Charged Particles for Cancer Treatment: The Benefits of Ions over Photons

Photon radiations such as X- and gamma-rays have been used effectively to treat cancer for many years although their use can be limited by damage to normal tissues in the path of the photon beams. Charged particle therapy is a rapidly emerging technique in radiotherapy because the exquisite dose localization of charged particles allows for higher doses to be given to tumor tissue while normal tissues are exposed to lower doses and decreased volumes of normal tissues are irradiated. To date, most particle therapy has employed energetic proton beams where the characteristics of the Bragg curve allow for dose distributions that are as good as or better than even the most advanced X-ray therapy approaches. Heavy ions can have even better dose distributions in the target because of reduced lateral scattering compared with protons. However, the substantial potential clinical advantages of heavier charged particles, such as carbon ions, lie in the additional biological effects of the particles which include greater cell killing effectiveness, decreased radiation resistance of hypoxic cells in tumors and reduced cell cycle dependence of radiation response. These biological advantages depend on many factors such as endpoint, cell or tissue type, dose, dose rate or fractionation, charged particle type and energy, and oxygen concentration. This talk will discuss the unique biological advantages of charged particle therapy and how the advantages can be further exploited by dose scheduling and combinations with molecularly-targeted agents to increase efficacy of particle therapy for cancer. In addition, areas of particular research needs, such as quantification of Relative Biological Effectiveness (RBE) for various tumor types and radiation qualities, role of genetic background of tumor cells, sensitivity of cancer stem cells to particles, and carcinogenic potential of charged particle therapy, will be highlighted.