For RT on prostate, everyone needs an empty bowl, but some people need a full bladder, and others an empty bladder, depending precisely which parts the beam is being directed to. I suspect it's all much less of an issue for the empty bladder folks - you can just go and dump everything out immediately beforehand, and bowl needs to stay empty for perhaps 10-15 mins max.
It's harder for the full bladder folks. The protocol is to dump everything out on arrival, and 45-60 minutes before they're ready for you on the machine, they ask you to drink a litre of water relatively quickly (within 10 mins I think they said, not slowly sipping it), and note the time you finish. They then wait at least 30 mins for it to make its way through to your bladder, and it may well be longer by the time the previous person has finished, and you go in with hopefully between 0.3 litres and a full bladder, and hope your bowls are still empty. Trouble is, it's probably an hour or more since you emptied your bowls, and it's quite difficult to tell if they're still completely empty when you have a full bladder. If they're not, it's pretty impossible to empty your bowls without emptying your bladder, although maybe it's something that can be practiced. Anyway, if you're in this situation, you get sent to dump everything out and wait some time for most of the water to work its way out (1 litre is typically 2 full bladder's worth, so 2-3 pees), and then you start going around the cycle again.
I had to do that on the planning session, but on the first real session today, it was OK, although I could feel my guts rumbling as I was about to go in and thought I might fail the empty bowl test again.