|Year : 2012 | Volume
| Issue : 3 | Page : 433-435
Primary extra-gastrointestinal stromal tumor of the omentum
Jeevan Divakaran, Bal Chander
Department of Pathology, Dr. Rajender Prasad Government Medical College, Kangra at Tanda, Himachal Pradesh, India
|Date of Web Publication||17-Nov-2012|
Department of Pathology, Dr. Rajender Prasad Government Medical College, Kangra at Tanda, Himachal Pradesh
Source of Support: None, Conflict of Interest: None
Gastrointestinal stromal tumors (GISTs) are mesenchymal tumors originating from interstitial cells of Cajal or related stem cell-like precursors present in the wall of the gastrointestinal tract. However, identical tumors originating from areas other than the gastrointestinal tract have been reported which are histologically identical to the usual GISTs. We are reporting a case of primary omental GIST in a 57-year-old female.
Keywords: Gastro intestinal stromal tumor, omentum, CD117
|How to cite this article:|
Divakaran J, Chander B. Primary extra-gastrointestinal stromal tumor of the omentum. J Can Res Ther 2012;8:433-5
| > Introduction|| |
Gastrointestinal stromal tumors (GISTs) are rare intra-abdominal tumors of mesenchymal origin, defined as expressing positive histochemistry stain to c-KIT (CD117) ligand. Mostly, these tumors arise within the muscularis propria of the gastrointestinal tract wall and therefore are found more often in the stomach (60%), jejunum and ileum (30%), and infrequently in the duodenum (5%), colorectal (<5%) and less than 1% in the esophagus and appendix.  Omental, mesenteric and retroperitoneal locations account for less than 5% of these tumors. Those arising from outside the gastrointestinal tract are labeled extra-gastrointestinal stromal tumors (EGISTs).  A review of the literature has shown that so far only 28 cases of primary omental GISTs have been reported.  Herein, we present yet another rare case of primary omental GIST which has been confirmed with immunohistochemical analysis.
| > Case Report|| |
A 57-year-old female patient was admitted with complaints of pain abdomen for the past one year. There was no history of loss of appetite or any gastrointestinal symptoms. She is a known case of hypothyroidism, on treatment for the last six years. Physical examination revealed a mass arising from the pelvis up to the umbilicus; firm, mobile and non-tender. Per vaginal exam showed the mass to be filling the whole of the pelvis, measuring about 22 weeks' size of period of gestation. A provisional diagnosis of fibroid uterus/ovarian tumor was made. All investigations including CA 125 were normal.
Ultrasonogram suggested that the uterus was grossly enlarged with a hypo-echoic degenerating fibroid showing cystic changes and measuring 9.55×11.48×11.20 cm.
A CT scan confirmed that the 13.8×10.7×9.9-cm size mass was arising from the left side of the pelvis extending towards the midline of the abdomen, anterior to the urinary bladder. The mass was predominantly cystic with presence of irregular solid enhancing elements in it. The uterus seemed uninvolved. The left ovary was not visualized separately and the mass appeared to be arising from the left ovary. Right ovary was normal. There was no lymphadenopathy. Kidneys, liver, spleen and pancreas were normal.
The scan concluded a possible malignant ovarian tumor. A mass measuring 10×10 cm was removed under epidural anesthesia. It appeared to arise from the omentum and was adherent to the mesenteric border of the distal jejunum. Omentectomy was performed along with jejunal resection.
A grayish brown tumor measuring 9×9×7 cm having a smooth external surface with attached omentum measuring 8×6×2 cm was received. The tumor was necrotic and hemorrhagic. Two segments of small intestine measuring 9 cm each were also received, with one of them displaying a grey brown tumor deposit measuring 3 cm on the serosal aspect [Figure 1].
|Figure 1: Gross photograph: Omental mass with large areas of necrosis and hemorrhage. Inset shows segment of small intestine with cut surface of serosal deposit|
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The microscopic examination of the sections from the tumor showed a spindle cell lesion with cells arranged in fascicles. Most of the nuclei were vesicular and had blunt ends with a proportion of them exhibiting prominent nucleoli. Cytoplasm was moderate in amount and cell boundaries were indistinct. There was moderate pleomorphism and mitotic rate was 3 to 4 per 10 HPF. There were foci of dense lympho-plasmacytic infiltrate with areas of hemorrhage and necrosis [Figure 2].
|Figure 2: Microphotograph low power H & E stain showing section from omental mass with areas of necrosis and hemorrhage along with tumor composed of spindle cells. Inset shows high-power view of tumor|
cells with mitosis (arrow)
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The histomorphological features of serosal tumor deposits were similar to the omental mass but without the features of necrosis and hemorrhage. The tumor was not seen infiltrating the muscularis propria.
The stain for CD117 was diffusely positive as was smooth muscle actin (SMA) in both the omental mass as well as in the serosal tumor deposit [Figure 3].
Based on the above mentioned features, a diagnosis of extra-gastrointestinal stromal tumor was concluded.
|Figure 3: Microphotograph high-power CD 117 stain (right) showing diffuse positivity in tumor cells whereas smooth muscle actin (left) shows focal positivity|
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| > Discussion|| |
It is generally accepted that GISTs arise primarily from the interstitial cells of Cajal or related stem cell-like precursors present in the wall of the gastrointestinal tract. Although they are characterized by the positive expression of the receptor CD117, also known as c-kit, some GISTs do not or only weakly express this marker. 
EGISTs arise outside the gastrointestinal tract but they share histological features with their gastrointestinal counterpart. The clinical, pathological and prognostic features of GISTs are widely known, while data about EGISTs are very few: incidence, histogenesis and histological predictors of outcome are not yet defined.
Sakurai et al., published their results on the cytological, immunohistochemical, and genetic analysis of five omental mesenchymal tumors. They found all tumors to be positive for CD117 and CD34 staining, while all were negative for smooth-muscle cell markers. More importantly, the authors reported finding c-KIT immunoreactive CD117 and CD34 cells within specimens of the omentum.  These findings and those of Yamamoto et al.,  underscore the fact that histologically EGISTs have a similar appearance to GISTs and that EGIST is a distinctive entity, different from leiomyosarcomas.
The Kit/PDGFRA tyrosine kinase inhibitor Imatinib has been successfully used in the treatment of metastatic GISTs for more than five years. ,
Microscopic features are site-dependent and the majority appears as spindle cell tumors. , Mitotic activity, cellularity and presence of necrosis have been found to be associated with worse outcomes. C-kit gene mutations were not found to correlate with prognosis in patients with EGISTs. 
The prognostic factors indicating the malignant potential of GISTs include mitotic rate, tumor size and location. GISTs with a large size (>10 cm in diameter) or with a high mitotic count (>10/50 HPFs) and GISTs with diameter >5 cm and more than 5 mitotic figures/50 HPFs are considered at high risk for recurrence. 
Based on the criteria advocated by Miettinen and Lasota, (tumor size and mitotic activity) the tumor being reported is placed in Group 3a, at intermediate risk. ,
Our case demonstrated strongly positive immunohistochemical expression of c-kit (CD117) but because of the rarity of primary omental GISTs, the existing data is not sufficient to assess their malignant potential and their overall prognosis. Surgery remains the standard treatment for non-metastatic EGIST in the greater omentum. Imatinib therapy as an adjuvant to complete resection has been carried out safely and may prevent relapse to prolong long-term survival. Because of its rarity, it is essential to analyze accumulating data from case reports for a better and more detailed understanding of primary omental GISTs.
| > References|| |
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[Figure 1], [Figure 2], [Figure 3]