Ho Young Yoon, et al. DISCUSSION The vast majority of spindle cell stromal tumors of the gastrointestinal tract were once considered to originate from smooth muscle. With the development of immunohistochemical staining and electron microscopy, various distinct origins have become apparent. These modalities allowed for a more precise classification among stromal tumors in the gastrointestinal system. For example, Daimaru et al. reported that 24 out of 306 diagnosed gastro- intestinal spindle cell tumor cases were determined to be schwannomas by immunohistochemical methods. When these 24 cases were re-examined with hematoxylin-eosin, only 9 cases were dia- gnosed as schwannomas.2 This marked difference supports the high likelihood for misdiagnosing schwannoma as spindle cell tumors before the emergence of immunohistochemical staining. Appelman et al. described a schwannoma as a benign neurogenic tumor under the name 'small intramuscular spindle cell nodule'.3 It is, in fact, a spindle cell mesenchymal tumor that is very rare in the gastrointestinal tract and originates in Schwann cells in the neural plexus. The most common gastrointestinal site is the stomach, constituting 0.2% of all gastric neoplasms. Malignant transformation of a gastric schwannoma is very rare, and only one has been reported in children by Bee et al.4 The tumors are usually encased by intact mucosa and principally involve the submucosa and muscularis propria. Furthermore, they generally present asymptomatically or with abdominal discom- fort.5 Bleeding may be present in the case of deep ulceration,6 and a mass may be palpated in the epigastric area when exophytic growth has occurred. Although the definitive diagnosis of gastric schwannomas is determined by pathological ex- amination, it may be helpful to gain limited infor- mation through gastrointestinal endoscopy, com- puted tomography, sonography, and magnetic resonance imaging. On computed tomography, a gastric schwan- noma tends to be homogenous, which distin- guishes it from leiomyomas and leiomyosar- comas.7 Sonography with sufficient resolution power enables the determination of the tumor's layer of origin.8 In addition, magnetic resonance Yonsei Med J Vol. 49, No. 6, 2008 imaging is important to define the exact location and extent of the tumor with displacement of surrounding organs or vessels.9 In pathological tests, there are characteristics of fascicular arrangement or palisading nuclei with H&E stain. Immunohistochemically, positive stai- ning for S-100 protein and vimentin and negative staining for smooth muscle actin, c-KIT, and CD34 supports the notion that the tumor originated from the Schwann cell. Because the tumor was benign, surgical resection of the tumor was sufficient. In this case, a 57 year-old woman with a pal- pable abdominal mass was diagnosed with a pathologically confirmed gastric schwannoma after subtotal gastrectomy. The tumor size was 6 × 5.5 × 4 cm, and immunohistochemical staining was positive for S-100 protein but negative for desmin and smooth muscle actin, which supports the diagnosis of a schwannoma. REFERENCES 1. Melvin WS, Wilkinson MG. Gastric schwannoma. Clinical and pathologic considerations. Am Surg 1993; 59:293-6. 2. Daimaru Y, Kido H, Hashimoto H, Enjoji M. Benign schwannoma of the gastrointestinal tract: a clini- copathologic and immunohistochemical study. Hum Pathol 1988;19:257-64. 3. Appelman HD. Smooth muscle tumors of the gastrointestinal tract. What we know now that Stout didn't know. Am J Surg Pathol 1986;10 Suppl 1:83-99. 4. Bees NR, Ng CS, Dicks-Mireaux C, Kiely EM. Gastric malignant schwannoma in a child. Br J Radiol 1997;70: 952-5. 5. Prévot S, Bienvenu L, Vaillant JC, de Saint-Maur PP. Benign schwannoma of the digestive tract: a clinico- pathologic and immunohistochemical study of five cases, including a case of esophageal tumor. Am J Surg Pathol 1999;23:431-6. 6. Kim JY, An JY, Choi MG, Noh JH, Sohn TS, Kim S. A Case of giant gastric schwannoma accompanied with bleeding. J Korean Gastric Cancer Assoc 2006;6:47-51. 7. Vinhais SN, Cabrera RA, Nobre-Leitão C, Cunha TM. Schwannoma of the esophagus: computed tomography and endosonographic findings of a special type of schwannoma. Acta Radiol 2004;45:718-20. 8. Fujii Y, Taniguchi N, Hosoya Y, Yoshizawa K, Yasuda Y, Nagai H, et al. Gastric schwannoma: sonographic findings. J Ultrasound Med 2004; 23:1527-30. 9. Karabulut N, Martin DR, Yang M. Case report: gastric schwannoma: MRI findings. Br J Radiol 2002;75:624-6.