Many condolences. .. so sorry for your loss.
I read today:
'PSA testing commonly misses the 15% or so of potentially lethal high-grade cancers as they often make little or no PSA'
'Additionally, the prostatic specific antigen (PSA) and digital rectal examination (DRE) are both highly unreliable tests'
So maybe science was unfortunately not advanced enough yet to help your dad. I am no expert and had no idea that it was so high (15%) that PSA screening wouldn't pick up. Also this could be wrong but someone else on here may verify.
I hope you get an answer to your question.
Clare