PSA, Prostate-Specific Antigen, does come from prostate cells - both healthy ones and cancerous ones. If the prostate is thoroughly removed, any rise in PSA must be due to cancerous prostate cells. Some cancerous cells put out a lot of PSA, some very little. Men with mets can sometimes have PSAs in the 1000s..
You may want to read the sticky:Newly diagnosed with PC? – read this thread first
. Even though you're not newly diagnosed, it has a lot of the kind of information you may be looking for.