As @northoftheborder says, what you are describing could be a result of advanced prostate cancer to the spine.

On the other hand, I have on and off bouts with lower back pain, But I’ve had it for over 25 years, Way before the cancer. I have a belt I can put on, and within a day it normally makes my back stop hurting and I’m back to normal for another six months or longer.

So the real question is, is your lower back pain a common problem that many men have or is it related to prostate cancer?. Do you feel anything Going down your legs when you’re having this lower back issue, it can cause sciatica.

You could talk to your doctor about doing an MRI, a PSMA pet scan probably won’t find anything since your PSA is so low. A CT scan might work as well if they’ve got one to compare it with. This is something you have to talk to your doctor about. It may involve an Orthopedic MD.