Abstract
Cranio-spinal irradiation (CSI) using protons has dosimetric advantages compared to photons and is expected to reduce risk of adverse effects. The proton relative biological effectiveness (RBE) varies with linear energy transfer (LET), tissue type and dose, but a variable RBE has not replaced the constant RBE of 1.1 in clinical treatment planning. We examined inter-patient variations in RBE for ten proton CSI patients. Variable RBE models were used to obtain RBE and RBE-weighted doses. RBE was quantified in terms of dose weighted organ-mean RBE ({overline{{rm{RBE}}}}_{{rm{d}}} = mean RBE-weighted dose/mean physical dose) and effective RBE of the near maximum dose (D2%), i.e. RBED2% = {D}_{2 % ,RBE}/{D}_{2 % ,phys}, where subscripts RBE and phys indicate that the D2% is calculated based on an RBE model and the physical dose, respectively. Compared to the median {overline{{rm{RBE}}}}_{{rm{d}}} of the patient population, differences up to 15% were observed for the individual {overline{{rm{RBE}}}}_{{rm{d}}} values found for the thyroid, while more modest variations were seen for the heart (6%), lungs (2%) and brainstem (<1%). Large inter-patient variation in RBE could be correlated to large spread in LET and dose for these organs at risk (OARs). For OARs with small inter-patient variations, the results show that applying a population based RBE in treatment planning may be a step forward compared to using RBE of 1.1. OARs with large inter-patient RBE variations should ideally be selected for patient-specific biological or RBE robustness analysis if the physical doses are close to known dose thresholds.
Highlights
Cranio-spinal irradiation (CSI) using protons has dosimetric advantages compared to photons and is expected to reduce risk of adverse effects
Due to the spatial variations in relative biological effectiveness (RBE) within the patient, the biologically effective dose distribution received during proton CSI may differ significantly from dose estimates based on the constant RBE of 1.1, especially for organs at risk (OARs) located distal to the target volume, where the linear energy transfer (LET), and RBE, typically is high[12]
Dose volume histograms (DVHs) using RBE1.1 and LET volume histograms (LVHs) for the OARs are shown in Fig. 2, illustrating the degree of inter-patient variations for different OARs in physical dose and LETd which both affects the RBE
Summary
Cranio-spinal irradiation (CSI) using protons has dosimetric advantages compared to photons and is expected to reduce risk of adverse effects. The proton relative biological effectiveness (RBE) varies with linear energy transfer (LET), tissue type and dose, but a variable RBE has not replaced the constant RBE of 1.1 in clinical treatment planning. Contemporary clinical proton treatment planning is based on a constant RBE of 1.1 (RBE1.1), the proton RBE is known to vary with linear energy transfer (LET), tissue type and physical dose[9,10,11]. Due to the spatial variations in RBE within the patient, the biologically effective dose distribution received during proton CSI may differ significantly from dose estimates based on the constant RBE of 1.1, especially for organs at risk (OARs) located distal to the target volume, where the LET, and RBE, typically is high[12]. The same study emphasizes the risks from elevated RBE in brain tissues
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