Dedifferentiated Gastric Gastrointestinal Stromal Tumor with Rapid Growth Mimicking Other Malignancies: A Case Report.

INTRODUCTION: Gastrointestinal stromal tumors (GISTs) are diagnosed based on morphological and immunohistochemical findings. However, in rare instances, GISTs may undergo morphological and immunophenotypic dedifferentiation, making diagnosis challenging. Here, we report a rare case of a rapidly growing gastric GIST with a dedifferentiated component that posed a diagnostic challenge, together with a review of the literature. CASE PRESENTATION: A woman in her 70s presented with abdominal distention and was found to have a rapidly enlarging 21 cm gastric tumor. Endoscopic US-guided biopsy showed Class V cytology; however, a definitive histologic diagnosis could not be established, and immunohistochemical staining was negative for all examined markers. Distal gastrectomy and partial resection of the transverse colon were performed. Histologically, the tumor consisted of a conventional GIST component with immunohistochemically positive atypical spindle cells and a dedifferentiated component with immunohistochemically negative pleomorphic atypical cells. The tumor was diagnosed as dedifferentiated GIST. Anastomotic recurrence and peritoneal dissemination developed 11 months postoperatively. CONCLUSIONS: Here, we report an extremely rare case with findings most compatible with dedifferentiated gastric GIST occurring without prior TKI therapy. GIST should not be excluded even when both KIT and DOG1 are negative; a careful integrated diagnosis based on histopathological findings, imaging findings, and the clinical course is essential.