1. A 72-year-old man presents with epigastric pain radiating to the back, weight loss, and jaundice. A CT scan reveals a hypodense mass in the head of the pancreas with an abrupt cut-off of a dilated main pancreatic duct. The patient undergoes a pancreatoduodenectomy. Microscopically, the tumor shows a haphazard pattern of growth. Glands are often incompletely formed, with luminal necrosis and occasional single cells infiltrating the stroma. There is an intense desmoplastic reaction. What is the most likely diagnosis?