# A Difficult-to-Diagnose Collision Tumor in a Patient With Gastric-Type Duodenal Cancer and Gastric Cancer: A Report of a Rare Case

## TL;DR

A rare case of a collision tumor involving gastric and duodenal cancers was diagnosed postoperatively in an 82-year-old woman.

## Contribution

This is the first reported case of a collision tumor involving gastric-type duodenal cancer and gastric cancer.

## Key findings

- The collision tumor involved gastric cancer and gastric-type duodenal cancer meeting at the pyloric ring.
- Both tumors were positive for MUC5AC, with the duodenal cancer also positive for MUC6.
- Lymph node metastasis was detected from both gastric and duodenal cancers.

## Abstract

Collision tumors, characterized by the coexistence of two distinct malignancies within the same area, are extremely rare. Herein, we report a collision tumor that was difficult to diagnose preoperatively in a patient with gastric cancer and gastric-type duodenal cancer. An 82-year-old woman with a history of melena and anemia was diagnosed with progressive gastric cancer of the gastric antrum at the lesser curvature. Part of the tumor infiltrated the pyloric ring but was not detected in the duodenal bulb within the observable range. Enhanced computed tomography revealed wall thickening from the gastric antrum up to the pyloric ring, with swelling of the lesser curvature and a subpyloric lymph node. The patient underwent a distal gastrectomy and D2 lymph node dissection for gastric cancer. Pathological findings showed gastric cancer (papillary adenocarcinoma and moderately differentiated tubular adenocarcinoma) positive for mucin 5AC (MUC5AC), and duodenal cancer (solid-type poorly differentiated adenocarcinoma > non-solid-type poorly differentiated adenocarcinoma) positive for MUC5AC and mucin 6 (MUC6), with both tumors meeting at the pyloric ring. Lymph node metastasis from gastric and duodenal cancer was also detected. No previous reports of collision tumors involving both gastric-type duodenal cancer and gastric cancer exist. In patients with gastric cancer with duodenal invasion, the possibility of duodenal cancer complications and the extent of lymph node dissection should be considered.