This may just be a 'so how are you feeling' chat Rich - a lot of oncos wouldn't be interested in a PSA test this soon after the RT as it is too early to really tell them anything. And unfortunately, there isn't a definite PSA level that you should or should not have fallen to. They say that you should hit your PSA nadir about 18 months after RT finished - the radiation continues to kill the cancer for a very long time after the zapping has ended.
Even if I am right, it isn't pointless though - it is an important opportunity for the onco to check how you are and for you to ask questions like 'so next time I have my PSA test, what will you be looking for?' 'how will we know this is working?' and to discuss any side effects you are concerned about.