A 51-year-old man was referred to our hospital for an examination of a pancreatic mass with cysts noted during a physical examination two years ago. Computed tomography showed a 12 mm large hyper vascularized tumor with cysts in the pancreas, and diffusion MRI showed low signal in the same area. As EUS showed a multifocal mass with cysts, we considered a diagnosis of pNEN and performed a subtotal gastric preservation pancreaticoduodenectomy. Pathological examination showed small round atypical cell clusters. Immunostaining was positive for Chromogranin A, Synaptophysin, CD56, and glucagon, and mitotic count was less than 2/10 HPF. This led to the diagnosis of NEN G1. In pNEN, cystic changes are more likely to occur in cases with positive glucagon production and immunostaining. In cases with cysts, it is important to differentiate between pNEN and MCN, and evaluation including immunostaining may be useful.
