I was told I could take a urine bottle into the treatment room and use it if really needed. In my hospital I kept my joggers and underwear on,they just pulled the waistband down a little without exposing my bits. So I had a tena pad in my underwear, a controlled leak will be absorbed, a sudden full gush may be more of an issue.
Although things may change during treatment, you could practice drinking water and holding it for a set period. Keep hydrated during the treatment days. By practicing you will also find out how long it takes to refill your bladder.
To see how effective my pads were I put a pad in my underwear and stood in the shower to see how well the urine was absorbed. If I could slow the flow, I could get to the bottle or loo in time ,if I just let the whole bladder go at once the pad didn't contain everything. You could experiment with bigger pads.
Having said all that, the RT staff have seen it all before.
Hope all goes well.
Thanks Chris