1. A 68-year-old man presents with abdominal pain and weight loss. He undergoes a pancreatoduodenectomy for a 4 cm mass in the head of the pancreas. Grossly, the tumor is soft and appears better circumscribed than most ductal adenocarcinomas. Microscopically, the tumor is composed of poorly differentiated, noncohesive cells with a syncytial growth pattern and focal necrosis. The tumor has pushing borders and lacks a prominent desmoplastic reaction. What is the most likely diagnosis?