Over the last few decades, particle physicists have developed all kinds of useful simulation tools to help them predict the behavior of subatomic particles interacting with matter, with valuable applications in medical and space science to boot.

Now Joseph Perl, a physicist with the Stanford Linear Accelerator Center (SLAC) is adapting one of those simulation tools to help make cancer therapy that much safer.

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Geant4 was designed to track subatomic particles in high-energy accelerators, but it’s also ideal for mapping out the paths protons take through the body during proton therapy treatments.

The “father of proton therapy” is Robert R. Wilson, the physicist who built Fermilab. He’d become interested in researching the effects of radiation damage on the human body as a result of his Los Alamos experiences on the Manhattan Project — especially the deaths of two fellow physicists from radiation sickness after accidents with the fickle plutonium cores, which affected him greatly.

X-rays are a form of ionizing radiation: they are of sufficient energy to damage and destroy cells by damaging DNA and other genetic material, thereby interrupting vital cell functions. X-rays can target cancer cells and prevent the out-of-control growth that invariably leads to death when untreated.

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Proton therapy works on similar principles. Wilson first suggested in 1946 that the energetic protons produced at the Harvard Cyclotron Laboratory might be an effective cancer treatment. The very first treatments were performed at particle accelerators originally built for physics research: Berkeley Radiation Laboratory in 1954, and Uppsala in Sweden in 1951.

So proton therapy has been around for about 50 years, generally reserved for the most complicated cancers, such as tumors in the head, eyes, or neck that have not yet spread to distant areas of the body — locations where collateral damage to surrounding tissue could have serious consequences.

That’s because proton therapy offers fewer side effects. In conventional x-ray therapy, the x-rays travel through the body and deliver radiation to all the tissues along the way to the actual tumor. To cut down on the damage to healthy tissue, doctors usually limit the dose delivered to the tumor.

But proton beams deposit almost all their energy to the tumor itself, with very little damage to surrounding tissue, so the side effects are minimal. It’s also easy to tweak the energy levels of the proton beams to target a tumor at a specific depth, while protecting tissue that is both shallower and deeper.


That’s not to say there isn’t room for improvement, and this is where particle physics tools can help. “To perfect this stuff, what we have to understand really is where are the particles going?” Perl told Symmetry Breaking. “We have to understand particle transport when we’re designing the medical linacs . We have to understand particle transport when we’re talking about how the beams actually penetrate the body.” To wit:

Geant 4 relies on a technique called Monte-Carlo simulation, which models each proton moving through the body in a series of random steps. At each step, the program essentially casts a die to guess where the particle will move next. Over many steps, the program shows where protons are most likely to end up.

Perl describes the simulation tool as “a really fancy techno-Lego kit.” But it’s not useful to proton therapy applications right out of the box. So Perl is collaborating with Massachusetts General Hospital and the University of California, San Francisco, to develop custom simulations — and to make the tool more user friendly. After all, he concludes, “If we can make it easier for people to use, the more likely they are to use things right.”

Illustration by Steven Hawkins, via Lawrence Livermore National Laboratory. (bottom) Israel Proton Therapy Initiative (creative commons)