At SLAC (Stanford University’s National Accelerator Laboratory), physicists are making cancer therapies safer by using “Geant 4” software — originally designed for tracking subatomic particles — to map proton paths through patients’ bodies during radiation treatment.

In conventional 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; however, a some healthy cells are also susceptible to damage.

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.

Geant 4 uses 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.

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. Such obstructions can block some radiation from the intended target, while simultaneously allowing some tissue to soak up unnecessary radiation. Geant4 can potentially help medical physicists program beams to track a moving target and deliver a constant dosage to the tumor.

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 to help medical physicists customize their simulations.