Diagnostic dilemma: a collision of pancreatic neuroendocrine tumor G3 and adenocarcinoma with extensive fibrosis.

An 82-year-old man presented with intermittent abdominal pain and elevated liver enzymes. Blood tests showed normal levels of tumor markers (CEA, CA19-9, NSE). Contrast-enhanced computed tomography (CE-CT) revealed a 20 mm hypovascular mass in the pancreatic head. Magnetic resonance imaging indicated low intensity on both T1- and T2-weighted images and high intensity on diffusion-weighted images. Endoscopic ultrasonography visualized an irregular hypoechoic mass. Initially, it was diagnosed as pancreatic ductal adenocarcinoma (PDAC) based on imaging. Subsequent histopathological analysis via endoscopic ultrasound-guided fine-needle aspiration revealed a neuroendocrine tumor (NET). The preoperative diagnosis was changed to a pancreatic NET grade1. Consequently, a pancreaticoduodenectomy was performed. Histopathological examination of the resected specimen unveiled a mixed tumor-NET-Grade1/Grade3 and invasive PDAC. No clear transition between the NETs and PDAC was observed. The high grade of NET with significant fibrosis contributed to decreased enhancement of CE-CT. Finally, we diagnosed this case as a pancreatic collision tumor involving both NET and PDAC components, with lymph node metastases attributed to the NET components. In this case, achieving an accurate preoperative diagnosis was challenging despite utilizing both imaging and biopsy diagnostics. This unique case underscores the difficulties encountered in the preoperative assessment of mixed tumors.