Take this dot " ? "    Do you see it?  Then, it's detectable.  We'll call it
2 tenths of one nanogram (0.2)  If it's cancerous, in a few months, it might
look like this; " . "  We'll call that 1 nanogram (1.0).  At this size, it
starts to become too much for your immune system to deal with.

Now, look at this "    "   If you cut it, and pasted it to a diagnostic
screen and magnafied you could probably find it.  We'll call it .04, but, in
reality, it is virtually undetectable.  And, if it is there, your body can
deal with it.

Two things are possible.  First, the surgery might have left a small
amount of prostate cancer behind in the prostate bed.  Second, the
cancer could have metastized before surgery, which means some cells
capable of living outside the area of the prostate took up residence at
remote sites.   In either case,  initially there might not be enough
cancer cells present to produce a measurable amount of PSA, but with
time those cells could multiply and after a while, that would be
reflected by rising PSA.  It is also possible that the body's defenses
could keep a minor amount of cancer under control for years but
eventually the cancer might figure out a way to grow.

If recurrence occurs in the immediate area of the prostate, after
surgery, it is often possible to treat it with follow-up radiation.  If
it has escaped to distant sites,  the standard treatment is hormone
suppressing drugs which rob the prostate cancer cells of the hormones
they need to grow.  Unfortunately, eventually hormone independent cancer
cells grow in number and the treatment fails.

Usually, if all the signs are good, e.g., negative margins and the rest,
 then the probability is very high that the cancer won't recur.   But
in case of positive margins or worse or high Gleason grade, the
likelihood of recurrence is higher, though it still won't happen in a
large number of cases.

In any case, there is usually a fairly long period between the time PSA
starts rising and the time the patient has clinically detectable
symptoms.  In some cases, the patient lives out his life without having
any symptoms except rising PSA.

"Undectable" is NOT "nonexistent".  Do you see that cold germ on your finger!
No? Then how can you get a cold by rubbing it on your nose?