Evaluation of Relative Biological Effectiveness for Proton Therapy
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Proton therapy for treating cancer patients has been evolved to become a highly desired choice of radiation therapy due to the physical characteristic of protons. Currently, the biologic effectiveness of protons relative to photons is considered to be a constant ratio of 1.1. However, experiments show that the RBE is higher in different portions in proton ranges. Since the usage of a constant RBE is justified based on experiments conducted using older methods of dose delivery, the re-evaluation of using a constant RBE and proposing models for determining RBE values is necessary now that recent experimental results, using new actively scanned beam delivery method, are available and suggest deviations from conventional data. To suggest a method for calculating RBE values, an experiment by Guan et al. is chosen to derive biologic response results with respect to proton beams. A widely used phenomenological RBE model by Wilkens and Oeflke is chosen for comparison purposes. The proposed RBE model is based on Wilkens and Oeflke model with revisions on tissue parameters behavior vs. LET based on Guan et al results. Three RBE models including the constant RBE, Wilkens' model and the suggested model based on Guan’s experiment are compared for three brain cancer patient cases. The comparisons are demonstrated using cumulative RBE weighted dose volume histograms. The sensitivity of models to tissue parameter changes are also analyzed. The suggested model shows escalated variable RBE weighted dose compared to constant RBE weighted dose, and it is sensitive to RBE model parameters in all three patient studies.