Learn about techniques used to accelerate a Monte Carlo particle physics simulator. The strategies discussed include sorting to minimize thread divergence and data structures for efficient memory access. The software, named MPEXS, is primarily focused on X-ray radiotherapy and has been recently extended to cellular and DNA levels. Simulation of DNA ionization is particularly challenging, because large numbers of low energy particles have to be managed. Implementation of these strategies has both improved the run-time performance and reduced the memory usage. The results from the performance analysis are likely to be of use in other domains that rely on discrete event simulation. Extension of physics coverage for proton and carbon therapy and neutron radiation protection is envisioned.