1. A 65-year-old man presents with abdominal pain and jaundice. He has a history of cancer. Imaging 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, and microscopic examination shows a haphazard pattern of growth, with poorly formed glands and an intense desmoplastic reaction. The tumor cells are positive for CK7, CK8, CK18, CK19, and CEA. What is the most likely diagnosis?