• Number 350  |
• November 14, 2011

SLAC develops physics simulation software to make cancer therapy safer

Tiny particles are making a big difference in the world of cancer therapy. And physicists at DOE's SLAC National Accelerator Facility—experts in particle transport—are using computer simulations to make those therapies safer.

Software developer Joseph Perl and his colleagues are turning the simulation toolkit Geant4 into a powerful application for medical physicists. Originally designed to track subatomic particles in high-energy physics experiments, Geant4 can also map proton paths through patients' bodies during radiation treatment.

In radiation treatment, subatomic particles inflict DNA damage on dividing cells—both healthy and cancerous—causing them to commit suicide. The technique works because rapidly growing cancer cells are more likely to be dividing at any given time, and thus are more likely to be killed; but a smaller proportion of healthy cells are also susceptible to damage.

Minimizing collateral damage is a tough problem for medical physicists who design radiation treatments.

“To perfect this stuff, what we have to understand really is where are the particles going?” Perl said. “We have to understand particle transport when we're designing the medical linacs,” the accelerators that deliver the particles to patients.

In contrast to the X-ray beams used in traditional therapies, which go all the way through the body, proton beams dump their energy at a specific depth. Medical physicists can target a tumor at one depth and avoid deeper and shallower tissues by tweaking the energy levels of one or more beams. Proton beam therapy may be particularly useful in children, for whom stray radiation can stunt growth and cause secondary cancers in adulthood.

Unlike many other tools, Geant4 can also simulate the effect of tissues, such as the rib bones, that may move in and out of the proton beam as a patient breathes. This capability helps medical physicists program beams to track a moving target and deliver a constant dosage to the tumor.

Geant4 is freely available to anyone who wants to use it, but in its current form may be challenging to some novices. To address this problem, SLAC’s Geant4 team has joined Massachusetts General Hospital and the University of California-San Francisco, in a four-year collaboration funded by the National Institutes of Health. The project, headed by Perl, will help medical physicists customize their simulations without disrupting the program’s innermost workings. “If we can make it easier for people to use,” Perl said, “the more likely they are to use things right.”

[Helen Shen, 650.926.8797,
hshen@slac.stanford.edu]