1. A 60-year-old man presents with abdominal pain and weight loss. Imaging reveals an ill-defined mass in the head of the pancreas with an irregular dilated main pancreatic duct. Microscopically, there is diffuse replacement of the normal lobulated parenchyma of the pancreas by fibrosis. The ducts are dilated and contain impacted calculi. Numerous islets are aggregated and form large, non-neoplastic, adenoma-like complexes. What is the most likely diagnosis?