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Pilot Study of Multi-Field Optimized (MFO) IMPT Dose Distributions Using Fixed and Variable Relative Biological Effectiveness (RBE) Models for Brain Cancer

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R Kohno

R Kohno*, W Cao , P Yepes , F Poenisch , D Grosshans , R Mohan , UT MD Anderson Cancer Center, Houston, TX


SU-I-GPD-T-104 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate relative clinical potential of brain MFO IMPT dose distributions in terms of fixed and variable RBE.

Methods: A test patient with grade III anaplastic astrocytoma was chosen. The prescribed doses were 57 and 50 Gy(RBE) to the GTV and CTV respectively. Maximum dose constraints for chiasm, optic nerve and the brainstem were 54 Gy(RBE). Multi-field optimized IMPT plans were optimized on an Eclipse TPS employing 3, 4, 5, 6 and 9 fields. Dose distributions, along with the associated LET distributions, for each plan were recomputed using an in-house fast Monte Carlo (FMC) system with the fixed RBE (fRBE) of 1.1 and using the published variable RBE (vRBE) model by Wilkens et al. Comparative analysis of dose distributions was performed.

Results: fRBE_TPS underestimated CTV D95 by 0.9% and 1.2% compared with fRBE_FMC and vRBE_FMC averaged over all plans studied. Homogeneity indices for fRBE_TPS, fRBE_FMC and vRBE_FMC were 1.058, 1.072 and 1.091. Conformity indices for RBE_TPS, fRBE_FMC and vRBE_FMC in the 9-field plan were 1.244, 1.193 and 1.252, and were the best compared with plans with smaller number of fields. Constraints for all normal structures were met.

Conclusion: In this pilot study, we evaluated relative biological effectiveness of dose distributions in a brain MFO IMPT plan using a variable RBE model. IMPT plans for additional patients for this and other sites are being studied.

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