You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
a 74-year - old female patient underwent a roux - en - y cystjejunostomy for pancreatic pseudocyst developed several melena episodes and she was surgically reappraised .
a 12  8.0  5.0 cm retro - gastric lesion was resected and pathology report indicated an unsuspected gastrointestinal stromal tumor ( gist ) . the report aimed to describe an atypical presentation of gist
medical literature has previously shown that a pancreatic cystic neoplasm could be mistaken for a pancreatic pseudocyst .
cystic tumors of the pancreas most commonly corresponds to serous microcystic adenoma , intraductal mucinous tumor , mucinous cystic tumor , and solid pseudopapillary tumor and , less commonly to cystic endocrine tumors , cystic metastasis , cystic teratomas , and lymphagiomas 1 .
the case reported aimed to redefine the differential diagnosis of a presumed pancreatic pseudocyst , emphasizing the possibility of gist .
a 74-year - old female patient was admitted to the internal medicine ward of so paulo hospital in april , 2012 .
the main complaint was gastrointestinal bleeding identified as melena , occurring some weeks after a roux - en - y cystjejunostomy for a pancreatic pseudocyst .
bleeding was not , initially , thought to be related to the previously done cystjejunostomy , as it is an allegedly uncommon complication of that procedure .
several gastric erosions were observed but thought to be unrelated to the patients bleeding , and the urease test was positive for helicobacter pylori ( hp ) infection .
although there was no evidence of gastric bleeding , the initial approach consisted of a proton pump inhibitor and antibiotic therapy , for hp .
because the melena persisted , the patient underwent colonoscopy , which revealed diverticulosis in both the ascending and transverse colon .
other diagnostic procedures included an abdominal ct scan and technetium-99 m - labeled rbc scintigraphy , both of which had nonspecific findings and were not diagnostic .
repeat uge showed complete resolution of the gastric erosions which led to the search for a lesion in the small bowel . by this point in her clinical course , the patient had required 20 packed red cell transfusions .
a small bowel barium study revealed findings compatible with a retro - gastric tumor causing extrinsic compression of the stomach . at that time
, the medical team considered that the main diagnostic concern was a pancreatic cystic neoplasm masquerading as a pancreatic pseudocyst and the patient underwent a laparotomy ( fig.1 ) . at surgery ,
a retro - gastric lesion measuring 12  8.0  5.0 cm was resected and the pathology report indicated an unsuspected gastrointestinal stromal tumor ( gist ) ( fig.2 ) .
histologic findings were an epithelioid and fusiform mixed pattern tumor , with high mitotic grade ( 21 of 50 high - power fields ) and central cystic degeneration .
by immunohistochemistry , the tumor expressed c - kit , vimentin , dog 1 , and ki 67 ( figs.3 and 4 ) .
gists are rare , with an estimated incidence of 1.5/100.000 per year 2 and encompass tumors primarily derived from cajal interstitial cells  pacemaker cells which regulate gastrointestinal tract motility 3 . by immunohistochemistry ,
in fact , the kit mutation leads to kinase activation , and experimental studies have demonstrated that the mutation is capable of causing cajal cells hyperplasia and gist - like tumors in mice 3 .
any part of the gastrointestinal tract  from esophagus to rectum  may be involved , although stomach ( 60% ) and small bowel ( 30% ) are the predominant organs affected .
the clinical expression of disease tends to be with gastrointestinal bleeding and nonspecific abdominal complaints .
some tumors from outside the gastrointestinal tract are called extragastrointestinal stromal tumors or egists ; most of them are from the omentum , mesentery or retroperitoneum 3 .
gist pathology report encompasses three tumor subtypes : a fusiform cell ( 70% ) , an epithelioid cell ( 20% ) , and a mixed pattern .
the fusiform cell subtype correlates with the kit mutation ( cd117 positivity ) , the tumor cells present a spindled phenotype , and the clinical course of such tumors is usually less aggressive .
the other histological patterns ( epithelioid cells and mixed pattern ) correlate with platelet - derived growth factor receptor alpha ( pdgfra ) expression ( cd34 positivity ) , although such correlation does not consistently occur .
tumors with a predominantly epithelioid pattern and pdgfra mutation tend to have a favorable prognosis . on the other hand
, an epithelioid or mixed pattern accompanied by the kit mutation  not pdgfra mutation  which occurs particularly in gists of the stomach , tends to have an unfavorable course . by immunohistochemistry ,
95% of gists express cd117 , and these kit mutated tumors respond to therapy with kit inhibitors ( imatinib or sunitinib ) .
less than 5% of gists are kit immune - negative ; many of them have the pdgfra mutation and respond to targeted therapy as well .
a third type , called  wild  type and without either kit or pdgfra mutation , has been described too .
other histological markers are s-100 protein ( 5% ) , desmin ( 2% ) , a recently described  discovered on gist  protein ( dog1 , 95% ) 3 and ki-67 , the expression of which correlates with a high mitotic grade of tumor cells and with a worse prognosis 4 .
by the time the patient had the second operation , the medical staff had already estimated gross probabilities of different diagnostic possibilities .
first , bleeding should be regarded as a potential surgical procedure complication . according to newell et al .
( 1990 ) , bleeding after roux - en - y cystjejunostomy is definitely an uncommon complication , occurring in only 2% of the cases 5 .
second , a colonic source of bleeding should be considered despite a presentation as melena .
previous studies have focused on the diagnostic yield of colonoscopy in the investigation of bleeding after a negative uge ; positivity rates of 23.5% to 30% were reported .
the main limitations observed were overvaluing the role played by lesions with low implication for bleeding and small sample sizes 6,7 . a recently published study with better design has reported a 14.2% positivity rate for colonoscopy ( 4.8% if only lesions with a high implication for bleeding are included ) 8 .
finally , melena should be considered to result from some previously neglected condition , for example , a pancreatic cystic neoplasm mistaken for a pancreatic pseudocyst 9 .
other possibilities would be small bowel carcinoid 11 , dieulafoys lesion 12 and other rare diseases .
combining data from those reports , it could be estimated a 2% bleeding probability related to the cystjejunostomy previously carried out ; a 4.8% melena probability from a colonic source ; finally , a 12% bleeding probability from an unsuspected pancreatic cystic neoplasm .
the upper gastrointestinal bleeding investigation points to the small bowel whenever uge and colonoscopy are unrevealing , and endoscopic capsule and small bowel study should be considered . in the case
presented here , a small bowel barium examination was able to uncover a suspicious lesion . by means of probabilistic reasoning ,
to our knowledge , gist masquerading as pancreatic pseudocyst is rather unusual , and we found only one prior report of such an atypical presentation in the medical literature 13 .
in fact , the case reported by aggarwal and colleagues was a metastatic gastric gist mistaken for pancreatic pseudocyst .
on the other hand , the case reported here seems to be a primary retroperitoneal gist .
this type of tumor , actually known as egist , seems to be a rare medical finding .
a case series from the national cancer institute of brazil ( inca ) reported only nine egist cases over a period of 13 years from 1997 to 2009 14 .
primary egist of the retroperitoneum seems even rarer , with only 58 cases reported in the medical literature according to casella and colleagues 15 . in conclusion , it is not an overstatement to say that any presumed pancreatic pseudocyst merits double attention . it might be a pancreatic cystic neoplasm  as the medical literature has previously shown  or even a gist , as this report has emphasized .
key clinical messagea 74-year - old female patient underwent a roux - en - y cystjejunostomy for pancreatic pseudocyst developed several melena episodes and she was surgically reappraised . 
 the main diagnostic concern was a pancreatic cystic neoplasm . 
 a 12  8.0  5.0 cm retro - gastric lesion was resected and pathology report indicated an unsuspected gastrointestinal stromal tumor ( gist ) . the report aimed to describe an atypical presentation of gist 
 .