Wondering about the rate in the drop of my PSA after HDR brachytherapy

I was diagnosed with prostate cancer approximately 18 months ago. My PSA was 4.7. Gleason scores were 3+4. 16 of cores were positive. PET scan showed that cancer was contained to the prostate. Elected for two factions of HDR brachytherapy. Four months after treatment my PSA dropped to 1.68. Four months later, it dropped to 1.33. Is this rate acceptable or should it have dropped lower?