"I think that Barry has had a new tumour grow in previously healthy prostate material."
I would qualify this in my case by saying "..had a new tumour grow in a seemingly cancer free Prostate that had been previously treated with RT as a primary treatment." I say 'seemingly', because sometimes there can be some cells that are radio resistant and their DNA is not sufficiently damaged to be destroyed/stop reproduction.. These cells can be so small so as not to be seen on MRI but eventually concern raised by the way follow up PSA changed. So although my Prostate was ostensibly cancer free for 2+ years after RT, it might have been harbouring some cancer cells all the time.
With increasing PSA over 6 years post RT, a subsequent MRI indicated a small tumour which was confirmed by biopsy. This tumour was treated by HIFU in 2015 and initially PSA reduced a little but then started to rise again, very slowly but persistently. In fact, it only reduced slightly following a TP biopsy earlier this year. I think this was because part of the tumour was removed in one of the cores that was taken in the biopsy. In retrospect, I am doubtful that the HIFU completely eradicated the tumour. Calcification and the original RT may have made the HIFU more difficult. However, a further attempt with HIFU was approved in this spring but has been cancelled twice due to reasons too involved to go into here. I do not wish to have a Prostatectomy at this stage with as I was told would be permanent incontinence, so I believe further HIFU is my only realistic chance of direct rather than systemic treatment.
So to summarise, after RT, residual cancer in the Prostate is likely to be due to radio resistant cells that have survived but with Focal Therapy, it could be that the procedure failed to kill all the intended tumour, an insignificant tumour became significant or a completely new tumour grew in a previously unaffected part of the Prostate.