Figure 4.

Double Strand Break (DSB) distributions and resulting misrepair rates. By converting the energy distributions from Fig. 3 into DSB distributions, the spatial distribution of damage around an ‘average’ DSB can be calculated. This is shown in (a), with a rapid rise of DSB count with range as greater portions of the particle track are encompassed. Panel (b) presents the resulting interaction rate, obtained by scaling the DSB density by $e^{-\frac{r^2}{2{\sigma }^2}}$, showing most intra-track interactions occur for DSBs separated by a few hundred nm. By integrating the interaction rates from (b), the total interaction rate can be obtained for different particle energies. The rate is plotted as a function of either proton energy (c) or LET (d). This rate increases steeply as the particle slows down, and is closely related to LET – although this relationship is slightly super-linear, due to the smaller total track radius at lower energies. The misrepair rate for 2 Gy of uniformly distributed DSBs is also plotted for comparison (dashed line), showing that even at moderate LETs (>5 keV/µm) intra-track effects can be a dominant contribution to misrejoining rates at clinical doses.