I don’t think they use proton therapy for salvage. Protons are very precise and are targeted to expend all of their energy at a given point - usually the prostate and a preset marginal area around it (Bragg Effect). The benefit is that the beam ends at that point and does not pass thru other structures.
Salvage radiation MUST pass thru in order to hit wayward cells that could be anywhere - behind the prostate bed or in front of the rectum. The beam, however, can be ‘shaped’ to avoid maximum effect to sensitive structures. Hope this helps.
Phil