Rapidly Progressive Gastric Poorly Differentiated Neuroendocrine Carcinoma Mimicking Gastrointestinal Stromal Tumor: A Case Report.

Poorly differentiated gastric neuroendocrine carcinoma (NEC) represents a rare but highly aggressive malignancy, typically diagnosed at advanced stages, which are associated with poor prognosis. We present a case of a 71-year-old male with a history of distal gastrectomy who developed recurrent gastrointestinal bleeding from a subepithelial gastric mass. The lesion initially measured approximately 2 cm and demonstrated endoscopic and radiological features reminiscent of gastrointestinal stromal tumors (GIST). Repeated endoscopic biopsies indicated weak c-KIT and DOG1 positivity; however, the mass enlarged rapidly to 8 cm within 2 months, accompanied by multiple hepatic metastases. Ultrasound-guided liver biopsy confirmed the diagnosis of poorly differentiated gastric NEC. Cisplatin-based palliative chemotherapy was initiated, but the patient died within 3 weeks due to persistent hemorrhage and rapid clinical decline. Although uncommon, gastric NEC may closely mimic GIST both clinically and pathologically. Accurate differentiation between these entities is essential, given their markedly divergent prognoses and therapeutic approaches.