Presence of Cytokeratin 19-Expressing Cholangiocarcinoma-Like Tumour in a Liver Metastatic Lesion of Rectal Neuroendocrine Tumour

INTRODUCTION: Tumours with adenocarcinoma and neuroendocrine components have often been reported, although the reason underlying the dual components remains unclear. CASE PRESENTATION: A 43-year-old woman with multiple liver metastatic lesions of rectal neuroendocrine tumour underwent primary tumour resection and subsequent liver transplantation. Pathological examination indicated a cholangiocarcinoma-like tumour with gland formation, adjacent to a liver metastatic lesion of the neuroendocrine tumour. This tumour comprised atypical columnar epithelium, and stained positively for neuroendocrine markers and the ductal marker cytokeratin 19, indicating amphicrine properties and a partial cholangiocarcinoma phenotype – features not observed in the primary and metastatic neuroendocrine tumours. CONCLUSION: The presence of adenocarcinoma only at the metastatic site indicated that neuroendocrine tumour cells acquired stemness and differentiated into adenocarcinoma through metastasis, or that the adenocarcinoma newly arose from the adjacent epithelium influenced by the neuroendocrine tumour. We propose a novel mechanism for the pathogenesis of mixed tumours in neuroendocrine tumours.