Extra gastrointestinal stromal tumour of omentum – A case report

Extra gastrointestinal stromal tumours (EGIST) are rare neoplasms of the abdomen which have similar morphological and immunohistological features of gastrointestinal stromal tumours. These tumours are not connected to either stomach or intestine and most commonly arise from mesentery, retroperitoneum, omentum etc. We describe a case of EGIST arising from omentum in a 40 year old male patient who presented with features of acute intestinal obstruction.


Introduction
GISTs are the most common mesenchymal tumours of the GIT arising from the interstitial cells of Cajal(ICC). Most of the tumours are located in stomach and small intestine. Some of the tumours that are histologically similar to GIST are found outside the GIT and are called as EGIST [1]. The majority of the tumours arise from mesentery, omentum and retroperitoneum. The other sites include ovary, pleura, uterus and abdominal wall. Positive immunohistochemical staining for CD117 is a characteristic feature for majority of GISTs.

Case report
A 40 year old male patient presented with clinical features of acute intestinal obstruction of three days duration. On examination he was found to have a 9x8 cm tender mass in left iliac fossa. There was no significant past history of abdominal illness. Routine laboratory investigations were within normal limits. Plain abdominal radiograph revealed signs of intestinal obstruction. Abdominal ultrasound showed 8.7x5.9 cm size hypoechoic mass in left side of abdomen anterior to tail of pancreas and adjacent to small bowel loops. The other findings on ultrasound were cholelithiasis, left renal cortical cyst and mild ascites.
CECT abdomen showed 10x9x7cm intensely enhancing, well defined mass with central necrotic areas and air pockets adherent to jejunal loops with minimal adjacent fat stranding ( Figure  1&2). Upper GI Endoscopy revealed grade B esophagitis with hiatus hernia. Exploratory laparotomy revealed a large friable necrotic mass in the omentum with multiple small intestinal adhesions. End to end anastomosis of jejunum with feeding jejunostomy was done. On histopathology gross specimen revealed multiple grey brown soft tissue bits altogether measuring 14x11x2cms. Multiple sections were studied which revealed a cellular spindle celled lesion arranged in fascicles, interlacing bundles interspersed by areas of spotty necrosis, haemorrhages and sparse lymphocytic infiltrate. Individual cells are plump to elongated spindle cells with scanty to moderate amounts of cytoplasm with a central oval to spindle vesicular nucleus along with occasional mitotic figures. Mild nuclear pleomorphism is noted. There was no evidence of any mucin/pigment production/ identifiable native tissue of either the bowel or lymph

Discussion
Majority of EGIST tumours are metastatic lesions of a primary GIST. 95% of GISTs carry an activating somatic mutation of CD117 [2]. CD117 is the product of proto-onco-gene c-KIT, a tyrosine kinase transmembrane receptor located on chromosome 4 [3]. The KIT proteins are detected by immunohistochemical assay for the CD117 antigen. EGISTs are thought to arise from stomach and small intestine but get separated from GIT during their development [4] and others found that multipotential mesenchymal stem cells are the cell of origin of these tumours [5].
Miettnesset et al compared the cases of omental and mesenteric EGISTs and found that omental EGISTs have a better prognosis than mesenteric EGISTs. Reith et al in the report on clinicopathological and immunohistochemical features of 48 EGISTs concluded that tumour size has no association with the final outcome of the patients. Yamanoto et al in the study of 39 cases of EGIST suggested the molecular characterization of GIST as a prognostic factor.
There are many reports of primary omental GISTs in the literature [6][7][8][9]. Though the age incidence in all published series is above 30 years, Mario reported a large EGIST from omentum in a 27 years old male which was surgically removed. Christian Franzini reported a case of EGIST from greater omentum in a 74 years old man and concluded that EGIST of greater omentum are slow growing tumours without clinical appearance. Dedemadi et al also reported a case of large EGIST from lesser omentum which was asymptomatic. Loiy et al reported a case of EGIST in the abdominal wall and proposed that it should be considered in the differential diagnosis of solid mass of the abdominal wall.
Barros A retrospectively analyzed 142 patients of GIST of which 9 were cases of EGIST arisisng from visceral sites like pancreas, mesocolon, spleen, pelvis and retroperitoneum. Other rare sites described in the literature include mesocolon transversum [10], pelvis [11]. Multiple recurrent EGISTs were also reported [12].
In the present case the patient came with acute intestinal obstruction which was a rare unusual presentation. These tumours have to be differentiated from other tumours like melanoma, desmoid tumours in which CD117 expression is detected. As per the national institute of health (NIH) modified classification these tumours have to be considered as high risk tumours as the size of the lesion was >5cms.

Conclusion
GISTs are not uncommon tumours and EGIST has to be considered in the differential diagnosis of solid masses of omentum or mesentery in an adult patient and pertinent IHC marker has to be done for accurate diagnosis.