I did not lose weight or gain weight after the treatments. My weight has always been managed reasonably over the years and I have stayed active.

Here is a link to Elekta’s treatment centers:
https://www.elekta.com/patients/treatment-centers/
You have done the spaceoar which should help and they will ask you to drink water each time about 45 minutes before your treatment to help separate your bladder from the radiation. You may under or over do that but they will let you know. If you believe that your doctor has the expertise to accurately map your treatment plan then given all the prep work you have done, you should feel good that you have taken steps to minimize side effects. Keep asking questions of your doctor and the people (physicist, dosimitrist…) that are in the treatment room with you. It’s scary but after you have had your first treatment, you will feel more comfortable.