Synchronous medullary carcinoma of the pancreas and non-ampullary duodenal adenocarcinoma.

Synchronous primary malignancies involving the pancreas and duodenum are rare and distinguishing them from direct extension or metastatic spread may be difficult in the periampullary region. We report a 73-year-old man who initially presented with upper gastrointestinal bleeding and was later investigated for weight loss, abdominal pain, dyspepsia, and abnormal liver biochemistry. Preoperative imaging and biopsy identified poorly differentiated adenocarcinoma involving the pancreatic and duodenal region, but the relationship between the lesions remained uncertain. Following pancreaticoduodenectomy, two macroscopically separate tumor sites were identified. Histological examination showed that the pancreatic tumor had features of medullary carcinoma, including syncytial growth, pushing borders, marked lymphocytic infiltration, and mismatch repair deficiency. In contrast, the duodenal tumor showed glandular differentiation, infiltrative invasion, and desmoplastic stroma, but also deficient mismatch repair. Lymph node involvement was seen with the pancreatic tumor but not the duodenal lesion. This case suggested that synchronous primary tumors should remain in the differential diagnosis when adjacent periampullary lesions show discordant pathological features, and that definitive classification may require integration of gross examination, morphology, selected immunohistochemistry, mismatch repair status, and nodal disease pattern on the resection specimen. The underlying diagnosis of Lynch syndrome provides a novel pathogenesis for these genetically related yet distinct tumors. This diagnosis was important not only for understanding tumor pathogenesis but also for ongoing surveillance and family counselling.