IS IMRT AS GOOD AS
PROTON?
Recently, Uwe Wolff, journalist and
author, interviewed Dr. Hans Rinecker (PhD) and Dr. Jörg Hauffe, from
the Rinecker Proton Therapy Center in Munich, Germany. The Rinecker
Center recently celebrated its first year of operation. Dr. Hauffe and
Dr. Rinecker offer some excellent responses to the reporter’s
questions, particularly with regard to claims made that advanced forms
of X-ray technology are as good as proton. Following are a few
excerpts from the interview.
Dr. Hauffe, you are
physicist by profession and come from CERN (Conseil Européen pour la
Recherche Nucléaire or European Council for Nuclear Research), the
huge research institution for elementary particles. You have dedicated
your entire work life to the highly demanding physics of this new type
of therapy facilities. Why should, in the opinion of a physicist,
proton radiation replace the X-ray therapy?
Hauffe: X-ray-radiation
is, simply spoken, always a shoot-through method. The X-rays
can be easily bundled and precisely targeted at the tumor in the
dimensions left/right and top/bottom. But not lengthwise in the
third dimension. The X-rays penetrating the body will cause
most of the damage just under the skin, with their intensity
decreasing until they leave the body: Some
50% of the X-rays leave the body again on the opposite side.
The X-rays cannot be targeted at all at the tumor’s position in the
depth. This has the effect that X-rays inevitably damage the healthy
tissue surrounding the tumor with a three to five times higher
integral radiation dose (depending on the body and tumor geometry)
when a certain therapeutic dose in the tumor is to be ensured.
New, more precise X-ray
irradiation devices, such as the IMRT method or the cyber knife, are
frequently the subject of discussions among experts. Will they not
improve the X-ray situation?
Hauffe:
Only partly. The currently best X-ray apparatus, the so-called Rapid
Arc from an American company, exposes the tumor to X-rays from all
sides. The dose overlapping effect achieved in the tumor can thus be
much better adjusted to the real shape of the tumor than before, when
the irradiation fields still had an e.g. square shape. However, even
these advanced X-ray devices cannot overcome their inherent
fundamental shortcoming, namely to be unable
to target in the third dimension, since X-rays are subject to their natural physical laws.
The modern apparatuses only further distribute the damaging radiation.
Therefore more healthy tissue is affected which is still
damaged by a three to five fold radiation volume compared to the
tumor.
And why is proton
radiation any better?
Hauffe:
The protons take advantage of an effect that was discovered by the
physicist Bragg some 100 years ago: These accelerated, charged
hydrogen atomic nuclei will have the same ionizing effect as X-rays,
but with a much more favorable local dosage distribution. When
penetrating the body, these particles will not be
absorbed, “soaked up”, so to say, like X-rays which are
electro-magnetic waves. They will rather be slowed down, although,
initially, they discharge relatively little energy. The slower they
get, i.e. the longer their transit time is in the atomic electron
shells of the living tissue, the more they will be decelerated, the
more energy they will discharge, the higher will be the local dose and
thus the biological effect. This will eventually reach a
pronounced peak which is called Bragg peak in honor of the discoverer.
Beyond this peak there is no more radiation, since the
protons’ energy has all been spent. We can
therefore aim directly at a radiation-sensitive organ and stop in
front of it. Before the peak, the radiation effect is lower in
comparison with the tumor dose, not higher as in the case of X-rays.
Moreover, we can target the Bragg peak at the tumor with millimeter
precision by adjusting the speed of the protons. We can penetrate the
body 38 cm at a speed of 180 000 km/sec, but we can also localize the
Bragg peak on the surface, e.g. in a tumor of the eye. We can apply
the radiation three-dimensionally and not just two-dimensionally, as
is the case with X-rays.
What does that mean,
biologically, so to say, for the patient? Will he respond to proton
radiation more favorably?
Rinecker:
Here at the RPTC, we do a lot of comparative calculations between
X-ray and proton radiotherapy for individual patients simply to show
how much better our method is. We would not therapy patients
with protons, if we could not reduce the radiation dose that hits the
healthy tissue in addition the higher effect on the tumor itself.
Depending on the tumor’s position, on its size and on the geometry
of the ambient area, we
can normally work with only a quarter of the damaging radiation,
also in comparison with the most modern conventional X-ray devices. The
patient feels better, he does not suffer from acute and subsequent
side effects, or at least much less, and our therapists are free to
use an optimized tumor dose. We can then also reduce, in many
cases, the number of necessary radiation sessions, as is the case with
prostate cancer. This is certainly to the patient’s benefit as well.
Critics of proton
therapy are known to say that the proton therapy does not necessarily
achieve better results than state-of-the-art X-ray units…
Rinecker:
This was, indeed, a point of criticism in former times. However, the
proton results were, expectedly, equal to X-rays in a historical
comparison only then, when the tumor dose in proton therapy was not
increased or distributed more efficiently. This was the case during
earlier treatments in the first large-size facility at Loma Linda,
USA, because of restrictions of the then valid permit. Unfortunately,
these outdated publications are still being quoted. However, the
recovery results of protons have never been worse than those from
X-ray despite the concurrent much more gentle treatment of the ambient
tissue. Wherever the dose in the tumor has been increased or
administered more effectively over time however, proton results have
definitively been better, as has been the case with prostatic tumors,
but also with chordomas at the base of the skull and in the body
elsewhere as well as with other types of tumors.
Hence, the direct
comparison between these two types of radiation is legally not
admissible?
Rinecker:
The question with regard to comparative research between proton
therapy and conventional X-ray would be, whether the three to
five-fold higher exposure of the patient’s healthy tissue in the
case of X-rays would be tolerable, or admissible, nothing else.
Such kind of research objective is neither ethical nor scientifically
justifiable – apart from the fact that it is illegal
Bob’s comment: Well, there you have
it. With all forms of conventional (X-ray) radiotherapy—including
the most advanced forms—three to five times more radiation is
deposited on healthy tissue than with proton radiotherapy. And all
oncologists and physicists agree that the only safe dose of radiation
to healthy tissue is a zero dose. Therefore, the chances of collateral
damage causing debilitating side effects and delayed secondary cancers
is clearly greater with X-rays than with protons.
The entire interview can be found at Rinecker
Proton Therapy Center website.