  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.
