I think your radiation oncologist was suggesting that your PSA cannot be undetectable when not being treatment with hormone treatment. If the patient has had a prostatectomy they can have a undetectable PSA after successful treatment whereas a patient who chooses the radiation path will never be undetectable (assuming they are not being treated by hormone treatment).

I have been treated by Mayo and several other cancer centers. I believe Mayo chooses to use the less precise PSA test because they believe that discerning recurrence with low levels can be unnecessarily stressful for the patient without a real value. Treatments do not change when patients PSA goes up from .02 to .03 as an example. But a patient's anxiety certainly does. PSA is more volitile at very low levels. For instance, my PSA once increased from < .015 to .09 and then went down to < .015 months later.