Collision Tumor Composed of Adenocarcinoma and Malignant Lymphoma in the Remnant Stomach after Distal Gastrectomy

A 65-year-old man who had undergone distal gastrectomy with Billroth-II anastomosis for a duodenal ulcer 45 years previously was referred to our hospital because of weight loss and anemia. He was given a diagnosis of remnant gastric cancer with cT4aN (+) M0 cStage III. After neoadjuvant chemotherapy, we performed hand-assisted laparoscopic total remnant stomach resection and partial transverse colectomy. Macroscopic examination of the resected specimen showed a large type 3 tumor that had spread widely in the remnant stomach and a 3-cm prominent lesion on the anterior wall. Histopathological examination revealed the type 3 tumor to be an adenocarcinoma, while the prominent lesion was diagnosed as diffuse large B-cell lymphoma (DLBCL). Postoperative PET-CT revealed abnormal uptake in the periaortic lymph nodes. We also suspected extranodal DLBCL lesions. The final diagnosis was collision tumor composed of the remnant gastric cancer ypT4b (transverse colon) N2M0 ypStage IIIB and DLBCL. The patient was subsequently treated with chemotherapy for DLBCL.