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Proton Radiation


Rinecker Proton Therapy Center

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?

Dr. Hauffe

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?

Dr. Rinecker

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.