Hi OM,
I note your situation and hope the RT works well for you along with your RT. Prostate Cancer is a very complex disease and there are different types, some of which tend to progress faster than others and the RT/HT combination may slow advancement in some cases for years, however, it may be necessary to change the form of HT to a more advanced form when original HT becomes inadequate. RT treats specific areas and can cause cancer cells in the treated areas to die over up to 24 months. However, RT is not going to stop possible to permanently stop spread to non RT treated areas, that is down to HT or Chemo, which treats systemically helping to restrain cancer cells wherever they are. So it's likely more down to how well systemic treatment works than the RT, particularly as this is being described as palliative, but may work to a degree quite well. Some men do well but I suggest you discuss your situation with your Oncologist because men can react very differently to treatment and apart from being medically qualified ie lack access to your histology and scans, which we don't have in both respects.
You should be regularly monitored so that your treatment should be updated as needed to restrain the cancer for as long as possible.