A Case of Coexisting Malignant Lymphoma and Adenocarcinoma Occurring as a Collision Tumor in the Remnant Stomach after Distal Gastrectomy

A collision tumor involving malignant lymphoma and adenocarcinoma in the remnant stomach is extremely rare. A 79-year-old man had undergone distal gastrectomy for malignant lymphoma of the duodenum (non-Hodgkin lymphoma, diffuse, small B cell type) 12 years earlier. An ulcerating tumor was found at the gastrojejunal anastomosis and endoscopic tumor biopsy indicated poorly differentiated adenocarcinoma, necessitating total remnant stomach resection. Pathological examination showed a collision tumor of malignant lymphoma (non-Hodgkin lymphoma, diffuse small B cell type, fSM) and adenocarcinoma (por1, fMP) with no nodal metastasis. Clinically, no evidence of recurrence was seen during follow-up. The man died of severe pneumonia two years after the second operation.