What happened, in summary

A 60-year-old man sought care for 2 months of stomach discomfort. Gastroscopy showed a 2 cm lesion on the lesser curvature of the stomach that was covered by normal-looking mucosa. Endoscopic ultrasound and CT suggested that the mass might be a gastrointestinal stromal tumor rather than a typical gastric carcinoma. He underwent endoscopic submucosal dissection to remove the lesion and obtain a full tissue diagnosis. Pathology instead showed poorly differentiated adenocarcinoma confined beneath the surface mucosa. Further imaging did not identify a separate primary tumor. Because the diagnosis was gastric cancer, he then underwent subtotal gastrectomy with regional lymph-node dissection. The surgical specimen confirmed poorly differentiated adenocarcinoma extending into the muscular layer of the stomach. The margins were clear, indicating complete resection. After surgery, follow-up CT imaging and upper gastrointestinal endoscopy were planned. His case highlights how an unusual growth pattern can make gastric adenocarcinoma resemble a submucosal tumor before tissue examination establishes the diagnosis.