Gastric Epithelioid Mesenchymal Tumor with the EWSR1::CREM Fusion Gene: A Case Report.

INTRODUCTION: In recent years, new molecularly defined tumor groups have been reported among tumors previously considered unclassifiable. Among them, gene fusions involving the CREB family of transcription factors, including cAMP-responsive element modulator (CREM), with genes encoding FET family RNA-binding proteins, such as Ewing sarcoma breakpoint region 1 (EWSR1), have recently been shown to be implicated in driving the pathogenesis of various tumor types. Here, we report our experience with a gastric mesenchymal tumor with epithelioid histology and an EWSR1::CREM fusion, which is rare but requires caution.

CASE PRESENTATION: A 58-year-old man with epigastric pain underwent esophagogastroduodenoscopy, which revealed a submucosal tumor, 40 × 30 mm in size, at the greater curvature of the upper gastric body. Surgical resection was scheduled because of easy bleeding from the tumor and because biopsy could not establish a diagnosis. The tumor was clinically considered benign because there was no significant accumulation on positron emission tomography scans. Therefore, we performed a local resection of the stomach. Histologically, the tumor consisted of a proliferation of keratin-positive, relatively uniform epithelioid cells arranged in sheets, with a scattering of lymphoid follicles in the surrounding area. Based on a pathology consultation, the tumor was diagnosed as a mesenchymal tumor with EWSR1::CREM fusion.

CONCLUSION: We experienced a gastric epithelioid mesenchymal tumor with EWSR1::CREM fusion genes. Since a malignant course has been reported in similar tumors in the stomach and abdominal cavity, such patients require careful follow-up.