Tracks to therapy

Studies of the structure of particle tracks have led to models of track eff ects based on radial dose and radiobiological target theory that have been very successful in describing and predicting track effects in physical, che mical, and biological systems. For describing mammalian cellular inactivati on two inactivation modes are required, called gamma-kill and ion-kill, the first due to synergistic effects of delta rays from adjacent ion paths thu s resembling the effects from gamma rays, and the second to the effects of single ion transits through a cell nucleus. The ion-kill effect is more sev ere, where the fraction of cells experiencing ion kill is responsible for a decrease in the oxygen enhancement ratio, and an increase in relative biol ogical effectiveness, but these are accompanied by loss of repair, hence to a reduction in the efficiency of fractionation in high LET therapy, as sho wn by our calculations for radiobiological effects in the "spread out Bragg Peak".