Primary eGISTs are distinctly uncommon, according to previous reports[4, 5]. An extensive literature review was undertaken, using the key words ‘GIST,’ ‘Cajal cells,’ ‘KIT,’ and ‘Imatinib.’ To the best of our knowledge, there have been no previous reports of a GIST arising from the mesoappendix. This case was positive for CD34 and CD117, which supports the diagnosis, as this test distinguishes a GIST from mesenchymal tumors arising from smooth muscle cells, such as leiomyomas, leiomyoblastomas, and leiomyosarcomas. The origin of GISTs was at first attributed to the ICC, but it is now recognized that they arise from multipotential mesenchymal stem cells. ICC or ICC-like cells have also been described in various organs, excluding the gastrointestinal tract. In the case we present, the tumor may have originated from ICC or ICC-like cells and multipotential mesenchymal stem cells in the mesoappendix.
It can be difficult to confirm a diagnosis of GIST, and specific morphologic, immunohistochemical, and molecular analyses are required. The majority of GISTs have a uniform appearance that falls into one of three categories: spindle cell, epitheloid cell, and mixed cell. On immunohistochemistry, 90% to 95% of GISTs are diffusely and strongly positive for CD117 (c-Kit), but this is no longer considered an absolute criterion. Not all CD117-positive tumors are GISTs, as melanomas, synovial sarcomas, desmoid tumors, and schwannomas can also be positive for this marker. Some CD117 negative GISTs have an epithelioid morphology and arise in the stomach or outside the gut. Therefore, CD34 has been proposed as a more reproducible marker, and is positive in 80% to 85% of cases. The spindle cell type is more likely to stain with CD34 than the epithelioid type, and the mixed spindle-epithelioid type is more likely to stain with CD34 than the nonmixed type. Smooth muscle actin is expressed in 30% to 40% of cases. S-100 positivity is present in up to 5% of these tumors, but is relatively more common in small intestinal GISTs. In general, GISTs tend to be negative for desmin, or weakly positive in 1% to 2% of cases.
The spectrum of the clinical presentation of GISTs is broad and depends on tumor location and size, with approximately 70% of patients developing such symptoms as abdominal pain, gastrointestinal bleeding, and mass effects. The remaining 20% to 30% present incidentally during radiological imaging or surgery for some other cause. The tumors occur with equal frequency in both sexes, although some studies have shown a male preponderance[1–3].
Since the classification of GIST as a distinct entity, there has been an increased interest in defining the imaging characteristics. The tumors usually commence in the bowel wall, but may extend to involve either the mucosal or the serosal surfaces. Tumor size ranges from 1 cm to 35 cm, with a median of 5 cm. The tumor margins are well-defined in approximately two-thirds of cases. They can be of any size, but large tumors, in particular, can have areas of hemorrhage and necrosis that demonstrate a heterogeneous appearance on imaging. The enhancement pattern can vary from homogeneous to heterogeneous[8–10]. According to the World Health Organization criteria, malignant potential is assessed by tumor size, mitotic count, and the cell proliferative index. Tumors smaller than 5 cm in diameter are usually benign, those between 5 and 10 cm are of uncertain malignant potential, and those larger than 10 cm are usually malignant. This case was a low-grade malignant GIST according to these criteria, and to the criteria of a previous study[8–11].