Good questions.  I would guess they cut away as much as they can
without jeopardizing surrounding tissues such as the bladder, rectum,
and the precious final sphincter at the bottom that keeps us continent.
Often, a surgeon can see cancer at the edge of the sample and and will
dig deeper to try to get it all.  My original interpretation was that a
prostate is like a walnut, you just go in and pluck it out.  It turns
out, the edges of the gland aren't so well-defined so there is a lot of
educated guesswork involved.  Experience sure helps too.  I don't
believe they sample tissue during surgery - at least my doctor didn't
and I've not heard of them doing anything like that in prostate
surgery.  I read back in 2003 when I was diagnosed that a suspicious
lymph node is sent up to see if cancer has spread and if so, the
surgery is discontinued - no point in taking the prostate out at that
point.  I assume they still do that.
Dave Perry

They take the entire prostate -- margins and all.  The entire prostate is
then coated with ink, frozen, sliced into layers and examined under a
microscope by pathologists in a  lab. The ink marks the location of the
margin so the pathologist can determine if cancer cells have breeched the
margins.

Presuming they do not suspect spread beyond the prostate, they start
with cutting out the entire prostate plus a prescribed set of parts
which a) are prone to have cancer and b) will be useless once the
prostate is gone so will never be missed. As they cut, every border of
the removed meat is examined by the surgeon's eye and touch for
real-time evaluation of spread (an advantage of RRP over robotic RP);
appearance and feel (e.g., sticky) can distinguish cancer beyond some
certain level of advancement. They will also send suspected or likely
snippets/slices of marginal tissue (margins = borders of the removed
meat, whether it's the prostate capsule or your left butt-cheek if they
had to follow sticky cancerous tissue that far; my point is that a
"margin" is not a sack the prostate lives in -- it's merely the outer
surfaces of whatever got cut out, even if it's butt-cheek meat) to the
pathologist for real-time evaluation. If s/he finds cancer at the
margins, s/he picks up the phone and tells the surgeon to keep cutting
at the specific site where s/he found positive (cancerous) margins. (At
some point, of course, the surgeon's got to stop if she feels  it's a
lost battle. I'm guessing no RRP has progressed to the patient's
gluteous maximus)

OTOH, if they suspect cancer beyond the prostate in advance, or maybe
even if they don't, they will sample some lymph nodes for, again,
real-time assessment by the pathologist waiting in the lab. If s/he
phones back with cancer in the nodes, it's decision time; Is the spread
bad enough to render surgery useless, in which case surgery will stop?
(Some docs always abort if any lymph nodes show cancer; Walsh and many
others say that's an overreaction. I made sure in advance that my
surgeon's prime directive was, "If in doubt, cut it out". I consider
surgery a nuisance, not a catastrophe, in that it's temporary and it's
pain-free (DRUGS, remember?). I also infinitely prefer curative
impotence and incontinence to certain death by PC, given a choice.)

I.P.

Ha!  And that's why you want someone with lots and lots of experience.  They
cut it where they can.  It's a very small piece of tissue buried deep in a
cave of tissue.