I'm not an expert on this disease I was diagnosed with two months ago, but "Salvage Radiation Therapy" can come in many forms I suppose, except for brachytherapy obviously as your prostate is gone after surgery.
My takeaway was, and I think it's HUGE and I'm glad TA made me think of it, is since you no longer have a prostate there's one less organ (walnut size) there to absorb the radiation so it makes it difficult to avoid the rectum, bladder, urethra.
When it's beaming down though the skin and tissue it will certainly affect the bladder and perhaps the rectum more than otherwise. I would say even with precice IGRT it would be hard to just radiate the bed of the prostate with any pin-point accuracy.
I believe this happened to my dad, though I can't be 100%. He had to have cancer scrapped from his bladder and the urologist commented that it was "gummy" from the radiation. If my memory serves, he had his prostate removed, had SRT and then got bladder cancer. They tried scrapping and it eventually came to where they removed his bladder and made a new one inside his body. (he has a bag for both urine and defecate). He's still alive at nearly 82 though...