I'm ahead of you with RT treatment and I also went through similar.
After a week or two (time blurs after a while...) I noticed that I had trouble peeing, especially early morning or if I got up in the night.
Mentioned it to the RT team and after some ultrasound and infection tests put me on Tamsulosin which relaxes the bladder muscles.
This cleared the symptoms but I'm now 'back to square one' with early morning issues.
Since mentioning it during one of my weekly meetings with the support nurse they put me on daily bladder scans, to see what the situation was. Turns out I am not fully emptying and retaining circa 200ml. 250ml is the limit where they call in the catheter, but I've not needed that yet!
Checking on a daily basis I've found that drinking barley based drinks (Robinsons et al) and cranberry juice has assisted it lowering my retention troubles.
I've also found that finding a drinking routine pre RT that suits has helped me, I've mastered the full but not bursting level for when I'm on the table. Previous times I've been counting down the RT positions and clicks in preparation for leaping off the bed...
I empty out, drink one cup of tea, hit the road one hour before the RT time, sign in, drink 500ml of water and I'm bang on for my treatment.
If you can find a similar working routine it can help.
Number one job is mention it to the RT team, they will be aware of it and have solutions available I'm sure.
cheers
Kevin