For prostate SBRT, what bladder and bowel prep strategy do you use?  

In our CT simulations and MRIs for prostate SBRT, we notice that the bladder can be significantly different in appearance between the two scans with our current strategy, even when the patient follows instructions exactly. Many of our patients are not able to drink water 2 hours prior as done in the RTOG protocol and hold their bladder for this amount of time. Currently we are asking them to drink the water 30 min to 1 hour prior. On our scans we see that it's not just the position of the prostate that varies, but the dimensions of the prostate can change as well with this variation in bladder filling.