You may not be aware that for radiotherapy, the beam is shot from many different directions, at the prostate. The reason for this is to reduce damage to surrounding tissue.
The following is a very simplified explanation of a radiotherapy plan:
To delivery a full dose of RT to the prostate. 25% is fired from above, 25% from below, 25% from the left and 25% from the right.
This means that any part of the body only gets a quarter dose, except the prostate which gets hit by all four doses.
The plan gets more complicated, because the radiotherapist wants to avoid hitting the rectum and some other organs, so the beam from below won't be fired directly from below it will be fired from slightly one side. Also no two paths should be exactly opposite each other, but I won't explain why. In reality they will use more than four paths, probably up to ten if they can.
As the hip prosthesis are probably made of metal they will block or scatter x-rays so the radiotherapy plan becomes much more difficult to design, as they have to avoid any x-rays hitting the hips.
Also the metal will make any scans of the prostate very blurred, so it will be harder to know where the target actually is.
I would not have thought this would make radiotherapy impossible, just more difficult, but I'm not a radiotherapists so how would I know.
I googled hip prosthesis radiotherapy, and this article came up.
https://humanhealth.iaea.org/HHW/MedicalPhysics/Radiotherapy/Topicsofspecialinterest/ManHipProsRT/index.html
I guess the fact I have a degree in physics might be the reason I found this easy to read and understand.