classical gastrointestinal stromal tumor ( gist ) is a non - epithelial neoplasm arising from the muscularis propria layer of the gastrointestinal tract leading to its tendency for exophytic growth . this is in contrast to other mesenchymal tumors like leiomyomas that involve the muscularis mucosae . gists most frequently occur in the stomach ( 70% ) , followed by the small intestine ( 20 - 30% ) , anorectum ( 7% ) , colon , and esophagus . extragastrointestinal stromal tumors ( egists ) are neoplasms with histopathologic features similar to gist , but are found outside the gastrointestinal tract with no evidence of a concurrent neoplasm in the gastrointestinal tract . this distinction from gist is essential since extragastrointestinal spread of a proven gist has been frequently documented . only 58 cases of pathologically proven egist in retroperitoneal location have been documented in current english medical literature and less than 10 of these had a preoperative imaging with confirmed histopathology . a 40-year - old female patient presented to the surgical outpatient department of our institution with a 4-month history of a gradually increasing generalized abdominal swelling , more on the left side . on palpation , generalized distension of the abdomen was noted , especially involving the left side quadrants . on the basis of clinical assessment , the patient was referred for cross - sectional imaging . patient underwent a contrast - enhanced computed tomography ( cect ) scan on a 16-slice ct scanner ( general electric brightspeed , milwaukee , wi , usa ) . cect was performed by injecting 90 ml of intravenous contrast iohexol ( omnipaque , general electric healthcare milwaukee , wi , usa ) through an 18-gauge needle in the antecubital vein at a rate of 3.5 ml / s . scanning parameters used were tube current of 105 ma s and peak tube voltage of 130 kilovoltage ( kvp ) . in addition to a baseline non - enhanced scan , image acquisition was done during venous phase ( 70 s ) and the excretory phase ( 3 min ) . the 5-mm - thick axial images were reformatted into thinner sections in three orthogonal planes ( 0.6 mm thick ) . non - enhanced computed tomography ( nect ) images showed a very large well - defined mass in the abdominal cavity , predominantly on the left side . these scans showed the lesion had predominantly low attenuation with fine linear calcification forming a reticular pattern . few foci of punctuate calcification were also seen in the posterior part of the lesion [ figure 1 ] . on post - contrast images , rest of the lesion showed areas of non - enhancing fluid attenuation [ figure 2 ] . few septae in the periphery of the mass appeared thicker with tiny calcified foci [ figure 3 ] . the mass caused compression of liver , spleen [ figure 3 ] , pancreas [ figure 4 ] , and descending colon . however , no evidence of infiltration or invasion was noted [ figure 5 ] ( negative embedded organ sign demonstrated , for comparison see [ figure 6 ] ) . the pancreas was noted as being displaced anteriorly and to the right of the midline and lay along the right lateral margin of the mass . a subtle but distinct flat plane was noted between the mass and pancreatic tissue . above findings show that the mass did not originate from any of the structures in close contact , indicating a lesion of retroperitoneal origin . both kidneys demonstrated normal parenchymal enhancement and contrast excretion , suggesting absence of distal obstruction [ figures 4 and 5 ] . 40-year - old female with abdominal mass subsequently diagnosed as extragastrointestinal stromal tumor ( egist ) in the retroperitoneum . nonenhanced computed tomography of abdomen ( axial section ) shows a large well - defined lesion with low attenuation areas and reticular pattern fine linear calcification ( small arrow ) involving the abdominal cavity , predominantly on the left side . few rounded foci of calcification are also noted in the posterior part of the lesion ( thick white arrow ) . contrast - enhanced computed tomography ( cect ) of abdomen ( coronal section ) during venous phase demonstrates multiple enhancing septae ( blue arrow ) radiating out from a central enhancing soft tissue attenuation structure ( red asterisk ) . cect abdomen ( axial section ) during venous phase shows the lesion compressing spleen ( solid arrow ) , left lobe of liver ( solid double arrows ) without invasion ( negative embedded organ sign ) . few tiny foci of calcification are also noted in the periphery ( arrowhead ) 40-year - old female with abdominal mass subsequently diagnosed as egist in retroperitoneum . cect abdomen ( axial section ) during venous phase shows the lesion compressing head and body of pancreas ( p ) without infiltration ( negative embedded organ sign ) and a distinct intervening fat plane with abdominal aorta ( a ) . lk : left kidney , rk : right kidney , i : inferior vena cava , sv : splenic vein . cect abdomen ( axial section ) in excretory phase demonstrates contrast excretion with adequate opacification of ureters ( thin black arrows ) . the mass lesion shows negative embedded organ sign with descending colon ( solid triple white arrows ) . embedded organ sign , schematic diagram ( reproduced with permission from ref . [ 4 ) . schematic diagram shows embedded organ sign , negative and positive . in order to better characterize the lesion , on t1-weighted spin - echo ( se ) images , the mass showed areas of low signal intensity ( isointense to skeletal muscle ) and multiple markedly hypointense ( dark ) linear bands and punctuate foci [ figure 7 ] . non - enhanced magnetic resonance imaging ( mri ) ( axial section ) of abdomen shows the mass with low signal intensity areas , multiple markedly hypointense linear bands ( blue arrow ) in a reticular pattern , and few hypointense round foci ( white arrowhead ) on t1-weighted spin echo ( se ) images . t2-weighted fast spin echo ( fse ) images demonstrated an extremely heterogeneous lesion composed of multiple well - defined loculations of varying sizes and signal intensity . most of the locules demonstrated iso- to mildly hyperintense signal intensity ( isointense to renal parenchyma , hyperintense to liver parenchyma ) [ figure 8 ] , but few of these also showed cyst - like bright fluid signal intensity [ figure 9 ] . the central area of the lesion showed irregular markedly hypointense signal intensity [ figure 9 ] . mri ( axial section ) of abdomen shows multiple loculated structures with iso- to mildly hyperintense signal intensity on t2-weighted fast spin echo ( fse ) image ( dashed arrow ) [ pancreas ( p ) , aorta ( a ) ] . mri ( coronal section ) of abdomen shows a few of the loculated structures with bright fluid signal intensity ( curved white arrow ) and the central region of the lesion with large hypointense signal ( red asterisk ) on t2-weighted fse images . intraoperatively , after mobilizing the descending colon and splenic flexure , the relationship of the mass with pancreas , left kidney , and surrounding vessels was noted . pancreatic body and tail was freely dissected from the tumor with the help of blunt and sharp dissection . there was no infiltration of mass into the retroperitoneal structures including left kidney and pancreas . , the mass appeared grayish white to brownish in color showing solid areas and partly necrotic surface . histopathologic examination using hematoxylin and eosin stain at ( 10 ) magnification , revealed extensive hyalinization , scanty cellular areas with areas of hemorrhage , and foci of osseous metaplasia [ figure 10 ] probably corresponding to the calcification in the lesion . further analysis of the stained sample at ( 40 ) magnification showed fascicles of uniform spindle cells with pale eosinophilic cytoplasm and perinuclear vacuoles indenting the nucleus [ figure 11 ] suggestive of a spindle cell sarcoma of low to intermediate grade - possibly leiomyosarcoma . photomicrograph [ hematoxylin and eosin ( h and e ) stained sample ( 10 ) ] shows extensive hyalinization , scanty cellular areas with few areas of hemorrhage ( black arrowhead ) , and foci of osseous metaplasia ( thin long black arrow ) . photomicrograph of hematoxylin and eosin ( h and e ) stained sample ( 40 ) shows fascicles of uniform bland spindle cells ( thin long red arrow ) with pale eosinophilic fibrillary cytoplasm and extravasated red blood cells . immunohistochemical staining revealed diffuse positivity for vimentin , but the actin and desmin markers were found to be negative . the strong and diffuse immunoreactivity for cd117 ( proto - oncogene c - kit ) [ figure 12 ] confirmed the lesion as gist in retroperitoneal location . immunohistochemical staining for cd117 ( c - kit ) ( 40 ) shows diffuse reactivity for tumor cells ( thick yellow arrow ) . by definition , the term gist applies only to those gastrointestinal mesenchymal neoplasms with immunoreactivity for cd117 marker , normally expressed on the myenteric plexus and interstitial cells of cajal in normal adult gastrointestinal tract . retroperitoneal egist , like typical gist , also shows positive staining for cd117 for confirmation . staining for other immunological markers is variable : b - cell lymphoma 2 ( bcl-2 ) ( 80% ) , cd34 ( 70% ) , smooth muscle actin ( 35% ) , s100 ( 10% ) , and desmin ( 5% ) . retroperitoneal egists have been shown to have two main histological patterns - spindle cell variety ( cigar - shaped cells with elongated nuclei ) and epithelioid type ( polygonal cells with centrally placed nuclei ) . egists in the retroperitoneum grow silently and consequently are only discovered once they start causing compression symptoms . understanding of the embedded organ sign ( both positive and negative ) helps in localizing the lesion to its possible organ of origin . gist of primary origin in the retroperitoneum has been often discussed in the surgical and pathologic literature , but rarely in the radiologic literature . on cross - sectional imaging , retroperitoneal egists mostly appear as well - defined , non - infiltrative , inhomogenous soft - tissue density masses with heterogeneous contrast enhancement . . they may show areas of cystic degeneration , necrosis , or calcification . the calcification pattern in the case reported by takao et al . , was peripheral rim - like , in contrast to the reticular pattern noted in our patient . the general pathologic consensus for grading and assessing metastasis risk of classical gastric gists is based on tumor size and mitotic count . no such consensus appears to exist for retroperitoneal egist as of yet , since metastases have been rarely reported . on the basis of imaging findings alone it may be very difficult to distinguish gist from other retroperitoneal mesenchymal neoplasms such as malignant fibrous histiocytoma , liposarcoma , leiomyosarcoma , and fibrosarcoma . on cross - sectional imaging , when a large well - defined retroperitoneal mass lesion with atypical calcification patterns ( peripheral rim like or reticular ) is encountered , gist should be considered in the differential diagnosis , even if the lesion shows poor contrast enhancement .
neoplasms with histology and immunohistochemistry similar to gastrointestinal stromal tumors may occur primarily outside the gastrointestinal tract , usually in the omentum and mesentery . 
 these are referred to as extragastrointestinal stromal tumors ( egists ) . 
 retroperitoneum is a very rare site for such neoplasms . 
 we report a patient with egist in the retroperitoneum , elaborating the cross - sectional imaging and histopathologic findings .