MRCP2-1385

A 75-year-old man presents to his GP with left flank pain and intermittent sweats that improve with eating. He has also fainted between meals. A technically difficult ultrasound reveals a 3 cm mass in the tail of the pancreas. A CT scan confirms the mass and blood biochemistry is normal. A blood glucose test after an overnight fast and exercise bike session shows a reading of 2 mmol/l. A CT guided needle biopsy of the pancreas shows ill-defined nests of cells that stain positive for chromogranin and synaptophysin. What is the most likely diagnosis?