After I'd been diagnosed and put on to hormone therapy, I started reading about it and that Tadalafil could be useful to try and preserve erectile function and prevent long term damage to penile tissue while you have no Testosterone. At the same time, my GP asked me in for an appointment (Cancer Care Review 1, which everyone is supposed to get after a cancer diagnosis). I prepared myself to ask for and justify the Tadalafil, having heard many people struggle to get it. I walked in and sat down, and after exchanging hellos, how are yous, etc, first thing GP said was "Right, you've started the hormone therapy, let's get you on Tadalafil" - I never had to say a word.
I had expected erections to stop working, but I was lucky and they continued working on hormone therapy, so although I had the Tadalafil at home, I didn't start taking it. Some 6 months later, I did mention this to a urologist, and he said I should be taking it anyway - it's beneficial even if you don't need it for erections, or even if it doesn't work for erections, so I started taking it at that point. About 20% of men can still get erections on hormone therapy, and Tadalafil increases this (although no one has researched by how much). It is important to continue having regular erections to maintain penile tissues, but the involuntary ones will vanish while on hormone therapy. If you can't get them regularly naturally or via Tadalafil, you should as for a pump. (The number of men who can still have sex on hormone therapy is much lower than the number who migt manage an erection.)
I'm not sure Tadalafil is much use regarding the radiotherapy, but it can help protect against the hormone therapy.
Radiotherapy doesn't normally have any impact on erections at the time (except possibly temporarily) although if you're on hormone therapy, you won't be aware of this. If radiotherapy is going to damage erections, what you will find is the erectile function starts deteriorating within 2 years of the radiotherapy, and continues getting worse. Tadalafil will help initially, but you may reach the point where Tadalafil (and the other PDE5 inhibitors) no longer work. An expert in this field said to me if you get to 2 years after radiotherapy and have no deterioration in erectile function, then you swerved that bullet. Erectile function can still deteriorate after that, but the incidence of this drops back the the same level as similar men who never had prostate cancer, so that will be some other reason. (Unexpected erectile dysfunction should always be investigated, because there are a number of potentially serious conditions for which it's a side effect.)