Contribution of Nuclear Fragmentation to Dose and Relative Biological Effectiveness (RBE) Across Common RBE Models in Carbon Radiotherapy
S Hartzell*, F Guan, P Taylor, C Peterson, R Howell, S Kry, The University of Texas, MD Anderson Cancer Center, Houston, TX
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WE-IePD-TRACK 5-5 (Wednesday, 7/28/2021) 12:30 PM - 1:00 PM [Eastern Time (GMT-4)]
Purpose: Variable RBE in carbon radiotherapy may be calculated using several models, including the microdosimetric kinetic model (MKM), stochastic MKM (SMKM), and Local Effect Model I (LEM), which have not been thoroughly compared. This work compares how models handle the nuclear fragmentation of carbon beams to provide insight to where model differences arise.
Methods: Geant4 Monte Carlo was used to simulate various monoenergetic and SOBP carbon beams incident on a water phantom. From these, input parameters for each RBE model (microdosimetric spectra, double strand break yield, kinetic energy spectra, dose fragment contributions) were calculated for each contributing fragment of the carbon beam (namely H, He, Li, Be, B, and secondary C). The spectra for each fragment was used to calculate the linear (α) and quadratic (β) portions of each RBE model, which were combined with α and β values of reference radiation and physical dose to calculate RBE. The contribution of RBE by each fragment was then compared across models.
Results: Calculations found that secondary fragment contributions could exceed 20% of total physical dose. When calculated using identical beam parameters, RBE varied greatly in magnitude across models and was typically lowest using MKM. When compared across fragments, RBE typically decreased with atomic number when Z˂3 and increased when Z≥3 for RBE(MKM) and RBE(SMKM). Using LEM, RBE increased with Z until Z=6, where RBE dropped sharply. Trends of RBE by fragment varied by region (entrance vs. SOBP) for microdosimetric models only.
Conclusion: This study demonstrated that secondary fragments could contribute notably to physical dose, indicating that fragmentation is an important factor in treatment delivery. Similar trends were seen in fluctuations of RBE by atomic number for microdosimetric models, which differed from those of RBE(LEM). Further insight into model differences may drive future cell studies and increase consistency in model use.
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