@heavyphil Yes, they can give photon after photon (or proton after proton) – re-radiation can be done. But, there are concerns, (I’m not sure that the prostate being “fried” is a correct representation.)

Dr. Rossi goes over all of this —>

The concern with re-radiation – and this gets to the unknowns — is that even though that normal prostate tissue may not have been clinically damaged the first time it was treated, there may be some microscopic injury that may or may not be repaired (or reparable). They’re still learning more about how much of that actually is repaired as time goes by.

Historically, the primary concern is that if you re-radiate, you're going to place patients at a high risk for complications impacting nearby tissues and organs: for instance the rectum – because rectal tissue does not tolerate radiation well. But, these days you have rectal spacers that can create big margins, that way you avoid re-treating the rectum. (SpaceOAR reduces radiation to the rectum by 70%.)

If (for example) you gave me 70 Grays of radiation the first time and it didn't work, giving me another 30 – 40 more Grays doesn't make any sense; we not only risk exceeding the prostate’s radiation tolerance, but also causing complications of the colon/bowel. It’s gotten to be much better these days because of better delivery and excluding the rectum away.

So, the biggest concern is what happens to the rectum, bladder, urethra, and other surrounding tissues that might accumulate permanent radiation injury. Once these tissues reach their tolerance dose, additional full-dose radiation becomes unsafe.

Again, re-radiation is generally not the preferable salvage route. If there is a recurrence after primary radiation there are different ways to address that – and re-radiation is just one of them. What’s preferable and probably the safest way to do it is focal cryo surgery – freeze the area where the recurrence is because it's more controllable than doing re-radiation; also HDR brachytherapy is an excellent way to do it because you have more control over where the dose goes.

I personally know 2 guys who had their recurrence re-treated with SBRT, because the recurrent lesions were localized, small and well-defined. In those two cases using advanced techniques, administering a tightly focused and administered photon dose was done. And, it’s worked out well for them.