Tumor size & spread. Stage i: the cancer is not touching the major blood vessels near the pancreas. Stage ii: the cancer extends beyond the pancreas, or the cancer has spread to a nearby lymph node. Stage iii: the cancer has wrapped around the major blood vessels near the pancreas. Stage iv: the cancer has spread to other organs. Surgery is usually not recommended for stage iii or IV tumors, but there are exceptions.
One must discern. Resectability: any distant disease in the peritoneum, liver, or distant organs rules that out, but local infiltration - posterior to aorta, and take-off arteries, or engulfing superior mesenteric artery, vein or portal vein, preclude surgery. Imaging, endoscopic ultrasound, arteriography, mri, and laparoscopy are all used after physical and history. Blood work helps too, .