@clevelandguy
You point out an important issue, Proton can be used in some cases where the maximum radiation was given previously.

There do seem to be some limitations.

Time Since Last Treatment: Generally, a significant amount of time (often several years) must have passed since the first round.

Location of Recurrence: If the cancer is strictly confined to the prostate, it is easier to treat. If it has spread to surrounding tissues that were also fully irradiated, it is more complex.

While proton therapy is a viable option for recurrence, it is often considered experimental or specialized in this context. It should be performed by a team experienced in re-irradiation to ensure the best balance between killing the cancer and avoiding damage to surrounding healthy organs.