Often after Prostatectomy some unseen cancer cells are left in the bed where the Prostate was removed and this together with some lymph nodes in the pelvic area are radiated with the hope that this will remove remaining cancer cells. The location chosen is what you might call an educated guess. Clearly there is a limit to the extent that RT can be directed. In your case the RT doesn't appear to have reached the cancer cells, even though RT does take months to work. So as no specific area could be found you have been put on HT which systemically puts the cancer cells to sleep wherever they are. While HT works, it makes it more difficult to identify where the offending cells are. That is why sometimes RT is not given until the cancer cells congregate to the extent that a tumour is identified to which RT can be directed. But if waiting longer there is a risk that the cells advance further and gain extra heterogeneity. In your case the cancer cells could be in a number of places, including bladder, liver, lung, bone or lymphatic system among them, perhaps too small or dispersed to be seen by scan until HT no longer constrains them. So at present it may not be possible to know where these errant cells are.