inflammatory myofibroblastic tumors ( imts ) are rare neoplasms that occur preferentially in children and young adults.1 imts most commonly occur in the lungs , mesenteries , omentum , and retroperitoneum.1,2 imts involving the stomach are extremely rare . these distinctive tumors are composed of myofibroblastic cells accompanied by an inflammatory infiltrate of plasma cells , lymphocytes , and eosinophils.3 imts present with nonspecific clinical symptoms , and their preoperative diagnosis can be difficult . imts in the gastrointestinal tract can present with bleeding , obstruction , anemia , or intussusception , depending on the site.4 herein , we present a case of an imt that involved nearly the entire proximal stomach . his symptoms were not associated with abdominal pain , dyspepsia , fever , or weight loss . the patient had no notable medical or operational history except an admission for bronchial asthma during childhood . emergency gastric fibroscopy detected a protruding mass 44 cm in size , with central ulceration at the posterior wall of the fundus and two slightly protruding submucosal lesions with central erosion on the lesser curvature side of the upper stomach body ( fig . a biopsy of the fundic protruding lesion revealed no diagnostic information except that it was a spindle cell - type lesion ( tests for cd34 , cd117 , actin , and s-100 were all negative ) . computed tomography showed a 3.6-cm enhancing ulcerofungating mass with a 4-cm eccentric wall thickening and an enhancing mucosal layer on the lesser curvature side of the upper stomach body ( fig . although the diagnosis was uncertain , an operation was planned due to the risk of rebleeding . because the fundic mass and the thickened portion of the lesser curvature formed one continuous , massive lesion , laparoscopic proximal gastrectomy was performed to resect the entire area involved . the cut surface of the resected specimen revealed two lesions and involvement of the muscle layer ( fig . the tumor was mainly composed of spindle - shaped cells in a hyalinized stroma with lymphoplasmacytic infiltration compatible with a diagnosis of imt . although rearrangement of the anaplastic lymphoma kinase ( alk ) gene was not found , the tumor was focally positive for actin , and other immunohistochemical results ( cd34 , cd117 , and s-100 tests were all negative ) excluded gastrointestinal stromal tumor , leiomyoma , and neurogenic tumor . in addition , immunoglobulin g4 ( igg4)-related sclerosing disease could be ruled out because igg4/igg ratio was less than 0.4 ( fig . the patient showed an uneventful postoperative course , and no local recurrence or distant metastasis was found during the 18-month follow - up . imt was first described in 1937 as a primary lung tumor and has since been reported in various sites , including the mesenteries , the retroperitoneum , and the gastrointestinal tract.4 it mainly occurs in young adults and children , and the lung is the most frequent location . various terms have been used to describe this entity , such as ' inflammatory pseudotumor , ' ' pseudosarcomatous myofibroblastic proliferation , ' and ' plasma cell granuloma . ' primary gastric imts have been reported in only 17 adult patients.5 the pathogenesis of imt is unclear . allergic , immunologic , and infectious reactions have been postulated.6 approximately half of imts harbor a clonal cytogenetic aberration that activates the alk tyrosine kinase receptor gene on the 2p locus of chromosome 23.7 rearrangement of the alk gene is another postulated pathogenesis . manifestations of gastric imt may include abdominal pain , hematemesis , melena , a palpable mass , anemia , and high fever.6,8 hemoperitoneum has rarely been reported as an initial finding in patients with gastric imt.9 because gastric imts localize in the submucosal layer , endoscopy can not distinguish them from other submucosal tumors . thus , surgical resection with an adequate margin is the most reliable diagnostic and treatment method . abdominal computed tomography is helpful for determining the necessary extent of resection.6 in the present case , a proximal gastrectomy was performed because the gastric imt involved nearly the entire proximal stomach . the histological appearance of gastric imts is characterized by proliferation of spindle - shaped myofibroblastic cells and lymphocytic infiltration among tumor cells . differential diagnoses for imt include inflammatory fibrinoid polyp , gastrointestinal stromal tumor , leiomyoma , and follicular dendritic cell sarcoma.3 alk expression distinguishes imt from its differential diagnoses . however , not all gastric imts are positive for alk rearrangement ; only about 50% of imts in children and young adults show such rearrangement.1 in addition , imts test positive for actin with most immunohistochemical tests.3 immunohistochemical tests for cd117 , cd34 , and s-100 can help distinguish imt from differential diagnoses . the typical characteristics of an imt were observed in the present case ( myofibroblastic proliferation , lymphoplasmacytic infiltration distributed among the tumor cells ) . although our case failed to show the alk mutation , the diagnosis was made based on morphological characteristics , the focally positive actin test , and negative results for cd34 , cd117 , and s-100 staining . imt has intermediate biological potential , frequently recurring but rarely metastasizing.10,11 among 17 reported gastric imt cases in adults , only one case recurred , as a peritoneal dissemination following total gastrectomy , distal pancreatectomy , splenectomy , transverse colectomy , and lymph node dissection , which were necessitated by the large size of the tumor . resection with an adequate margin is the only treatment option , and regular follow - up is mandatory .