Standardizing Monte Carlo simulation parameters for a reproducible dose-averaged linear energy transfer

Dose-averaged linear energy transfer (LET_D) is one of the factors which determines relative biological effectiveness (RBE) for treatment planning in proton therapy. It is usually determined from Monte Carlo (MC) simulation. However, no standard simulation protocols were established for sampling of LET_D. Simulation parameters like maximum step length and range cut will affect secondary electrons production and have an impact on the accuracy of dose distribution and LET_D. We aim to show how different combinations of step length and range cut in GEANT4 will affect the result in sampling of LET_D using different MC scoring methods.

In this work, different step length and range cut value in a clinically relevant voxel geometry were used for comparison. Different LET_D scoring methods were established and the concept of covariance between energy deposition per step and step length is used to explain the differences between them.

We recommend a maximum step length of 0.05 mm and a range cut of 0.01 mm in MC simulation as this yields the most consistent LET_D value across different scoring methods. Different LET_D scoring methods are also compared and variation up to 200% can be observed at the plateau of 80 MeV proton beam. Scoring Method one has one of the lowest percentage differences compared across all simulation parameters.

We have determined a set of maximum step length and range cut parameters to be used for LET_D scoring in a 1 mm voxelized geometry. LET_D scoring method should also be clearly defined and standardized to facilitate cross-institutional studies.

Establishing a standard simulation protocol for sampling LET_D would reduce the discrepancy when comparing data across different centres, and this can improve the calculation for RBE.