Abstract
PurposeWhile a large amount of experimental data suggest that the proton relative biological effectiveness (RBE) varies with both physical and biological parameters, current commercial treatment planning systems (TPS) use the constant RBE instead of variable RBE models, neglecting the dependence of RBE on the linear energy transfer (LET). To conduct as accurate a clinical evaluation as possible in this circumstance, it is desirable that the dosimetric parameters derived by TPS (DRBE=1.1) are close to the “true” values derived with the variable RBE models (DvRBE). As such, in this study, the closeness of DRBE=1.1 to DvRBE was compared between planning target volume (PTV)‐based and robust plans.MethodsIntensity‐modulated proton therapy (IMPT) treatment plans for two Radiation Therapy Oncology Group (RTOG) phantom cases and four nasopharyngeal cases were created using the PTV‐based and robust optimizations, under the assumption of a constant RBE of 1.1. First, the physical dose and dose‐averaged LET (LETd) distributions were obtained using the analytical calculation method, based on the pencil beam algorithm. Next, DvRBE was calculated using three different RBE models. The deviation of DvRBE from DRBE=1.1 was evaluated with D 99 and D max, which have been used as the evaluation indices for clinical target volume (CTV) and organs at risk (OARs), respectively. The influence of the distance between the OAR and CTV on the results was also investigated. As a measure of distance, the closest distance and the overlapped volume histogram were used for the RTOG phantom and nasopharyngeal cases, respectively.ResultsAs for the OAR, the deviations of DmaxvRBE from DmaxRBE=1.1 were always smaller in robust plans than in PTV‐based plans in all RBE models. The deviation would tend to increase as the OAR was located closer to the CTV in both optimization techniques. As for the CTV, the deviations of D99vRBE from D99RBE=1.1 were comparable between the two optimization techniques, regardless of the distance between the CTV and the OAR.ConclusionRobust optimization was found to be more favorable than PTV‐based optimization in that the results presented by TPS were closer to the “true” values and that the clinical evaluation based on TPS was more reliable.
Highlights
Most newly built proton therapy centers worldwide are implementing the pencil beam scanning technique because of its distinct advantages of dose conformity to targets and neutron exposure reduction compared to the more conventional passive scattering methods
Robust optimization was found to be more favorable than planning target volume (PTV)‐based optimization in that the results presented by treatment planning systems (TPS) were closer to the “true” values and that the clinical evaluation based on TPS was more reliable
When evaluated with a variable relative biological effectiveness (RBE), the values of D99 decreased from those evaluated with RBE = 1.1, but the size of ΔD99 was similar between optimization techniques in both Radiation Therapy Oncology Group (RTOG) phantoms (OAR radii of 15 and 12 mm)
Summary
Most newly built proton therapy centers worldwide are implementing the pencil beam scanning technique because of its distinct advantages of dose conformity to targets and neutron exposure reduction compared to the more conventional passive scattering methods. The dose distributions for multiple uncertainty scenarios (e.g., setup and range uncertainties) are calculated, and treatment plans are optimized simultaneously with respect to all the scenarios.[2–6]. Both techniques are implemented in commercial treatment planning systems (TPS) and have been used in clinical practice. Extensive preclinical evidence shows that the RBE varies across treatment fields. It depends on linear energy transfer (LET), tissue‐specific parameters (α and β), dose per fraction, and other factors.[7]. As far as the authors’ knowledge holds, no commercial TPS has so far been able to provide any option of utilizing LET during the optimization process, or to compute dose distributions weighted by a variable RBE
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