The don't have any contact with my GP on treatment for my PCa. Instead I have regular reviews with oncol at a major local hospital. It may be that because I don't have mets my experience may not be helpful: I am on HT having recently completed RT. However, I've been told that if my PSA rises by more than 2 they will intervene with other treatment. I believe the low intervention threshold is as Max203 says: the more aggressive types of PCa do not necessarily generate large volumes of PSA.
Viv