Physicists Join Fight Against Cancer, By Understanding How Best To Use Radiation To Kill Tumor Cells

Radiation is thought to kill tumor cells by damaging their DNA. In the form of a double helix with two strands, DNA is the part of the cell that carries the genetic code. Radiation may break either one or both of the DNA strands. If only one strand is broken, the cell can often repair the damage. If both strands are broken, the repair is more difficult and the cell is more likely to die.

Reading and graduate students Jun Fu, Mathew Fitzpatrick and Bill Smith, have been working for seven years on the mechanisms that occur when a particular type of radiation, made of a beam of ions, hits the living target cells.

"Once you understand the processes," says Reading, "you can act in a more informed manner, devise a more effective radiation, and therefore hopefully reduce the peripheral damage to healthy cells caused by radiation."

Working in collaboration with a group of physicists led by Annie Chetioui, professor of physics at the University of Pierre and Marie Curie of Paris in France, the Texas A&M physicists are developing computer programs to describe in detail the effects of beams of ions on tumors.

A focus of Chetioui's work is to explain the existence of an optimal ion speed associated with the highest killing rates. "As you slow the speed of the incoming ions, they have much more time to interact with the atoms on the DNA strand, so the killing rate increases" says Reading. "But past a critical point, even though the energy deposited still increases, the killing stops."

Chetioui theorizes that a special atomic reaction called an Auger process is the key to understanding this. As a projectile hits one DNA strand, it may cause an explosion of energetic electrons that scythe through the second DNA strand at an adjacent site causing the double break.

Further experiments conducted in France have led to some support for this mechanism but many free electrons with a wide range of speeds are always produced by the impact, not just the Auger electrons. All of these electrons eventually deposit their energy in the target and may also damage the cell. To show that fast Auger electrons are responsible for cell death, scientists need to accurately determine the distribution of electron speeds.

The Texas A&M group is now trying to provide these details. Graduate students Smith and Fitzpatrick have written Ph.D. theses that predict the onset of the Auger process. Fu's master's thesis builds on this work to determine the electron speeds. If the projectile is a fast proton and the target atom is hydrogen, the speeds can be measured. These experimental results are in good agreement with Fu's work. He will now go on to study DNA carbon and oxygen atoms.

"We successfully calculated the simplest case: the proton-hydrogen reaction with one ejected electron," Fu says. "We are now extending the calculation to many-electron cases, and we would need to consider reactions between a proton and carbon and oxygen atoms as well."

Although work is still ongoing, the Texas A&M physicists are well on their way to developing a tool that can help sort out precisely what happens when living cells are impacted by ion beams.

"If Chetioui is right, then by choosing the right beam energy, you could greatly enhance the effectiveness of radiotherapy," says Reading. "You could deliver much smaller doses directly to the cancer, cutting down on peripheral damage to healthy cells."