Professor Whocannotbenamedhere does quite a number of salvage RS-RALP after both HIFU and RT.
Based on something he's found and something else I communicated back to him following this years Pelvic Radiation Disease Association conference, it's looking like salvage RT following hypofractionated RT (e.g. 20 sessions) is significantly more difficult than following standard RT (around 37 sessions).
It is also looking like PRD is more of an issue after hypofractionated RT too. Radiotherapy departments are moving to hypofractionation quickly because it helps with the country wide shortage of theraputic radiographers, and fewer sessions is preferred by patients.
RT doesn't turn the prostate to mush - quite the opposite - it makes it harder and unable to compress due to fibrosis to the smooth muscle. It's more difficult to remove because the fibrosis also ends up joining it on to neighbouring organs, where it wouldn't ordinarily be conjoined.