Hi Kristvet86 - sorry you need to be here but glad you found us. Your father is lucky to have you on his side. It's not unusual for wives or other family members to post here, since we guys are usually all too good at ignoring health issues! We're here to help if we can.
The Gleason score is a measure of how degraded the cancer cells are. It's a sum of 2 scores seen in a microscopic exam of biopsy tissue, types ranging from 1 (nearly normal) to 5 (barely prostate cells anymore). The G9 usually means either 4+5, or 5+4, the first number being the predominant one seen. The type 5 cells make very little PSA, so if they're dominant it is surely possible to have rather advanced cases with low PSA. Other types of prostate cancer exist too, some make very little PSA indeed.
So in short, yes it's possible to have advanced prostate cancer with a low PSA. Frustrating to diagnose, but it happens. And even more exasperating, there may be few symptoms too, if any. It's shocking to hear such a diagnosis when you feel no real problems!
Hormone therapy (Firmagon) is no picnic, but is the usual first step in the treatment plan for this situation. He's off to a good start.
I've taken the liberty of adding you to the illustrious G9 Crew roster. Here's a link to the thread we set up for us, since the G9 folks have some fairly specific concerns. By definition, G9 is high risk, so we're commonly looking for aggressive treatments for the best shot at remission. You are likely to have your dad around for quite a while, as the present treatments are pretty good at managing this for a long time. Every single case is different, but if you look through this thread you'll find many people there with most still doing pretty well.The Gleason 9 Crew -- Part 2, continued.