Why would it not be practical to use ct scans to detect pancreatic cancer?

Size, detectability: Ct scans will indeed show the mass of a pancreatic cancer once it reaches a certain size, but most cases are advanced by the time the primary tumor is large enough to see on a ct scan. So it isn't a question of "practicality", but one of what the ct is useful for. It can't be relied upon as a screening tool (plus it involves radiation exposure).