There are no guarantees and the best you can do is seek the opinion of a radiologist as to how likely and to what extent the RT might affect you and then make your decision whether to go ahead. There are systemic alternatives but how well and for how long these might be effective can't be predetermined and you still might need these after salvage RT anyway even if later on. Do you have the RT and improve your chances of ridding yourself of this cancer permanently or pushing it back albeit with some risk of organ damage or rely on down the line treatments as necessary? A decision only you can make.