I believe there are varying opinions on this. I had 20 sessions of IMRT (photon). 8 months later, PSMA PET scan showed increased uptake in a portion of my prostate. I asked my RO if it could be radiated again. A flat "no" was the answer. "You received a lifetime dose"

RO left the practice and I asked my new RO the same question. "While we don't like to, in some cases it can be done, however, I wouldn't even consider it until 3 years after the initial treatment since side effects can take that long to appear." So, the answer I got was a definitive maybe. This was with photon though. Don't know if proton makes a difference.