I had both as a treatment in IMRT. It was a 6 month ADT dose. This after a prostatectemy that looked promising but had BCR 13 months later and salvage radiation to the pelvic area that too looked promising. But the reason your Docs are recommending both IMRT is that the ADT helps in the killing of the cancer cells the radiation hits. They call it "sensetzing" them. There's disagreement on how it helps exactly with some postulating it jams the dying cells' repair mechanisms.
But in addition it kills or lames up the micro cancer cells that even the PSMA Pet scan can't detect.
I'm very athletic and my Doc said that's a major reason my side effects from ADT were relatively mild - some night sweats no loss in libido, etc. Full recovery of testosterone in about 2 months.
The procedure dropped my PSA undetectable for a 4 month period. This was in 2021. Still being treated for oligometastatic lymphtropic pc with a PSA that's never gone above 6. Classified as an indolent oligometastatic metachronous lymphtropic hormone-sensitive pc which means I'll probablybe fighting rear guard actions with it till I die. But if I keep those up I'll probably die with the disease and not of it. I'm 78 and survived rocket attacks and an ambush in Vietnam where I probably picked it up via Agent Orange. So no complaints here.
Talk to your Docs about this- but the key is to avoid as much of the ADT as possible to avoid or forestall becoming castration resistant. But as I understand it takes about 2 years of continous ADT for that to occur. So 6 month dose now isn't likely to even contribute to that issue even if there's more intermittent doses down the road. But you might ask you Docs about using an ARPI like enzalutimide instead of the ADT.
In any case good luck
Jim