Radiation Species determine the importance of the classical radiobiological factors. (A) The oxygen enhancement ratio (OER) has an inverse relationship with the linear energy transfer (LET). While the cell killing effect of photons (grey) and protons (brown) are dependent on the oxygen tension, carbon ions (blue) are able to induce the same cell kill effect with a significantly lower degree of dependence on oxygen tension. (B) The attractiveness of protons (orange) and other heavier ion species, such as, neon (red), helium (purple) and carbon (green) is the existence of the Bragg Peak, which allows for minimal damage to the surrounding tissue, while low LET radiation, which does not exhibit this peak can induce greater damage to the surrounding tissue. Carbon ions have become a more popular option as it has the lowest entry RBE of other heavy ion species, and unlike protons, does exhibit fragmentation tails intermediary of the other heavy ion species, however more importantly, at the Bragg Peak has a significantly higher RBE compared to protons. (C) Under normoxic conditions low LET photons hydrolyze water and induce breaks in the phosphodiester bonds of DNA. Subsequently the DNA radicals in the presence of molecular oxygen will be fixed or become permanent. Under hypoxic conditions, however, the DNA radical becomes reduced by sulfahydrl groups and the DNA breaks become repaired. With high LET radiation (carbon) the particle directly acts on the phosphodiester bond of DNA inducing clustered damage which is less amenable to repair.