Surg Today (2001) 31:715–718

Gastrointestinal Stromal Tumor of the Lesser Omentum:Report of a Case

Hiroyuki Fukuda1, Toshikazu Suwa

1, Fumio Kimura

1, Toshiyuki Sugiura

1, Tokuzou Shinoda

1,

and Kou Kaneko2

Departments of 1 Surgery and 2 Pathology, Omiya Red Cross Hospital, 8-3-33 Kamiochiai, Saitama 338-8553, Japan

of both muscle and nerve (mixed GISTs), or lack differ-entiation (GISTs not otherwise specified).5,6 This reportdescribes a case of an omental GIST without lightmicroscopic or immunohistochemical evidence of myo-genic or neurogenic features. Only 14 cases of this typeof omental GIST have previously been reported,1,3 al-though radiological findings were documented in justone case. We describe the radiological and pathologicalfindings of the tumor from our patient.

Case Report

A 45-year-old Japanese man was admitted to our hospi-tal for investigation of an intra-abdominal tumor whichhad been identified on abdominal ultrasonography.Physical and laboratory examinations were unremark-able. Abdominal computed tomography (CT) and mag-netic resonance imaging (MRI) showed a well-definedsolid mass, about 4 cm in diameter, lying adjacent to thestomach (Fig. 1a). Alimentary examination of the gas-trointestinal tract and gastroscopy showed that thelesser curvature of the corpus ventriculi was com-pressed extraluminally without mucosal abnormalities.Angiography revealed a hypervascular mass fed bybranches of the left and right gastric arteries locatedadjacent to the stomach (Fig. 1b). The mass was pre-operatively diagnosed as an extramucosal tumor of thestomach.

At laparotomy, a well-encapsulated tumor was foundin the lesser omentum. It was slightly adherent to thestomach wall, but was removed without difficulty. Theexcised tumor, which was 4.5 � 2.5 � 3.0cm in size, hada white cut surface without necrosis or hemorrhage(Fig. 2). Histologically, the tumor was composed ofspindle–shaped cells with an interlacing bundle patternexhibiting microcystic changes and moderate cellularity(Fig. 3). No mitotic figures were observed. Immunohis-tochemical examination showed that the tumor was

Abstract We describe herein an extremely unusual caseof a gastrointestinal stromal tumor (GIST) of the lesseromentum. A 45-year-old man was admitted to our hos-pital with an intra-abdominal mass that was subse-quently misdiagnosed as a submucosal tumor of thestomach. The tumor arose from the lesser omentumand was removed without difficulty. Histologically, thetumor was composed of spindle–shaped cells with aninterlacing bundle pattern, and immunohistochemicalexamination showed that it was positive for myeloidstem cell antigen (CD34), but negative for HHF35 andS-100 protein. These findings were consistent with aGIST lacking myogenic features and neural attributes.The patient had an uneventful postoperative course,and was free of recurrence when last seen 11 monthsafter his operation.

Key words Myogenic tumor · Neurogenic tumor ·Omentum · Gastrointestinal stromal tumor

Introduction

Gastrointestinal stromal tumors (GISTs) are mesen-chymal tumors that arise mainly in the stomach andsmall intestine. They rarely originate in other intra-abdominal tissues such as the omentum and mesentery.1

Until recently it was believed that most GISTs weresmooth muscle neoplasms; however, with the advent ofimmunohistochemistry and electron microscopy, it hasbecome apparent that some GISTs have myogenicfeatures (smooth muscle GISTs), neural attributes (gas-trointestinal autonomic nerve tumors), characteristics

Reprint requests to: H. FukudaReceived: April 3, 2000 / Accepted: March 6, 2001

716 H. Fukuda et al.: Gastrointestinal Stromal Tumor

been reported.1 The omental GIST in this patientlacked differentiation of myogenic features and neuralattributes. Only 15 omental GISTs without myogenic orneurogenic features, including the present case, haveever been reported (Table 1). The tumors arose in sevenmen and eight women ranging in age from 31 to 89 years(median 61 years). The tumor diameters ranged from2.5 to 36cm (median 15.7cm). Macroscopically, mostwere large, solid masses exhibiting cystic changes. His-tologically, they were classified as spindle cell type (n �9) or epithelioid cell type (n � 6). The spindle celltumors had plump nuclei and a prominent vascular pat-tern, or elongated nuclei with focal nuclear palisadingand perinuclear vacuolization. In contrast, the epithe-

Fig. 1. a Abdominal computed tomography scan demon-strated a well-defined solid mass about 4 cm in diameter adja-cent to the stomach. b Angiography revealed a hypervascular

mass (arrows) fed by branches of the left and right gastricarteries

Fig. 2. The excised tumor, 4.5 � 2.5 � 3.0 cm in size, had awhite cut surface without necrosis or hemorrhage

Fig. 3. Microscopic findings. The tumor was composed ofspindle-shaped cells with an interlacing bundle pattern. It ex-hibited microcystic changes and moderate cellularity. Mitosiswas not observed (H&E, �40)

positive for myeloid stem cell antigen (CD34), but nega-tive for HHF35 and S-100 protein (Fig. 4). A few tumorcells were positive for MIB-1. These findings wereconsistent with a GIST lacking myogenic features andneural attributes. The patient had an uneventful post-operative course and was free of recurrence when lastseen 11 months after his operation.

Discussion

GISTs are mesenchymal tumors that usually develop inthe stomach and small intestine, although GISTs origi-nating in the omentum and mesentery have recently

a b

717H. Fukuda et al.: Gastrointestinal Stromal Tumor

lioid cell tumors contained sheets of polygonal cellshaving abundant, granular cytoplasm with a peripheralhalo-like clearing and sharp cell borders. Both cell typeshad a very low mitotic rate in 10 of the 15 cases, al-though a moderate mitotic rate was seen in 2 cases.Immunohistochemically, most of the tumors had similarfeatures immunohistochemically. Of the 15 tumors, 12were positive for CD34, which is expressed in endothe-lial cells, some fibroblasts, and GISTs,7 while 8 of the15 exhibited α-smooth muscle actin reactivity. Bothdesmin and S-100 protein were positive in only one case.The transmembrane receptor for stem cell growth fac-tor (CD117) was positive in all cases tested.

Table 1. Reported cases of gastrointestinal stromal tumors of the lesser omentum

Age (years)/ Size MitosisAuthorsRef. Year sex (cm) Cell type (per 50 HPF) Metastasis Outcome

Takahashi et al.3 1998 71/M 17 Spindle 1–3 None Died of coloniccarcinoma

Miettinen et al.1 1999 58/F 2.5 Epithelioid 1 Unknown Died of unknowncause

89/M 2.5 Spindle/epithelioid 7 Unknown Alive, 3.5 years31/F 7.5 Spindle 19 Unknown Alive, 2 years80/F 10 Epithelioid 7 Unknown Alive, 1.6 years44/M 12 Epithelioid �1 Unknown Alive, 1.5 years72/M 15 Spindle/epithelioid 26 Unknown Unknown67/F 16.5 Spindle 5 Unknown Unknown56/F 20 Epithelioid 0 Unknown Alive, 2 years64/M 20 Spindle 4 Unknown Unknown34/M 23 Spindle 1 Unknown Alive, 3.4 years60/M 24 Epithelioid 1 Unknown Alive, 8.5 years68/F 26 Spindle 2 Unknown Unknown70/F 36 Epithelioid �1 Unknown Alive, 1 year

Present case 2001 45/M 4.5 Spindle 0 None Alive, 0.9 year

HPF, high-power field

Fig. 4. Immunohistochemical find-ings. The tumor was positive forCD34 (immunohistochemical stain,�10)

The radiological features of omental GISTs withoutmyogenic or neurogenic features have not been estab-lished. In both of the cases examined by abdominal CTand angiography, a hypervascular mass with clear mar-gins was demonstrated. This suggests that the radiologicfeatures may be similar to those of omental leiomyomasand leiomyosarcomas; however, it is difficult to distin-guish omental GISTs from these tumors radiologically.Most GISTs with myogenic features are demonstratedas hypervascular tumors with clear margins on CT andangiography.8–10 Despite the use of advanced radiologi-cal imaging techniques, it is difficult to differentiate aGIST in the lesser omentum from a GIST in the lesser

718 H. Fukuda et al.: Gastrointestinal Stromal Tumor

curvature of the stomach. In fact, about half of all omen-tal GISTs are misdiagnosed as extramucosal tumors ofthe stomach.8–11

Surgical resection is the only effective treatment as noadjuvant therapy has been established.2 Furthermore,no postoperative recurrences of omental GISTs withoutmyogenic or neurogenic features have been reported,1,3

although 5 of the 10 patients with mesenteric GISTslacking myogenic and neurogenic features had a pooroutcome despite surgical resection.1 It has been re-ported that tumor size, cellularity, mitotic index, distantmetastasis, invasion, and necrosis affect the prognosis ofpatients with gastric or small intestinal GISTs.12 Thesefactors may also affect the prognosis of patients withomental GISTs, in which case, according to the criteriaof gastric or small intestinal GISTs, the omental GISTin our patient is considered to have been a borderlinemalignancy based on tumor size and moderate cellular-ity. However, the malignant potential of omental GISTshas not been established and therefore, follow-up isneeded to rule out malignant potential, late recur-rences, or metastasis.

In conclusion, GISTs without myogenic or neuro-genic features that arise in the omentum are very rareand surgical resection is the only effective modality. Themalignant potential of this unusual tumor has not beenestablished.

References

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