Dick,

There have been some studies, but I don't know how long term,
complete, or objective they are.

As I understand it, due to an effect called the "Bragg Peak"
(see: http://en.wikipedia.org/wiki/Bragg_peak) carefully aimed
and energized protons deliver a significantly higher percentage
of their total energy to the target area than to the areas in
front of and behind the target as compared to x-rays.  As a
result, in theory and I presume also in practice, they do less
damage to areas around the prostate than x-rays emitted by
external beam devices.

Does this lead to better cancer control?  I'm no expert, but I
THINK the theory is that it does NOT lead to better cancer
control, but DOES lead to a lower incidence of unwanted side
effects.

Unfortunately, proton accelerators cost about $140,000,000.
That's a lot of bucks, and recouping that money requires that
prices be much higher than for other forms of radiation.

Before deciding that it's worth the money and the travel to get
to a proton center, you also have to take into account a couple
of other factors.

1. Modern external beam x-ray equipment is pretty good at
targeting tumors.

Using IMRT or IGRT, radiation oncologists can route the x-rays in
ways that give relatively small doses to those structures that
are most sensitive to damage.  So, while proton therapy may
deliver even less dosage to those structures, conventional x-ray
therapy may already deliver small enough doses that the side
effects are not a problem for most men.  In other words (I'm
going to make up some numbers here) if proton beams cut side
effect damage to non-target areas by 50%, but the incidence of
deleterious side effects is already down to 10%, you may increase
your chances of being side effect free only from 90% to 95%, not
from 100% to 50%. -- Remember, those numbers are made up, I don't
know the real numbers.

2. There are other forms of x-radiation that do also avoid
zapping regions outside the prostate.

Brachytherapy implants radioactive "seeds" directly in the
prostate.  I have read (don't know if it's true, I'm no expert
here) that those seeds only deliver significant radiation to an
area 1-2 millimeters around each seed.  That's why they need so
many of them and don't just plant one seed in the prostate.

So it seems to me that, in theory, a good radiation oncologist
working with brachytherapy can also greatly reduce dosage to
areas outside the prostate.  Whether it's more or less than what
proton beams do is a question for the rad oncs.

Brachytherapy also has the advantage that it's often done as a
one day treatment instead of 40 days, and the disadvantage that
it's a surgical procedure, usually requiring anesthesia and an
overnight hospital stay.

For more serious cases, many rad oncs prescribe a combination of
external beam and brachytherapy.  So even if you choose brachy,
you could still need some EBRT.  However that EBRT is delivered
with significantly lower dosage than when there are no seeds.
Its main purpose is to treat areas around the prostate which
could have extra-prostatic tumor extensions, beyond the reach of
the seeds.  If a proton therapist suspect such extra-prostatic
extension, he may radiate those areas too.  And it is my
understanding that they often use regular x-rays for that, not
protons.

Hope that helps.

   Alan