Teaching Case

A 41-year-old woman was transferred to the surgical department due to epigastric discomfort. There was no medical history concerning malignant diseases.
Esophagogastroduodenoscopy (EGD) was performed and revealed a submucosal tumour (Fig. 1). For further diagnostics computer tomography ( CT) was recommended.

Contrast enhanced CT detected a homogeneous exophytic mass measuring 3.7cm at the lesser curvature of the gastric body (Fig. 2 and Fig. 3). Fluorine-18-fluorodeoxyglucose (18F-FDG) positron emission tomography/CT showed an FDG-avid lesion in the gastric mid-body.
These findings highly suggested gastrointestinal stromal tumour (GIST). We therefore decided to perform partial gastric resection after obtaining informed consent from the patient.
The final pathological report revealed gastric schwannoma located at the mid-body of the lesser curvature of the stomach. No cancer cells were identified. The neoplastic cells had immunoreactivity with S100 protein but no immunoreactivity with c-Kit.

Mesenchymal tumours of the gastrointestinal tract are mainly spindle cell tumours, which include gastrointestinal stromal tumours (GIST), leiomyoma or leiomyosarcoma, and schwannomas. Among these tumours, GIST is the most common (60–70%) occurring in the stomach [1]. The differential diagnosis for this neoplasm is difficult due to the macroscopic and microscopic similarities with other typical lesions of the stomach.
Gastric schwannomas are rare stromal tumours that consist of spindle cells of the gastrointestinal tract and arise from the Schwann cells of the gastrointestinal neural plexus. The stomach is the preferential site of localization and schwannomas represent ~0.2% of all gastric neoplasms [2]. Usually these tumours are benign. The incidence of sporadic malignant peripheral nerve sheath tumour is low with a life time risk of 0.001% [3].
The treatment for GIST and schwannoma is surgical resection. However the prognosis for benign schwannoma is much better.

Immunohistochemical staining can differentiate various spindle cell tumours. Schwannomas can be distinguished from other spindle cell tumours of the stomach by S100 protein. Tumour cells that are positive for S100 protein and negative for smooth muscle actin, c-Kit, and CD34 confirm the diagnosis of schwannoma [4]. S100 protein and glial fibrillary acidic protein positivity is evidence of nerve sheath differentiation [4].
The differentiation between gastric schwannomas and GIST can be difficult in the preoperative workup. FDG-PET/CT is a very useful metabolic imaging tool for the evaluation of malignancy of various tumours. In the evaluation of bioactivity and malignant potential of gastric GIST [6]. However, in the diagnosis of schwannomas, some benign tumours show high FDG uptake, so the evaluation of the malignant potential of schwannomas with FDG-PET scan might be insufficient. Fluorine-18-alpha-methyltyrosine PET/CT scan may be the most reliable method for the differentiation of benign schwannomas from other malignancies [6]. The reason for high 18F-FDG uptake in benign tumours like schwannomas remains unclear [7]. The patient in this case revealed a high standardized uptake value lesion in the gastric midbody, which did not provide differential diagnosis before operation. Our case suggested that 18F-FDG PET/CT has only a limited role in making preoperative differential diagnosis between benign schwannoma and malignant GIST [7] [8].

Benign gastric schwannoma cannot be differentiated savely from malignant GIST preoperatively by FDG uptake. In our case the final diagnosis could only be made by pathological report after partial gastrectomy.

In conclusion, the correct histomorphological diagnosis of GIST, is fundamental, as misdiagnosis may prevent the patient from specific target therapies.