As we see it here, in the non-medical professional world we have, bone scans seem to be "useless" unless you have a very high PSA. But is that really true? We do know that "in general" bone mets do not show up until PSA gets high. But then at a high PSA, how do you know what is a met and what is an old injury? How do you know that your PCa is not one of those rare ones that produces little or no PSA?
You do not know. Your doctors each have an opionion as to what they need. You will need to choose a doctor you trust, and do what that doctor believes is needed.
I can only speak to my situation. I had an inital PSA of 7. Bone scan was lit up like a Christmas tree, primarily because of many injuries over the years, but there were some vertebrae that were suspicious, for which I had no corresponding injury or break over the years.
Advance 3 years. Another bone scan on orders of my new medical oncologist, with BCR and a PSA of 3.8. The original Christmas tree remains, but the vertbrae are much worse, and collapsing. Are they mets? We don't know that yet, but they contribute heavily to my osteoporosis diagnosis, and how my HT is dealt with. If they are worse in March, they will do biopsies.
What did the "useless" bone scan get me? A baseline, and time to get all the records for the various bangs and breaks in the pre-PCa years, so that the new scans can tell us something. I have no reason to say that I should not have had the initial bone scan, even if it did not find PCa.
Choose your doctor, then work with him. If you don't like his requests, find someone else you can trust, then let them do their job.