GISTs are well-defined masses arising within the muscolaris propria layer of GI wall,
with variable size.
Intramural in origin,
they may grow exophytically,
projecting into the abdominal cavity and displacing adjiacent organs,
or grow intraluminally,
potentially compromising lumen patency.
They often show overlying mucosal ulceration; focal areas of hemorrhage,
cystic degeneration and necrosis may occur,
particularly in larger lesions.
Different imaging modalities can be used to accurately detect and characterize GISTs.
Ultrasonography
- On ultrasound,
GISTs appear as hypoechoic,
well-demarcated masses,
heterogeneous because of the presence of cystic and solid components.
- On endoscopic ultrasonography (EU),
these hypoechoic masses appear contiguous with the fourth hypoechoic layer of the GI wall,
which corresponds to muscularis propria,
and it is possible to understand whether the lesion arises from the bowel wall (intramural growth) or from a structure outside the bowel wall (extramural growth) just compressing the gastrointestinal wall.
Moreover,
EU provides an excellent mean for guiding fine-needle aspiration or tru-cut biopsy of the lesion.
Radiography
- Abdominal radiography may show a non-specific soft-tissue density mass displacing gastric air shadow and/or bowel loops,
sometimes causing intestinal dilatation.
- In case of cavitary masses conteining air,
irregular gas collections may be evident.
- Fistulization may occur,
resulting in luminal enlargement and communication of the lesion with peritoneal cavity or adjacent organs.
- Rarely calcifications may be visible.
Fluoroscopy
- In barium studies,
GISTs commonly appear as submucosal masses,
usually sharply defined,
with smooth margins forming right or obtuse angles with adjacent mucosa.
- Overlying mucosal surface is coated with barium with the exception of focal ulcerated areas,
giving a typical "bull-eye appearance".
- Sometimes gastric GISTs may appear as intraluminal polypoid mass.
Computed Tomography (CT)
- On CT,
GISTs appearances vary with size,
location and aggressiveness of the tumor.
Usually the mass appears as intramural or intraluminal solid soft-tissue lesion,
with smooth,
sharply defined margins.
- These tumors are usually highly vascular and contrast enhancement (CE) is more often rimlike with a peripheral enhanced pattern,
even if homogeneous CE is possible,
generally in smaller lesions.
The most typical feature of GISTs is the presence of focal areas of lower density due to hemorrhage,
necrosis and cystic degeneration within the mass,
especially in larger tumors (Fig.1)
- Lesions with extensive hemorrhage or necrosis may form large cavities which may communicate with gastric or bowel lumen and contain air,
air-fluid or fluid-fluid levels. Ulcerating GISTs may be identified by the so called Torricelli- Bernoulli sign,
defined as a stream of air bubble arising from a deep crescent-shaped ulcerated neoplasm localized in a hollow viscus,
reflecting the active behavior of the ulcer that fills alternately with fluid or gas,
depending on the position of the patient.
- To identify the organ of origin,
especially in larger masses displacing anatomic structures,
some radiological signs may be useful:
-the "embedded organ sign": part of organ of origin is embedded in the mass with adjacent desmoplastic reaction;
-the "beak sign": consisting of sharp angles between the organ of origin and the lesion;
-the "prominent feeding artery or draining vein sign": feeding artery to hypervascular masses or draining vein may be prominent enough to be visible on CT images.
- CT may demonstrate local extension to near anatomic structures: larger tumors may displace adjacent organs,
generally without direct invasion; ulcero-infiltrative pattern is rare,
although it may be present in more aggressive lesions.
- CT is also sensitive for the detection of metastatic lesions which show similar features to those of primary tumor generally spread to liver,
peritoneal cavity,
lung and bone.
Liver lesions may show hypervascular pattern or appear as cystic multilocular lesions with fluid-fluid levels and thick walls.
Magnetic Resonance Imaging (MRI)
- On MRI,
solid components of GISTs appear as sharply delineated masses,
hypo- to isointense compared with muscle intensity on T1-weighted images (T1-w) and iso- to hyperintense on T2-weighted images (T2-w),
showing predominant peripheral contast enhancement after administration of gadolinium.
- The masses tend to be heterogeneous if cystic and necrotic areas are detectable and the degree of necrosis and hemorrhage greatly affects the signal-intensity pattern:
- foci of high attenuation in T1-w are generally related to internal hemorrhage is present,
however the signal intensity may vary depending on the age of the hemorrage;
- foci of high hyperattenuating with no contrast enhancement on T2-w correspond to cystic degeneration;
-signal intensity voids are visible if gas is present within areas of necrotic tumor.
- On diffusion-weighted magnetic resonance imaging (DW MRI) GISTs usually show high signal intensity because of high cellular density,
with lower ADC values detected in the solid components of larger lesions.
Complications associated with GISTs
Baseline CT scans should always be performed to avoid false positives after contrast administration,
depicting any hyperdense material (foreign bodies,
retained contrast material,
pills,
clips…) mimicking bleeding foci.
- Gastrointestinal bleeding occurs as a result of tumor mucosal ulceration; although the gold standard for diagnosing this condition is represented by endoscopic examination performed after laboratory tests,
bleeding GISTs may be inaccessible to routine endoscopy due to their localization. CT scan performed before and after intravenous contrast administration may easily detect endoluminal blood on baseline (40-60 UH) and extravasation of contrast agent in delayed arterial phase.
An additional venous phase scanning may provide a better depiction of late and low-rate bleedings,
as well as better tumor staging.
Baseline CT scans should always be performed to avoid false positives after contrast administration,
depicting any hyperdense material (foreign bodies,
retained contrast material,
pills,
clips…) mimicking bleeding foci (Fig.2)
- Perforation may be the subsequent complication of the ulceration.
The amount and location of free air depends to the perforation site; however,
in case of suspected perforation,
the first imaging study remains plain abdominal radiography which may show the presence of free gas under the diaphragm,
more often with crescentic morphology,
best seen in erect chest X-ray.
CT scan is more sensitive in detection of free extraluminal intraperitoneal or retroperitoneal air,
especially when the amount of air is small.
Moreover CT may show the discontinuity of the viscus lumen,
localizing the perforation site (Fig.3)
- Spontaneous rupture observed in around 1% of GISTs,
consequently to intratumor necrosis,
bleeding,
or ischaemia induced by tumor embolisation.
Predisposing risk factors to rupture are large size,
exophytic growth,
rapid growth and wide cystic degeneration areas and necrosis within the mass.
It occurs both within the GI lumen and as intraperitoneal rupture,
the last one resulting in haemoperitoneum or peritonitis.
This complication should be suspected when,
at CT,
necrotic or haemorrhagic portions are detectable within the tumor with the presence of ascites or haemoperitoneum,
which is rare even in peritoneal metastasis.
CT reveals a large amount of high density fluid in the peritoneal cavity associated with diffuse thickening of peritoneal lining (Fig.
4)
- intestinal obstructions caused by GISTs may show different patterns:
- intussusception,
induced by GIST behaving as leading mass.
The most typical imaging finding in this condition is the bowel-in-bowel configuration,
described as target sign on ultrasound scan (consisting in alternating echogenic and hypoechogenic layers,
representing respectively mucosa and muscolaris of the bowel and submucosa band,
forming concentric rings on axial scans) and as stratified aspect of the layers of bowel wall on CT examination,
formed by the inner bowel and the folded edge of the outer bowel (Fig.
5)
- -direct occlusion,
caused both by GISTs with exophytic and endophytic growth,
for ab estrinseco or ab intrinseco triggering mechanism,
especially when largest in size (Fig.6)
- -volvolus-like torsion,
generally occurring when the small bowel rotates on the mesenteric root,
leading to whirl of mesenteric vessels.
The most representative radiological sign is the whirlpool sign of the mesentery as the result of swirling strands of soft-tissue attenuation within a background of mesenteric fat attenuation.
Plain abdominal radiographs show dilated bowel loops,
whereas CT is sensitive in revealing the transition point between dilated bowel loops and collapsed or normal caliber bowel distal to the obstruction point.