Had extensive radiotherapy 50 years ago

The only thing is that there will be few patients from that time to share experiences with, and probably even fewer oncologists.

So my comments are speculation. But I think it is likely that even 50 years ago they will have taken efforts to ensure the radiation doesn't spread beyond their target - with testes that should have been easily achievable with lead shields even if the machines of the time weren't as tightly focussed as now. If that was the case, you probably remember, there is no obvious reason that you would be more in risk of incontinence than any other patient undergoing radiotherapy. In general with modern machines they can target the beam to minimise side effects, not that anything is guaranteed.

Good luck. As far as I know incontinence is not often a big issue with radiotherapy these days, and the odd bit of leakage easily managed by wearing a pad. But that will also depend on your particular situation and treatment, so you need to discuss with your oncologist.

I think there are two aspects to this.

Will anyone be able to find your previous radiotherapy dosing records?
I was supporting someone a few years ago who had exactly the same issue. The hospital where they had their previous radiotherapy still existed, but it was now a very different establishment and hadn't had any radiotherapy facilities there for decades. They didn't have the radiotherapy records from 50 years ago. He contacted the health authority and they went on a hunt, but initially it looked like the records were lost. However, they were eventually found in the radiotherapy department of the nearest district general hospital. This enabled the medical physicists to plan his prostate radiotherapy to miss the areas treated 50 years ago. Radiotherapy treatment for testicular cancer is often higher up than the prostate (i.e. nowhere near the testicles), but not always.

The second thing is that although the mantra has always been that you can only treat any tissue once for radiotherapy because it's given it's maximum lifetime dose, this is no longer considered as set in stone as it once was. Providing your original treatment was a number of years ago, retreatment is now being offered in some cases. In your case, the target is going to be different, so any overlapping areas probably didn't have the full dose anyway, and aren't going to with the prostate cancer treatment either, and again this increases the scope for treatment.

Also, depending on your diagnosis, something like brachytherapy might be suitable which doesn't have any entry/exit wound, and therefore that wouldn't overlap with previous radiotherapy.