Hi rdog, and welcome to this site.
I did not have brachytherapy, so cannot add first-hand knowledge...hopefully some others who frequent this site will add first-hand knowledge later.
To your question "How much should PSA drop after 6 months radiation seed implant?
What I do understand is that after radiation, either seeds, (brachytherapy), or external beam or cryotherapy the PSA will drop slowly, frequently reaching its nadir (low point) after several years
. Also, it is not uncommon to see a "bounce" in the first several years lasting several months, and then continuing to decline. Eventually, PSA levels should drop below 1.0 ng/mL and stay there.
After brachytherapy, an evenutal PSA nadir under 1.0 ng/mL is the goal according to Brachytherapy Made Complicated
, the textbook by Drs. Wallner, Blasko, and Dattoli.
Your drop after 6-months is a little less than what I've seen among other brachytherapy patients after tha much time. What is your doctor saying?
added later as an edit...
Read this online at a Brachytherapy treatment center site:
PSA Response to Brachytherapy
The PSA response to prostate brachytherapy may be more complicated to interpret than after other forms of prostate cancer treatment. The usual PSA response includes a steep drop in the PSA level during the first 3 – 6 months, followed by a more gradual reduction, which may evolve slowly over a number of years. Eventually, most brachytherapy patients see their PSA nadir in the 0.0 – 0.2 ng/ml range, though this may take 5 years or more to fully develop. Too, some patients will never see their PSA drop to a level this low, and yet still be apparently cured of their disease. Rarely, unfortunately, patients will develop a persistently rising PSA after the treatment. If a patient develops a persistently rising PSA after the brachytherapy procedure, this may be diagnosed as “Biochemical failure of their disease.” If this happens, the following tests, care and recommendations will vary, depending upon the exact patient situation.