The way it was explained to me is that the nerves surround the prostate.
Peeling them back is like peeling wet single-ply toilet tissue.  So long as
the cancer is contained within the prostate capsule, there is a good chance
of sparing the nerves - that is, if they can be successfully pulled away
from the prostate.  My take away is that once the cancer moves out of the
capsule, there's a risk that the nerves are already damamged and therefore
more difficult to spare.

At least, that's how I understand it.

Sandy K.