I had 28 sessions of proton radiation + 6 months of ADT. Though my libido went to zero (testosterone level dropped to 3.0), I never experienced ED. From what I was told by my medical oncologist, the key is to continue doing it, despite the “want to” not being there.

My experience of not having ED while on ADT might be related to me ramping up my resistance-training and cardio exercise programs to minimize the side-effects of ADT; that might(?) also have had the side-benefit of keeping the blood flowing “down there” as well; I don’t know. Libido eventually returned to normal when testosterone level returned to normal.

As for delayed ED following radiation treatments, some studies show that late ED may be avoided by minimizing radiation hitting the penile bulb —> https://www.icr.ac.uk/about-us/icr-news/detail/avoiding-penile-bulb-with-radiotherapy-could-save-men-with-prostate-cancer-from-harmful-side-effects

As always, discuss with your medical team.