@jc76 The way my onco team explained it to me, when prostate cancer has metastasised, the metastatic cells will often evolve to produce their own testosterone after androgen-deprivation-therapy like Degarelix (Firmagon) cuts off their supply. In that case, the cancer cells will start multiplying again and your PSA will start rising even after your prostate has been radiated or removed. That's called metastatic Castrate-Resistant Prostate Cancer (mCRPC), as distinguished from metastatic Castrate-Sensitive Prostate Cancer (mCSPC), which still responds to ADT. At that point, more-aggressive treatments like chemo might come into the picture.

Next-generation medications like Apalutamide (Erleada) are designed to help prevent and/or slow down that progression (by years), and have been very successful so far both with mCRPC and mCSPC.