Targeted beams of high-intensity
radiation can shrink early-stage tumors
with limited collateral damage to
surrounding healthy tissue. The
addition of robotics and image
guidance systems in recent years has
made these stereotactic, or directed
beam, radiosurgery systems an even
more versatile weapon against cancer,
attacking not only brain tumors (for
which they were originally designed)
but also other diseases virtually
anywhere in the body.

Radiosurgery (and radiation therapy,
which relies on lower doses of
radiation spread out over a longer
period of time) uses a beam of
energetic particles to ionize the atoms
that make up the DNA chain. A treated
cell becomes unable to reproduce and
loses its structural integrity.
Because the technology does not
discriminate between healthy and
cancerous cells, Accuray (the maker of
Cyberknife) developed a tracking
system to help CyberKnife maintain
accurate targeting of soft-tissue
tumors that shift position during

As a result, CyberKnife's linear particle
accelerator can produce a radiation
beam that moves in rhythm with a
patient's breathing, targeting the
correct spot at all times. Anesthesia is
not typically needed for a CyberKnife
procedures, and the treatment itself is
painless.  A doctor says, The number
one thing I hear from patients
afterward is,"Are you sure I got

A  CyberKnife radiotherapy treatment
usually last  for 30 to 40 minutes at a
time over one to five treatment
sessions, typically during a single week.


The first type: To generate a radiation
beam, CyberKnife and Varian Medical
Systems's Novalis Tx  use a linear
accelerator that can be moved to treat
tumors in both the head and the body.
Indeed, physicians found that
CyberKnife could not only treat brain
tumors but also prostate, neck and
other cancers.

A second type of stereotactic
radiosurgery is Elekta, AB's Leksell
Gamma Knife, a pioneering technology
in the radiosurgery field. It uses a fixed
beam generated by a cobalt 60
synthetic radioactive isotope.
When Gamma Knife was developed in
the late 1960s, it offered an alternative
to conventional radiation therapy,
which bathes portions of the body in
lower-dose radiation for longer periods
of time in an effort to kill cancer cells
while limiting damage to surrounding
healthy tissue.
Unlike CyberKnife, Gamma Knife's
beam cannot move during treatment,
making the technology suitable for
treating tumors in the brain but not
other areas of the body that move due
to respiration.

The third form of radiosurgery relies
on a beam of protons to irradiate
tumors. This proton (particle) beam
technique has not found wide
deployment in the U.S., in part
because the equipment can cost in
excess of $100 million, but also
because little research exists on the
technology's efficacy and safety.

Despite costing 10 times as much,
proton therapy has emerged as a
challenge to CyberKnife, because it
uses a different type of radiation at a
lower dosage.   Protons also do not
work well with a moving target. If you
could make a proton system that's as
accurate as a CyberKnife, it would be