AW: [ RadSafe ] Article: Heavy ions damage DNA

Tue Oct 18 07:29:44 CDT 2005

		       L I E F E R S C H E I N

Bestellnummer : E014297391  	       	  Datum : 14.10.2005        
Kundennummer  : K000156935  

John:

Thank you for that pointer. Yet rather than interesting the article is confusing for me.

Particle energies for therapy are usually at least around (some) 100 MeV/u (u=atomic mass unit). At 1 MeV/u, protons can travel in water for 26 micro-m, Neon for 17 micro-m and Argon for 19 micro-m, i.e., with this energy you could hardly irradiate properly a mono-cellular layer of mammalian cells not to speak of treating patients.

It is known 'since ages' that the main mode of energy loss for charged particles changes from mainly loss to electrons to loss by atomic collisions when particles approach their stopping end. Hence the occurrence of secondary 'heavy' recoils could only surprise someone who has never read the even most basic treatise on stopping of ions in matter.

Since the first half of the 20th century when the first biological experiments with alpha particles were analysed, it is known that heavy ions act quantitatively as well - as we now believe - qualitatively differently than X- or gamma-rays. I cannot image anyone seriously interested in radiobiology to think "that heavy ions caused the same amount of damage as the conventional X-ray or gamma-ray radiation".

How can "severe damage" be engendered by a proton with an energy of 0.25 eV, i.e., an energy insufficient to brake many important (bio-) chemical bonds?

Desorption experiments from thin layers of (bio-)molecules irradiated with low to medium energy heavy ions were performed since the early 1970s. The measurement of (Maxwell) velocity distributions of ejected atoms/molecules lead to the realization that in the particle track core of ionised atoms very high temperatures exist. In addition "coulomb-explosions" within the ionized track core were postulated to contribute to these energies in desorbed atoms/molecules. Calculations for such "thermophysical mechanisms", i.e, of pressure and temperature shock-waves from such heavy ions tracks were also performed in the 1970s already and later, e.g., 

Sun Y Y, Nath R, Pressure wave generated by the passage of a heavy charged particle in water. Med. Phy. 20#3(1993)633-638.

Whether the confusion is already present in the original work which is referred here or whether it stems from the limited understanding of the author I cannot tell. Anyhow, it reminds me of the "wheel of reincarnation" of many (scientific) discoveries.

Kind regards, Rainer

Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Linder Hoehe
51147 Koeln
GERMANY
Voice: +49 2203 601 3147 or 3150
FAX:   +49 2203 61970

-----Ursprüngliche Nachricht-----
Von: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] Im Auftrag von John Jacobus
Gesendet: Montag, 17. Oktober 2005 02:32
An: radsafe; know_nukes at yahoogroups.com
Betreff: [ RadSafe ] Article: Heavy ions damage DNA

This appeared in PhysicsWeb at
http://physicsweb.org/articles/news/9/10/8
You may have to subscribe to see this article, but the ciation is given at the end of the first paragraph.

Question: Does anyone have an idea if these effects compare to the decay alpha particles of radon progeny?

Note there is a related article "How particles can be therapeutic" from August 2003 at
http://physicsweb.org/articles/world/16/8/9

------------------

Heavy ions damage DNA
14 October 2005

Medical physicists in Canada have discovered that heavy-ion-beam cancer therapy can cause more damage to healthy DNA than previously believed. The harm is caused by low-energy secondary particles rather than the heavy ions themselves. The results could help medical physicists develop more accurate dose models for heavy-ion therapy (Phys. Rev. Lett. 95 153201).

Heavy-ion-beam cancer therapy employs protons or ions such as argon and neon that have energies of about 1 MeV per nucleon. One advantage of heavy-ion therapy over other techniques is that most of the energy is deposited in a small region of space, known as the Bragg peak, whereas X-rays, for example, deposit their energy continuously once they enter the body. However, little is known about how heavy-ion radiation damages DNA on the molecular scale, especially in the region beyond the Bragg peak. This damage might be caused by the heavy ions after they have lost most of their energy or by low-energy secondary ions. This is a worry because the tissue beyond the Bragg peak is often healthy. 

Previously it was thought that heavy ions caused the same amount of damage as the conventional X-ray or gamma-ray radiation routinely used in medicine. These types of radiation cause damage by simple ionisation of atoms in cells, cleavage of single bonds in molecules, and attack by chemical radicals. 

Michael Huels and colleagues at the University of Sherbrooke decided to look into this issue in more detail. They fired low-energy ions onto a film of biomolecules in an ultrahigh vacuum and analysed the ions that desorb from the film with a mass spectrometer. The results show that the initial damage caused by the ions at their track ends is significantly more complex, clustered and lethal than that induced by X- or gamma-rays. Severe damage can be caused by energies as low as 0.25 eV per nucleon -- which is very low when compared with the energy of a typical heavy-ion beam. 

The new work was prompted by previous experiments by Thomas Schlathölter and colleagues at Gröningen in the Netherlands. In 2003 Schlathölter noticed that low energy (1 to 200 eV) secondary particles could be produced by firing high-energy MeV-range heavy ions at DNA fragments. The latest experiments were made possible by the development of a machine that is capable of producing heavy ions with energies as low as just 1 eV in the Sherbrooke lab. 

The team is now investigating how secondary ions, created by the primary ions inside DNA, can also cause damage although they have even lower energies than primary ions. "Our dream is that some day doctors will be able to manipulate the heavy-particle radiation effects at the molecular level - for example, by developing DNA 'radiosensitisers' that are specific to the secondary particles created in DNA during ion therapy," says Huels.

About the author
Belle Dumé is science writer at PhysicsWeb

+++++++++++++++++++
On Oct. 5, 1947, in the first televised White House address, President Truman asked Americans to refrain from eating meat on Tuesdays and poultry on Thursdays to help stockpile grain for starving people in Europe. 

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com

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