Mimics in Gastrointestinal Pathology
Case 4 -
Desmoid Tumors and Other Gist Mimics
Alyssa Krasinskas, Jeffrey Goldsmith and Susan Abraham
This 31-year-old woman presented with abdominal pain. By CT scan, she was found to have an 11 cm mass
near the ileocecal valve.
Grossly, there was a firm, white, circumscribed-appearing mass lesion that stretched the overlying
ileal and colonic mucosa, which was otherwise normal. Sections showed a uniform low-power appearance,
with spindled cells set in a stroma that was mostly collagenous. Areas of mildly myxoid stroma were also
present focally. The spindled cells had wavy nuclei with tapered ends. There were occasional mitotic
figures. By immunostaining, lesional cells were negative for both CD117 and CD34, with appropriate
internal positive controls.
In 1998, gastrointestinal stromal tumors (GISTs) were shown to harbor gain-of-function mutations in
the KIT gene.  It is now known that ~80-85% of GISTs have KIT mutations, 5-10% have
mutations of the platelet-derived growth factor receptor A (PDGFRA) gene, and the remaining 10% are
wild-type.  In 2001, the use of imatinib mesylate (Gleevec®) to target activated KIT protein
was reported to have a dramatic clinical benefit in a patient with metastatic GIST.  This
potential to respond to targeted therapy has heightened the need to correctly diagnose these tumors and
separate them from other soft tissue mimics. Not all GISTs are immunoreactive for KIT (CD117), not all
tumors that express CD117 are GISTs, and there are a variety of mesenchymal tumors in the
gastrointestinal tract or abdominal cavity that can overlap with GIST in their gross characteristics,
histology, and immunophenotype (Table 1). This talk will review several of the lesions that most
commonly cause confusion in the differential diagnosis of GIST.
Desmoid tumors (mesenteric fibromatosis) are uncommon (only ~900 new cases occur each year in the
U.S.),  but they can mimic GISTs because of their similar intra-abdominal location, size
range, and gross features. In one study, 52% of desmoid tumors seen in consultation were initially
diagnosed as GISTs by the referring pathologists.  Theoretically, the locations of the two
tumors should allow for their distinction; desmoids arise in the mesentery or retroperitoneum, while
GISTs arise in the bowel wall. However, the infiltrative border of desmoid tumors often results in their
extension into the bowel wall, and some cases even show lamina propria involvement. A few desmoids even
seem to arise in the muscularis propria.  Conversely, some GISTs are primary in the mesentery
and omentum, or metastasize to these sites. 
The distinction between desmoid tumor and GIST is important for two reasons. First, ~10% of
desmoid tumors arise in patients with familial adenomatous polyposis (FAP) and therefore screening for
FAP should be considered in patients who develop desmoids. Second, the biologic behavior, treatment, and
prognosis of these tumors are different. Unlike GISTs, desmoids do not metastasize but tend to recur and
infiltrate within the abdominal cavity, potentially resulting in obstruction or even perforation of the
bowel wall.  The tendency to recur after surgery is stronger in patients with FAP, where
desmoid disease is the leading cause of death in those who have undergone prophylactic
The link between desmoids and FAP led to understanding the molecular abnormalities of these tumors,
and also led to the elucidation of an immunophenotype that can be helpful in differentiating them from
GIST. In FAP, desmoid tumors arise because of second-hit mutations of the adenomatous polyposis coli
(APC) gene superimposed on germline APC mutations. Bi-allelic inactivation of APC results in cytoplasmic
and nuclear accumulation of b-catenin protein, which in turn leads to increased cell proliferation rates.
Some sporadic desmoids also arise through bi-allelic APC inactivation,  but more commonly
APC is wild-type and they instead harbor activating mutations of the b -catenin gene that lead to b
-catenin protein stabilization.  Regardless of whether APC or b -catenin is mutated,
accumulation of b -catenin protein in neoplastic spindle cell nuclei can be demonstrated by
immunohistochemistry in most (>90%) desmoids. b -catenin staining is negative in GISTs, sclerosing
mesenteritis, and leiomyosarcomas and so can help to distinguish desmoid tumors from their
The use of CD117 immunostaining to differentiate desmoids from GISTs can be difficult. Various
studies have reported CD117 staining in 0-75% of desmoids (Table 2), depending on the antibody used, its
dilution, and antigen retrieval method.
Optimization of the immunostaining method
has been shown to result in a very low level (only ~5%) of CD117 expression in desmoids.
 Smooth muscle actin (SMA) and desmin are not helpful in distinguishing desmoids from GISTs.
CD34, however, can be helpful since it is not expressed in desmoid tumors but is present 40%, 54%, 57%,
59%, 82%, and 94% of GISTs from the ileum/jejunum, duodenum, omentum/mesentery, colon, stomach, and
Immunohistochemistry is not needed for the diagnosis of most desmoids, which can be diagnosed by
recognition of several characteristic histologic features. Unlike GISTs, desmoids usually show
histologically infiltrative borders even in cases where they appear grossly circumscribed, and they lack
the necrosis or cystic degeneration that can be seen in GISTs. Desmoids are lesions of only low- to
moderate-cellularity, composed of uniform spindle cells in a collagenous stroma; keloid-like collagen
fibers may be present. They lack the high cellularity, nuclear atypia, atypical mitoses, skeinoid
fibers, nuclear palisading, and epithelioid component of some GISTs.
pattern also differs. A typical low-power feature of desmoids is the presence of small muscular arteries
and dilated, thin-walled veins with slight perivascular lymphocytes. Mitotic rate is not a helpful
feature in distinguishing desmoids from GISTs. The mitotic index of desmoids is usually low (mean of 2-4
mitoses per 50 high power fields [HPFs]) but can reach 10-11 mitoses per 50 HPFs.
Unlike the situation in GISTs, size, mitotic index, and mucosal invasion in desmoids do not serve as
prognostic features. Management is aimed at local control, since desmoids do not metastasize. Surgical
resection has historically been the first-line approach to desmoid tumors but this might not be feasible
in cases where a large portion of the small bowel is involved. Additionally, surgery itself is one of
the risk factors for occurrence and recurrence of desmoids in patients with FAP, who can develop multiple
tumors.  Because of the potential problems with surgery, and because clean margins are
difficult to achieve with the infiltrative nature of these tumors, other approaches to management have
been attempted. These include primary or adjuvant external beam radiation, cytotoxic and noncytotoxic
chemotherapies, tamoxifen, and NSAIDs.  Even though desmoids do not have KIT mutations,
treatment with imatinib was recently shown cause partial tumor regression or stabilization of disease in
a minority of patients with advanced desmoid tumors. 
Smooth Muscle Tumors
In the years before CD117 and CD34 immunostaining, many mesenchymal tumors called
leiomyomas or cellular leiomyomas were actually low-grade GISTs. However, there are true leiomyomas of
the colorectum (and rarely, small bowel) that differ from GISTs in their anatomical location, histology,
immunophenotype, and biologic behavior. In fact, true leiomyomas are several times more common than
colonic GISTs.  Colorectal leiomyomas do not tend to cause symptoms and nearly all are
incidental findings. They can be discovered either during endoscopic screening for colorectal cancer,
where they present as small polyps covered by an intact mucosa, or as incidental polyps in resection
specimens. Their main importance is in correct classification so that they are not mistaken for GIST.
The largest study of colorectal leiomyomas is by Miettinen et
al encompassing 88 tumors.  They range from 1 mm – 2.2 cm with a median diameter of 4
mm, and display a mild male predominance (2.4:1). They arise almost exclusively from the muscularis
mucosae, which is in contrast to the muscularis propria origin of most GISTs. Therefore, leiomyomas are
easy to biopsy and resect endoscopically. Histologically, leiomyomas contain plump smooth muscle bundles
that merge with the muscularis mucosae. They are usually even less cellular than the adjoining
muscularis mucosae. In distinction to GISTs, leiomyomas are always negative for CD117 and CD34 and are
diffusely positive for muscle markers including SMA and desmin. Occasional leiomyomas demonstrate
significant nuclear atypia akin to "symplastic" leiomyomas of the uterus. In Miettinen's series, 2 of 88
lesions showed cellular atypia, and one of these also had a single mitotic figure. Leiomyomas of the
muscularis mucosae behave in a benign fashion, although follow-up to ensure complete endoscopic excision
of "atypical" leiomyomas was recommended. 
True leiomyomas also commonly arise in the distal two-thirds of the esophagus and GE junction region.
Leiomyomas are the most common intramural soft tissue tumor of the esophagus and outnumber esophageal
GISTs by 2 to 3-fold.  Unlike colonic leiomyomas, esophageal leiomyomas usually arise in the
muscularis propria and only rarely (~1%) in the muscularis mucosae.  They can lack the
rounded contour of colorectal leiomyomas and instead can have lobulations, a serpiginous outline, or a
horseshoe shape.  Esophageal leiomyomas can be minute ("seedling" leiomyomas of <1 mm – 1
cm are recognized in 22% – 46% of surgical resections for esophageal carcinomas
giant (>10 cm).  Small leiomyomas are asymptomatic while larger tumors (>5 cm) can
result in epigastric discomfort and dysphagia. 
The distinction between esophageal leiomyoma and GIST cannot be based on anatomical location or size,
but instead requires evaluation of the morphology and immunophenotype. Grossly, leiomyomas have a firm
consistency, white or tan color, and whorled cut surface that is similar to uterine leiomyomas.
Histologically, they are low- to moderate cellularity with bland, pink spindle cells and only rare
mitoses. 24% of esophageal GISTs are epithelioid rather than spindled  and they are more
cellular than leiomyomas. Immunohistochemistry easily separates esophageal leiomyomas from low-grade
GISTs. SMA and desmin are almost always globally positive in leiomyomas, but only 12.5% of esophageal
GISTs show SMA positivity and only 19% are desmin positive.  Leiomyomas are always negative
for CD34 and CD117. It is important not to over-interpret CD117 reactivity in mast cells or the rare
leiomyoma that has a few entrapped CD117+ interstitial cells of Cajal. 
Esophageal leiomyomas are benign regardless of their size. In a 40-year review of esophageal smooth
muscle tumors at Massachusetts General Hospital, there was no malignant degeneration nor any tendency
toward recurrence.  In contrast, 53% of patients with esophageal GISTs in Miettinen's study
died of disease, including all patients who had tumors larger than 10 cm.  When symptomatic,
leiomyomas usually can be treated conservatively with enucleation. Open enucleation by thoracotomy has
been the traditional approach, but more recently video-assisted thoracoscopic and laparoscopic approaches
to enucleation are now low-morbidity alternatives. 
Rarely, intramural leiomyomas arise in the duodenum or anorectum. Most of these are asymptomatic but
they can cause duodenal obstruction or rectal bleeding.
 Intramural leiomyomas
of the duodenum and anorectum are similar grossly, microscopically, and immunophenotypically to
esophageal leiomyomas, but have not been reported to reach the large sizes of some esophageal tumors.
Sizes in the duodenum have ranged from 2 – 4 cm and in the anorectum from 1.5 – 4
GI leiomyosarcomas are much less common than either leiomyomas or GISTs. Only rare esophageal
leiomyosarcomas have been described. Of 3 well-characterized esophageal leiomyosarcomas, all were large
(9 – 16 cm) with mild nuclear pleomorphism and high mitotic rates (>50 mitoses/50 high power fields),
and ultimately resulted in death.  One case report described a pedunculated intralumenal
esophageal leiomyosarcoma arising from the muscularis mucosae,  but probably most esophageal
leiomyosarcomas (like leiomyomas) originate in the muscularis propria.
In the remainder of the GI tract, leiomyosarcomas are generally larger than leiomyomas and they do not
arise from the muscularis mucosae. Duodenal leiomyosarcomas range from 10 – 19 cm and most are
transmural.  Colonic leiomyosarcomas range from 3 – 8 cm and tend to form polypoid
intralumenal masses.  Leiomyosarcomas of the anorectum are smaller (2 – 5.5 cm) at
presentation, and also tend to form polypoid intralumenal masses.  Ulceration of the
overlying mucosa commonly occurs. Histologically, they are spindle cell tumors resembling smooth muscle.
In contrast to leiomyomas and in contrast to many GISTs, the mitotic rate of gastrointestinal
leiomyosarcomas is high, with >50 mitoses per 50 high power fields in most reported cases.
Leiomyosarcomas are uniformly positive for S MA, although occasionally the staining is focal. Desmin
positivity is less consistent. Of 19 leiomyosarcomas from the esophagus, duodenum, colon, and anorectum
studied by Miettinen and colleagues, only 12 (63%) were desmin positive and the labeling was often
All of Miettinen's cases were negative for CD117 and for CD34, with the
exception of a single esophageal leiomyosarcoma which showed CD34 labeling in 40% of the tumor cells.
All leiomyosarcomas studied for KIT mutations have been negative.
GI schwannomas are rare tumors that are often misdiagnosed as GISTs. Of 191 gastrointestinal
mesenchymal tumors evaluated by Kwon and colleagues, only 12 (6.3%) were schwannomas. 
Schwannomas are most common in the stomach, then colorectum, esophagus, and small intestine.
Histologically they are distinctive. Unlike schwannomas in other sites, GI schwannomas typically lack
well-defined nuclear palisading, Verocay bodies, xanthoma cells, and hyalinized vessel walls. Instead,
the usual GI schwannoma is a moderately or highly cellular tumor containing interlacing bundles of
spindle cells with wavy, tapered nuclei. "Benign nuclear atypia" is a characteristic finding consisting
of nuclear variability and hyperchromasia.  Cell borders are often indistinct and the
tapered nuclei appear to be interspersed among parallel collagen strands or in a myxoid stroma with
inflammatory cells. Mitotic activity is rare, in most studies <1 mitosis per 50 high power
and never over 5 per 50 high power fields.
Grossly, GI schwannomas can not reliably be distinguished from GISTs. Similar to GISTs, they are
well-circumscribed but unencapsulated lesions and most involve the muscularis propria. Size is variable,
from <1 cm up to 12 cm; among gastric schwannomas the mean size is 2.8 cm.  Schwannomas
lack necrosis and they do not invade the lamina propria; however, mechanical ulceration of the overlying
mucosa can occur. Cytologic features do not separate schwannomas from GISTs, especially since nuclear
palisading is reported in one-third of gastric GISTs.  A low-power feature that should alert
the pathologist to the presence of schwannoma is a peripheral lymphoid cuff, often containing germinal
centers. This is a characteristic finding in GI schwannomas
and is uncommon in
Immunostaining is necessary for an accurate distinction between schwannoma and GIST. GI schwannomas
are strongly and diffusely S100 positive in both nuclei and cytoplasm. Positive staining for glial
fibrillary acidic protein (GFAF) is seen in 63.6%,  70%,
and can be focal. Schwannomas are negative for muscle markers, CD117, and
CD34. In contrast, 91% of gastric GISTs stain for CD117, 82% for CD34, and <1% for S100.
 Correctly diagnosing schwannoma is clinically important because these tumors are all benign.
The behavior of GISTs, in contrast, is hard to predict and depends on tumor size, location, and mitotic
activity. In the largest study of gastric GISTs, overall tumor-specific mortality was 17%. 
GI schwannomas have no relationship to gastrointestinal autonomic nerve tumors (GANTs), a subtype of
stromal tumors with distinctive ultrastructural features (neurosecretory granules, well-developed cell
processes, and rudimentary cell junctions) but identical molecular characteristics and biologic behavior
to GISTs. 
Patients with neurofibromatosis 2 (NF2) are at increased risk of CNS and soft tissue schwannomas,
particularly bilateral vestibular schwannomas. However, none of the reported GI schwannomas has occurred
in a patient with NF2. Lasota and colleagues studied GI schwannomas for NF2 mutations and loss of
heterozygosity (LOH) at the NF2 locus on chromosome 22q.  Unlike conventional schwannomas –
where inactivating NF2 mutations are commonly (40-60%) identified – GI schwannomas had no NF2 mutations
and only a low rate (5%) of LOH at 22q. This underscores the differences (both morphologic and genetic)
between GI schwannomas and conventional schwannomas.
Inflammatory Fibroid Polyps:
Inflammatory fibroid polyps (IFPs) are rare, benign tumors of unclear origin. They are most common in
the stomach, particularly antrum and prepylorus, where they tend to form semipedunculated polyps covered
either by normal mucosa or – when large – by a central erythematous depression, ulceration, or
inflammatory cap.  IFPs also occur in the small bowel, particularly in the terminal ileum,
where they can cause intussusception or obscure bleeding. Uncommonly, they arise in the colon,
esophagus, or gallbladder. IFPs are usually based in the submucosa, but they often extend into the
overlying mucosa and can occasionally be transmural. Microscopically, there is a proliferation of
spindled and stellate stromal cells which tend to condense around blood vessels in whorled, perivascular
cuffs. These stromal cells are embedded in a granulation tissue-type stroma or fibromyxoid stroma with
numerous blood vessels and inflammatory cells. Prominent eosinophils are characteristic, but
lymphocytes, lymphoid aggregates, plasma cells, macrophages, and mast cells are also present.
IFPs can be mistaken for GIST because of their spindle cell proliferation. Their abundant small
vessels and inflammatory cells can be mistaken for nonspecific granulation tissue. In cases where
morphology is not diagnostic, immunophenotyping can be helpful. Stromal cells of IFP are positive for
– which is the most commonly used immunostain to confirm the diagnosis – as well as
fascin, CD35, cyclin-D1, and calponin.  A minority stain for smooth muscle actin.
Positivity for CD34 can lead to confusion with GIST. However, IFPs are negative for
although it is important not to over-interpret CD117+ mast cells.
Solitary Fibrous Tumors:
Solitary fibrous tumors (SFTs) are rare neoplasms that are derived from submesothelial cells.
Although most common in the pleura, SFTs can occur in a wide variety of extrapleural locations including
the mesentery and peritoneum.
The fact that SFTs only rarely occur in the abdominal
cavity can accentuate their tendency to be misdiagnosed as GIST, since pathologists can forget to include
SFT in the differential diagnosis of a spindle cell tumor. Grossly, SFTs form circumscribed firm masses
with pale, sometimes whorled, cut surfaces. The spindled cells are resemble fibroblasts growing in
fascicular, storiform, or myxoid patterns.  Hyalinized collagen fibers are characteristic of
SFTs. Features predicting malignancy are nuclear pleomorphism, >4 mitoses per 10 HPFs, and hemorrhage
or necrosis.  Pleural and extrapleural SFTs are CD34 and CD99 positive, and negative for
CD117. Immunostaining for CD99 does not help to distinguish SFTs from GISTs since 89% of GISTs are CD99
positive.  CD117 is the best marker to help make the distinction. However, relatively few
immunophenotypic studies of SFTs have included abdominal cases. Miettinen and colleagues evaluated 5
abdominal SFTs and confirmed that all 5 were CD34+ and negative for CD117, SMA, desmin, and
Remaining aware of the many different mesenchymal tumors of the GI tract and abdominal cavity is the
first step in avoiding an over-diagnosis of GIST. Knowing the locations and histologic features of these
different tumors is the second step, and in some cases (for example, leiomyomas of the muscularis
mucosae, classic desmoid tumors, inflammatory fibroid polyps, or sclerosing mesenteritis) histology alone
may be enough for a confident diagnosis. In many cases, immunophenotyping is needed as a third step to
exclude/confirm GIST. Immunostaining only for CD117 is insufficient and a panel of immunostains is
required (Table 3). Several studies have shown that a basic panel consisting of CD117, CD34, desmin, and
S100 differentiates most mesenchymal tumors of the GI tract.
b -catenin can be added
to the immunopanel when separation of desmoid from GIST is problematic. 
TABLE 1. Soft Tissue Lesions in the Differential Diagnosis of GIST
|Lesion Type ||Usual Location|
|Discussed in handout|
| Leiomyoma (colorectal) ||Muscularis mucosae|
| Leiomyoma (esophagus/GEJ) ||Mural|
| Leiomyosarcoma ||Mural|
| Schwannoma ||Mural|
| Inflammatory fibroid polyp ||Submucosal|
| Desmoid ||Mesenteric|
| Solitary fibrous tumor ||Subserosal or peritoneal|
| Benign fibroblastic polyp ||Mucosal|
| Ganglioneuroma ||Mucosal/rarely mural|
| Granular cell tumor ||Submucosal|
| Glomus tumor ||Submucosal|
| Neurofibroma ||Submucosal|
| Sarcoma (e.g., liposarcoma) ||Mesenteric|
| Sclerosing mesenteritis ||Mesenteric|
| Inflammatory myofibroblastic tumor ||Mesenteric/occasionally mural|
| Follicular dendritic cell tumor ||Mesenteric or mural|
TABLE 2. CD117, CD34, Smooth Muscle Actin, and Desmin in Desmoid Tumors
|Study ||CD117 (%) ||CD117 Antibody, Dilution, and Pretreatment ||CD34 (%) ||SMA (%) ||Desmin (%)|
|Miettinen  ||0/14 (0%) ||Santa Cruz, 1:400, heat ||- ||- ||-|
|Leithner  ||0/13 (0%) ||Dako, 1:1000, not specified ||- ||- ||-|
|Hornick  ||1/20 (5%) ||Dako, 1:250, none ||- ||- ||-|
|Lucas  ||1/19 (5.3%) ||Dako, 1:250, none ||- ||- ||-|
|Yamaguchi  ||15/25 (60%) ||Dako, 1:50, heat ||0/25 (0%) ||21/25 (84%) ||5/25 (20%)|
|Montgomery  ||6/10 (60%) ||Dako, prediluted, steam ||0/10 (0%) ||5/9 (56%) ||3/8 (37%)|
|Yantiss  ||9/12 (75%) ||Dako, 1:30, heat ||0/12 (0%) ||9/12 (75%) ||-|
TABLE 3. Immunohistochemistry for GIST and Other Mesenchymal Tumors of the GI Tract
|Tumor ||CD117 ||CD34 ||Desmin ||S100 ||β-catenin|
|GIST* ||+ (>90%) ||+ (47-100%) ||- (<5%) ||- (1-8%) ||-|
|Leiomyoma ||- ||- ||+ ||- ||-|
|Leiomyosarcoma ||- ||- ||+ (70-100%) ||- ||-|
|Schwannoma ||- ||- ||- ||+ ||-|
|Solitary fibrous tumor ||- ||+ ||- ||- (0-24%) ||- (0-24%)|
|Desmoid ||- ||- ||+/- (19%) ||- ||+ (90-100%)|
|Inflammatory fibroid polyp ||- ||+ (>70%) ||- ||- ||N/A|
*The staining pattern of GISTs depends upon the specific GI site. The numbers given above in parentheses for GIST and other mesenchymal tumors represent averages from refs 18, 47, and 50.
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