is a Postgraduate Year 5 Resident,
is a Research Assistant, and
is an Assistant Clinical Professor, Department of Radiology,
University of California San Diego, San Diego, CA.
Computed tomography (CT) has become the mainstay for noninvasive
diagnosis and staging of many gastrointestinal (GI) tumors. With
proper technique, CT permits evaluation of the bowel wall and
extramural tissues (Figure 1), a distinct advantage over
conventional fiuoroscopic and radiographic examinations that
primarily portray the mucosal surface (Figure 2).
Depiction of the bowel wall and extramural tissues affords accurate
assessment of lesion size and is essential for tumor staging. This
article will review: 1) optimal imaging techniques and interpretive
pitfalls in GI tumor imaging; 2) imaging features that help
differentiate benign from malignant pathology; 3) characteristic
findings and complications of common GI malignancies; and 4)
patterns of tumor dissemination.
Optimal technique is essential for accurate detection and
staging of GI tumors. A fundamental requirement is luminal
requiring the administration of enteric contrast agents (typically
approximately 450 mL of enteric contrast given 1 to 2 hours before
the CT examination). Historically, barium- or iodine-containing
liquid preparations were used. These preparations increase the
attenuation of the bowel lumen and are called positive contrast
agents. One disadvantage of positive contrast agents is that they
interfere with mucosal visualization. More recently, several
investigators have advocated the use of hypoattenuating or negative
contrast agents, such as water, milk,
or synthetic preparations. These agents permit direct mural
evaluation after intravenous (IV) contrast administration.
If luminal distention is inadequate, one of several ancillary
maneuvers should be attempted: 1) delayed imaging to permit passage
of enteric contrast into the region of interest; 2) administration
of additional oral contrast and/or effervescent material to distend
the proximal GI tract; 3) rectal administration of gas or enteric
contrast material to distend the colorectum; and 4) patient
repositioning to shift intraluminal material.
Unless it is contraindicated, IV contrast material should be
used, as it helps to depict and characterize the primary tumor,
assess normal bowel wall, stage metastatic disease, and detect
Typically, a 125- to 150-mL IV solution (approximately 40 g of
iodine) is administered at a rate of 3 to 4 mL/sec. For most GI
tumors, venous-phase imaging is sufficient, as these tumors and
their metastases tend to be hypovascular. However, some GI tumors,
particularly those of mesenchymal and neuroendocrine origin, are
hypervascular. If such a tumor is suspected, arterial-phase imaging
should also be performed.
Specific imaging parameters are institution- and
scanner-dependent. The authors obtain 5-mm images on a 4-slice CT
multidetector scanner and, in select cases, retrospectively
reconstruct overlapping 2.5-mm images to increase detection of
small nodes and peritoneal implants. If necessary, multiplanar
reformations are performed.
CT colonography is a specialized procedure for polyp screening.
Discussion of CT colonographic techniques is beyond the scope of
Bowel-wall thickening: Differentiating benign from
malignant GI pathology
Normal bowel-wall thickness is ≤ 3 mm.
On CT, bowel-wall thickening >3 mm is a frequent finding. The
most common cause is pseudothickening related to incomplete
distention. Assuming adequate luminal distention, wall thickening
connotes important pathology. The differential diagnosis is broad
and includes both neoplastic and non-neoplastic processes. Careful
analysis of 4 CT features of the thickened bowel wall usually
permits reliable differentiation: 1) bowel-wall attenuation and
enhancement, 2) degree of wall thickening, 3) length of
involvement, and 4) morphology. Other important clues include
distribution of bowel involvement, clinical presentation, patient
demographics, and ancillary findings (Table 1).
Bowel-wall attenuation and enhancement
The bowel-wall enhancement pattern helps in the differentiation
of benign from malignant disease. Bowel-wall stratification (target
sign or double halo sign), in which smooth, alternating low- and
high-attenuation layers conform to the circumferential strata of
the bowel wall, indicates benign pathology.
One important exception is scirrhous adenocarcinoma, a tumor that
infiltrates the submucosa and muscularis but spares the mucosa.
By comparison, heterogeneous wall attenuation and enhancement, in
which low-attenuation and high-attenuation areas do not conform to
the bowel-wall layers, is suggestive of malignancy.
In these cases, the hypoattenuating regions probably represent
ischemia, necrosis, and/or accumulation of mucinous material.
Homogeneous bowel-wall enhancement is indeterminate, and other
factors (below) need to be considered.
Degree of bowel-wall thickness
In general, mild bowel-wall thickening (<1 cm) suggests
although early malignancies may have a similar appearance. Marked
bowel-wall thickening (>2 cm) suggests malignancy,
although certain benign conditions (especially mycobacterial
infection, cytomegalic virus infection, Crohn disease, and severe
ischemic colitis) may also cause marked wall thickening. Wall
thickening between 1 and 2 cm is abnormal, but not specific for
benignity or malignancy.
Length of involvement
Pathology that affects >20 cm of bowel suggests a benign
Wall thickening that involves <5 cm of bowel suggests
Pathology affecting between 5 and 20 cm is nonspecific, as many
malignant (eg, non-Hodgkin's lymphoma [NHL]) and benign (eg,
granulomatous disease and bowel hemorrhage) processes cause
segmental bowel involvement.
Wall thickening that is homogeneous, symmetric, smooth, and
tapered suggests a benign etiology.
Wall thickening that is irregular, asymmetric, eccentric, and
abrupt suggests malignancy.
A special morphologic pattern that merits further discussion is
cavitary wall thickening.
Cavitating masses of the GI tract are an uncommon but important
morphologic pattern. The differential diagnosis includes
adenocarcinoma, NHL, gastrointestinal stromal tumor (GIST),
hematogenous metastases (especially from melanoma), and
diverticulitis with abscess formation. These causes are often
indistinguishable on CT (Figure 3). Factors favoring malignancy
include wall thickness >2 cm, wall enhancement, wall
irregularity, bulky or necrotic lymphadenopathy, distant
metastasis, and an insidious clinical presentation.
Primary neoplasms of the abdominal GI tract
The most common primary neoplasms of the GI tract are
adenocarcinoma, lymphoma, and carcinoid and mesenchymal tumors. In
recent years, a fifth tumor type, squamous-cell carcinoma of the
anus and rectum, has emerged as an important entity in anoreceptive
homosexual patients who have tested positive for human
immunodeficiency virus (HIV). This topic is being actively
investigated and is not further discussed in this review.
Adenocarcinoma is the most common malignancy of the abdominal GI
tract and can arise anywhere from the stomach to the rectum.
Sites of involvement-
In descending order, the most frequent sites of adenocarcinoma
involvement are: Colon/rectum (Figures 4 through 6), stomach
(Figure 7), duodenum, jejunum, ileum, and anus.
Adenocarcinoma is extremely common in the colon and rectum and
accounts for well over 90% of colorectal cancers.
The ascending colon is involved in 25% of cases, the transeverse
colon in 10%, the descending colon in 10%, the sigmoid in 30%, and
the rectum in 25%.
In immunocompetent patients, primary rectal malignancy other than
adenocarcinoma is rare; thus, in the absence of HIV or another
other immune deficiency, a rectal mass is considered adenocarcinoma
until proven otherwise.
Gastric adenocarcinoma classically arises in the distal stomach.
For reasons that are still unclear, the incidence of proximal
disease is increasing.
This increase is unfortunate, as proximal lesions have a worse
At autopsy, adenocarcinoma is the second most common malignancy
of the small bowel after carcinoid.
It is more common than NHL in the duodenum
and jejunum, but less common than NHL in the ileum.
Imaging features characteristic of GI adenocarcinoma include
irregular and asymmetric wall thickening, luminal narrowing with
tendency for obstruction, and short-segment involvement with an
abrupt transition from unaffected bowel wall to mass
("shouldering," Figure 5).
Locoregional lymph node (LN) enlargement (Figures 4 and 7) is
Mucinous adenocarcinoma has a tendency to calcify.
Such calcifications can be seen in both the primary mass as well as
metastatic lesions (Figure 6). Other CT appearances of
adenocarcinoma include a lobulated, polypoid mass (Figure 8),
superficial nodular mass, and deep ulcerating mass.
Gastric adenocarcinoma may also diffusely infiltrate the submucosa
(linitis plastica), causing a rigid or fixed appearance (Figure 7).
Differential considerations are provided in Tables 1 and 2
Complications from adenocarcinoma are frequent and include
metastasis (most common), bleeding, bowel obstruction,
intussusception (Figure 8), direct invasion into adjacent
structures, visceral perforation, fistula formation, and
pseudomyxoma peritonei from perforation of mucinous adenocarcinoma.
Patterns of spread-
Adenocarcinoma of the GI tract can spread to other sites via
several mechanisms. Hematogenous metastases usually involve the
liver and, less commonly, the lungs or bones.
Regardless of location, hematogenous metastases are typically
hypovascular and are best seen on portal venous-phase imaging.
Lymphatic metastases (Fig-ures 4, 5, and 7) are common, involving
locoregional LN first and more distant LN subsequently.
Another important mechanism of spread is direct invasion of
adjacent structures, including regional fat, omentum, and viscera.
Exfoliation of tumor cells into the peritoneum can occur, usually
after serosal penetration, and can lead to drop metastases to
ovaries (Figure 9), peritoneal and omental implantation, and
Carcinomatosis, or diffuse infiltration of peritoneum by
adenocarcinoma, can occur in late stages of lymphatic or
The GI tract is the most frequent extranodal site of lymphoma.
In this location, NHL is much more common than Hodgkin's disease,
and its incidence is rising.
Sites of involvement-
In immunocompetent patients, the most frequent sites of primary GI
involvement are, in descending order, stomach (50%) (Figure 10),
ileum (Figure 11), jejunum, duodenum (small bowel 33%), and colon
(16%) (Figure 12).
In immunodeficient patients and those of North African, Middle
Eastern, and Mediterranean descent, small-bowel involvement is more
common than gastric involvement.
Anorectal involvement is distinctly unusual in the absence of HIV.
Characteristic CT features of primary GI lymphoma include
circumferential, homogeneous and smooth bowel-wall thickening
usually without obstruction (Figure 12), mid-to long-segment bowel
involvement (Figure 11), bulky LN, which may surround mesenteric
vessels ("mesenteric sandwich"), aneurysmal dilatation of involved
bowel (Figure 1), and splenomegaly (40%).
In larger tumors, mural hemorrhage and necrosis may cause
heterogeneous bowel-wall attenuation (Figure 10). In the stomach,
lymphoma may diffusely infiltrate the submucosa, causing linitis
plastica. Differential considerations include adenocarcinoma
(Figure 7) (Table 1), GIST (Table 1), metastases, and infiammatory
disease (mycobacterial infection, or Crohn disease).
Patterns of spread-
Non-Hodgkin's lymphoma is usually a systemic disease;
characteristically, LN, spleen, and other organs are involved at
Other than diffuse dissemination, complications from lymphoma are
less frequent than from adenocarcinoma. Complications include
bleeding, intussusception, bowel obstruction, and perforation
(which may be silent).
Carcinoid is a submucosal tumor of neuroendocrine origin.
Sites of involvement-
GI carcinoid most frequently arises in the appendix (35%) or distal
ileum (16%) and hence usually manifests in the right lower
Less common sites include the rectum (13%), colon (3%), proximal
small bowel (rare), and stomach (rare).
Tumors of the stomach, appendix, and rectum frequently have low
malignant potential and high 5-year survival rates.
Tumors of the ileum and colon are often highly malignant with
metastatic disease found at the time of presentation.
The primary mass is usually undetectable.
Occasionally, a submucosal mass, possibly with central ulceration
(bull's-eye lesion), may be visible.
The classic CT imaging manifestation of carcinoid is a metastatic
mass in the regional mesentery (Figure 13). This mass is often
hypervascular, partially calcified, and spiculated.
Serotonin secreted by this mass typically incites a desmoplastic
leading to lymphatic and venous obstruction and mesenteric and
Dif- ferential considerations include retractile mesenteritis,
lymphoma, granulomatous infection, sarcoid, metastasis, GIST,
in-fiammatory pseudotumor, and fibromatosis (desmoid).
Patterns of spread and complications-
Frank metastatic disease is usually present once the primary tumor
reaches a diameter >2 cm.
In this setting, local metastases to the bowel mesentery (Figure
13) and hematogenous metastases to the liver (Figure 14) can be
identified by CT. Classically, the metastatic deposits in the liver
are hypervascular (Figure 14)
and may lead to carcinoid syndrome (episodic cutaneous fiushing,
diarrhea, and bronchospasm).
Other complications include intussusception (intraluminal
protrusion of primary tumor) and bowel obstruction (fibrotic
reaction to the serotonin produced by the mesenteric mass).
GIST, leiomyoma, leiomyosarcoma
Tumors of mesenchymal origin in-volving the GI tract include
GIST, leiomyoma, and leiomyosarcoma. Gastrointestinal stromal
tumor, the most common GI mesenchymal tumor, is composed of several
Benignity and malignancy are determined by tumor size (stratified
by organ of origin) and histologic factors (eg, mitotic number).
In general, larger masses and those involving the colon and rectum
are more frequently malignant.
True leiomyomas and leiomyosarcomas, which are composed purely of
smooth muscle elements, are rare.
Sites of involvement-
Gastrointestinal stromal tumor occurs most frequently in the
stomach (70%), followed by small bowel (20% to 30%) (Figure 15),
and anus/ rectum (7%).
Infrequently, GIST arises in the omentum or retroperitoneum.
Leiomyoma typically occurs in the stomach or proximal small
bowel. Leiomyosarcoma occurs more frequently in the distal ileum
(50%) rather than in the proximal small bowel.
However, even in the distal small bowel, GIST is far more frequent.
Imaging features and patterns of spread-
These mesenchymal tumors commonly present on CT as bulky,
heterogeneously hyperenhancing mural masses (Figure 15). Tumor
necrosis, calcification, hemorrhage, ulceration, and cavitation are
common. The mass may be exophytic (and spare mucosa), intraluminal,
Liver metastases are common and frequently hypervascular.
Hence, arterial-phase imaging of the primary tumor and of the liver
should be performed if possible. Differential considerations
include adenocarcinoma, NHL, metastasis, and diverticulitis with
abscess formation. The hypervascularity of the primary GIST and the
metastases, if present, suggests the correct diagnosis (Figure
Other mesenchymal tumors
Gastrointestinal lipomas occur most often in the ileum and
On CT, identification of a well-defined fat-attenuating mass is
This benign tumor is usually an incidental, asymptomatic lesion.
Rarely, a lipoma may serve as the lead point for an
Other mesenchymal tumors, including liposarcoma and nerve sheath
tumors, are rare.
Miscellaneous primary tumors
Most appendiceal mucoceles are caused by chronic infiammation
(eg, chronic appendicitis) that incites mucosal hyperplasia and
focal obstruction. Neoplasia (mucinous cystadenoma or
cystadeno-carcinoma) is a less frequent cause.
On CT, distension of a fiuid-filled (high-attenuation) appendix is
apparent. Pseudomyxoma peritonei, a complication of mucocele
rupture, may be the only indication of the primary pathology.
Metastases to the GI tract
Metastases to the bowel may occur via hematogenous dissemination
(melanoma, lung, breast, and Kaposi sarcoma), peritoneal seeding
(ovary, GI primaries), or direct invasion (pancreatic and GI
primaries). Hematogenous metastases typically arise along the
Bowel metastases from peritoneal seeding form along the mesenteric
CT findings include rounded bowel-wall masses with luminal
protrusion (Figure 16). Metastases (especially breast metastases to
) may cause diffuse submucosal infiltration, circumferential
bowel-wall thickening, and poor distensibility, indistinguishable
from primary adenocarcinoma. An unusual but important variant is a
cavitating metastasis. This morphology is most common with
metastatic melanoma (Figure 3)
and may mimic NHL, adenocarcinoma, GIST, or diverticular abscess.
Complications of bowel metastases include intussusception,
obstruction, and perforation.
Several CT imaging features of GI tumors assist in
differentiation of benignity from malignancy and aid in
determination of tumor etiology. Location, morphology, specific
signs, and associated findings all play essential roles.
Complications of both benign and malignant pathology are frequent
and must be diagnosed. CT is an optimal modality to accomplish
these tasks. However, proper CT technique, including the directed
use of both oral and intravenous contrast material, is essential
for adequate visualization and interpretation of pathology.