What happened, in summary

A man in his 70s sought care for poor appetite and a 4-kg weight loss over 2 months. Endoscopy showed a large gastric tumor extending from the antrum into the duodenal bulb, and biopsy found moderately to poorly differentiated adenocarcinoma. CT also showed enlarged regional lymph nodes and a large retroperitoneal mass. The retroperitoneal lesion was initially thought to be metastatic gastric cancer, so the gastric cancer was clinically treated as advanced disease.

He received 2 courses of cisplatin plus S-1 chemotherapy. The stomach cancer remained stable or slightly smaller, but the retroperitoneal mass grew rapidly and developed central necrosis. Biopsy of that mass showed a high-grade spindle-cell tumor rather than adenocarcinoma. Over the following weeks it enlarged further, causing pain, ascites and compression of major abdominal vessels.

He died 3.5 months after admission from hepatorenal failure associated with severe jaundice. Autopsy clarified that the retroperitoneal tumor was a separate dedifferentiated liposarcoma. The gastric tumor remained an adenocarcinoma with regional lymph-node involvement and treatment effect, but there was no distant organ metastasis from the gastric cancer. The rapidly progressive liposarcoma caused the major abdominal complications in his final months.