide window settings that
are routinely used to review CTC images
[13] (Figs.
5A,
5B,
5C and
6A,
6B,
6C,
6D). However, the routine use
of both wide window and soft-tissue window settings may not be acceptable
because it will significantly increase interpretation time.
The overall morphology of a lesion is easier to understand on 3D view (Fig.
4A,
4B,
4C) than on 2D view, and some
flat lesions, including those that present as thickened folds, are more
apparent at 3D fly-through than at 2D review (Figs.
5A,
5B,
5C and
7A,
7B,
7C,
7D), which may suggest the
superiority of primary 3D fly-through to primary 2D review in the detection of
flat lesions.
Diagnostic Pitfalls
Flat lesions, especially those that present as irregular mucosal nodularity
(Fig. 1A,
1B,
1C), may be easily mistaken for
feces and vice versa. Residual fecal material that is present even after
vigorous bowel preparation can mimic flat lesions (Fig.
9A,
9B). Fecal tagging is helpful
in distinguishing pseudolesions (i.e., feces) from true lesions (Fig.
10A,
10B). Air bubbles that are
present on the colonic surface during CT can mimic flat lesions (Fig.
11A,
11B). The fluid shell of a
bubble is generally too thin to be visualized on CT, so the base of a bubble
(i.e., the bubble's attachment to the mucosa) may look like a depressed lesion
with a slightly elevated border when visualized on a 3D endoluminal view (Fig.
11A,
11B). A pseudolesion can be
distinguished from a true flat lesion by noting the bubble's characteristic
morphology of a smooth, thin, ringlike peripheral elevation and the lack of
colonic wall thickening on soft-tissue window views and also by noting the
disappearance of the pseudolesion on the other scan (Fig.
11A,
11B).
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Fig. 9A —59-year-old woman with 9-mm residual fecal material that mimics flat
lesion in transverse colon. Three-dimensional endoluminal CT colonography
image shows nodular elevation with central depression (arrowheads) in
transverse colon that was proven to be residual fecal material at colonoscopy
with chromoscopic examination and segmental unblinding. Colon was cleansed
vigorously with 4 L of polyethylene glycol as shown by clean colonic wall
except for pseudolesion.
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Fig. 9B —59-year-old woman with 9-mm residual fecal material that mimics flat
lesion in transverse colon. Targeted 2D transverse image obtained using wide
window settings (width, 1,500 H; level, -400 H) shows subtle nodularity
(arrowhead) in nondependent wall of transverse colon that corresponds
to pseudolesion shown in A.
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Fig. 10A —64-year-old woman with residual fecal material that mimics flat
lesion in cecum. Irregular mucosal nodular structure (arrows) that
mimics morphology of carpet lesion (e.g., Fig.
1A,
1B,
1C) is noted in cecum on 3D
endoluminal view.
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Fig. 10B —64-year-old woman with residual fecal material that mimics flat
lesion in cecum. Pseudolesion (arrowheads)—that is, residual
feces—can be clearly distinguished from true lesion on 2D transverse
image obtained using wide window settings (width, 1,500 H; level, -400 H) due
to bariumbased tagging.
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Fig. 11A —57-year-old man with air bubble that mimics flat lesion in sigmoid
colon. Three-dimensional endoluminal CT colonography image from supine scan
shows pseudolesion with thin, ringlike peripheral elevation and central
depression (arrow).
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Fig. 11B —57-year-old man with air bubble that mimics flat lesion in sigmoid
colon. Three-dimensional endoluminal image from prone scan of same location as
A shows pseudolesion is no longer present.
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Top
Abstract
Introduction
