The
total radiation dose (which is measured in Greys, expressed as Gy) delivered in
external beam therapies for prostate cancer is fractionated i.e. divided into
smaller doses. This means that the treatment is usually spread over a relatively
long period, normally once a day, five days a week for seven weeks. The intention
behind this approach is to limit damage to organs other than the targeted prostate
gland.
With
the advance that has been made in more
accurate tracking and aiming methods to deliver
the therapy there has been a move in some quarters to deliver what is termed a
hypofractionated radiation dosage.
This
means that the prostate gland and cancerous tumors associated with it are
given higher individual doses
(hypofractionated doses)
of radiation over fewer treatments with
a lower total dose of radiation. The
theory is that such a procedure may offer better results, because it creates a
greater differential between the prostate and normal tissue, i.e. is more brutal
to the cancer and kinder to the normal tissue that is irradiated. There
is considerable disagreement among scientists and radiation oncologists about
the efficacy and long-term side effects of this procedure.
The
potential benefits of hypofractionation are two fold. For the man receiving the
therapy, there is the convenience of having many fewer sessions of treatment;
for the facility providing the procedure there is the benefit of a higher turnover
of patients.
CyberKnife®, which is generically known as Stereotactic Body Radiotherapy (SBRT)
is a procedure
which gained FDA approval in 1991 and which has a good record in certain tumors
(e.g. nervous system). But it took many years before it was accepted for the treatment
of prostate cancer. As late as February 2008 it was generally regarded as 'experimental'
for prostate cancer because there were very few studies and no long term studies
as to its efficacy and morbidity results.
The potential benefits of
CyberKnife®
are based on the sophisticated tracking of prostate position - it is claimed that
the stereotatic heads with the use of fiducials can track very small movements
of the prostate and deliver doses more accurately than other methods. This sophisticated
method of tracking prostate location may be particularly necessary because the
patient has to lie for an hour or so for each CyberKnife® session. This contrasts,
for example, with a session of conformal PROTON
BEAM as currently practiced, where the sessions are a minute or
so, hardly giving the prostate a chance to move..
Since
the invention of CyberKnife®,
a number of similar items have been developed with similar claims.
There
is some evidence to support the idea of hypofractionation, but overall it cannot
be denied that the procedure is experimental. Hypofractionation may well have
merit, but long term (>10 year) results are not available for HDR
BRACHYTHERAPY, which has been practiced since about 1997, and neither
long nor short term (>5 year) peer reviewed results are yet available for CyberKnife®.
Maybe in the long run this version will prove to have similar (but possibly not
better) outcomes vis-a-vis non-hypofractionated EBRT (External Beam Radiation
Treatment), in which case, its selling point will be convenience. Any man considering
any hypofractionated
radiation
therapy should
make sure that his is a truly informed consent, and that he has chosen it over
methods with established records.
There is a CYBERKNIFE
FORUM That enables patients to get answers to questions from experts
and share their own experiences. The forum is apparently run by a manufacturer
of Cyberknife®
equipment , and should
be considered in that light as should the fact that most of the responders appear
to be men who have chosen this therapy. A feature of such single focus forums
on the Internet is the firm belief that men who have chosen the therapy have made
the only 'right' choice.
