TH‐F‐105‐05: Comparison of Results for RBE‐Weighted Dose From Two RBE Models for Proton Therapy Treatment Plans

Purpose: To evaluate the differences between relative biological effectiveness (RBE)‐weighted doses calculated with either the linear quadratic (LQ) or repair‐misrepair‐fixation (RMF) RBE model for intensity modulated proton therapy (IMPT) treatment plans. Methods: The commercial treatment planning system (TPS) used at MD Anderson Cancer Center (MDACC) does not provide the LET information necessary in order to utilize variable RBE models to calculate RBE‐weighted dose for treatment plans. To obtain this information the Monte Carlo code MCNPX was used to recalculate IMPT treatment plans for patients with central nervous system (CNS) disease taken from the TPS and record the track‐averaged LET in each voxel of the dose distribution. The dose‐averaged LET was approximated from the track‐averaged LET by use of an appropriate scaling factor. The Monte Carlo Damage Simulation (MCDS) software was used to parameterize the number of double‐strand breaks generated in cells by protons according to their stopping power. This information, along with relevant biological coefficients, was then used to generate RBE‐weighted dose distributions according to the LQ and RMF RBE models. Plan evaluation metrics, such as a conformality index and heterogeneity index, were used for plan comparison. Results: The LQ RBE model produces higher RBE‐weighted doses, particularly in the target volume, compared to the RMF model. Doses predicted by the RMF model in nearby organs at risk can at times be several Gray lower than those predicted by the LQ model. Both models predict equivalently heterogeneous target dose distributions, while the RMF model predicts more conformal target dose. Conclusion: Calculation of proton treatment plan dose incorporating either the LQ or RMF models of variable RBE produces observable differences in the resultant dose distributions. Further investigation, including utilization of different biological coefficients, is required to determine the implications for all of the associated normal tissues. UTMDACC Sister Institution Network Fund Grant (SINF MDACC‐DKFZ)