Radiobiological Characterization of Clinical Proton and Carbon-Ion Beams

Electromagnetic radiation (photons) or particle beam (protons or heavy ions) have similar biological effects, i.e. damage to human cell DNA that eventually leads to cell death if not correctly repaired. The biological effects at the level of organs or organisms are explained by a progressive depletion of constitutive cells; below a given threshold, cell division is no longer sufficient to compensate for cell loss, up to a point where the entire organism (or organ) breaks down. The quantitative aspects of the biological effects are modulated by the microscopic distribution of energy deposits along the beam or particle tracks. In particular, the ionization density, i.e. the amount of energy deposited by unit path length (measured in keV/{\mu}m), has an influence on the biological effectiveness, i.e. the amount of damage per energy unit deposited (measured in gray or Gy, equivalent to 1 joule/kg). The ionization density is usually represented by the Linear Energy Transfer or LET, also expressed in keV/{\mu}m. Photon beams (X-rays, g-rays) are low-LET radiation, with a sparsely ionising characteristic. Particle beams have a higher LET, with a more dense distribution of energy deposits along the particle tracks. Protons are intermediary, with a LET larger than the photon one, but still belong to the 'radiobiological' group of low LET. The higher the ionization density, the higher the biological effectiveness per unit of dose.