Medullary carcinoma of the pancreas radiologically followed up as a cystic lesion for 9 years: a case report and review of the literature

Background: Medullary carcinoma of the pancreas (MCP) is a rare malignant pancreatic tumor. The World Health Organization classification defines the tumor as a subtype of pancreatic ductal carcinomas. MCP is histologically characterized as having highly pleomorphic cells with syncytial morphology, expansive tumor growth, and necrosis. The pathogenesis and clinical course of MCP are largely unknown. Herein, we report an unusual case of MCP that was radiologically followed up for 9 years prior to surgical intervention.

Case presentation: A 73-year-old Japanese woman with a nonspecific disease history was found to have an asymptomatic cyst in the pancreatic duct by abdominal ultrasonography. Thorough radiological investigation suggested it was not an aggressive neoplasm, and she received periodic check-ups under a clinical diagnosis of "a cystic pancreatic lesion of uncertain malignancy." Nine years after initial presentation, she experienced acute pancreatitis and underwent thorough re-evaluation. Dynamic computed tomography revealed no cyst; rather, a solid tumor was detected. Cytology of the pancreatic duct suggested adenocarcinoma. Pancreatoduodenectomy with D2 lymph node dissection was performed. The resected tumor was a non-mucinous, solid mass measuring 22 × 10 mm. Microscopically, the tumor had a well-demarcated pushing border. Lymphocytic infiltration was abundant, and stromal component was sparse. The tumor cells were composed of highly pleomorphic cells, proliferating in sheets without glandular formation. Neither lymphovascular invasion nor lymph node metastasis was detected. The histopathologic diagnosis was MCP, pT1aN0M0. The tumor carried a KRAS mutation, and MLH-1, MSH-2, MSH-6, and PMS-2 immunostaining results were normal, suggesting microsatellite stability. The patient has remained free of disease for 29 months following surgical intervention.

Conclusion: A review of 20 previously reported cases plus the present case suggests that subsets of MCPs have genetic aberrations such as KRAS mutation and high microsatellite instability. MCP has been suggested to have a better prognosis than common ductal adenocarcinoma; however, 15 of 20 previously reported cases died from disease. Whether the asymptomatic cyst observed over 9 years contributed to the development of MCP in this patient is a subject for future study.