Difference in LET‐based biological doses between IMPT optimization techniques: Robust and PTV‐based optimizations

Abstract

Purpose: While 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 (D RBE¼1:1 ) are close to the "true" values derived with the variable RBE models (D vRBE ). As such, in this study, the closeness of D RBE¼1:1 to D vRBE was compared between planning target volume (PTV)-based and robust plans.Methods: Intensity-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 (LET d ) distributions were obtained using the analytical calculation method, based on the pencil beam algorithm. Next, D vRBE was calculated using three different RBE models. The deviation of D vRBE from D RBE¼1:1 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.Results: As for the OAR, the deviations of D vRBE max from D RBE¼1:1 max 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 D vRBE 99 from D RBE¼1:1 99 were comparable between the two optimization techniques, regardless of the distance between the CTV and the OAR.---

Incorporating variable RBE in IMPT optimization for ependymoma

“…One limitation of the present study is that robust optimization was not incorporated in the tested planning approaches. It has been suggested that robust optimization (against physical uncertainties) might reduce the impact of variable RBE 50,51 . Variable RBE optimization is particularly suitable for existing robust optimization methods developed for IMPT planning.…”

“…It has been suggested that robust optimization (against physical uncertainties) might reduce the impact of variable RBE. 50 , 51 Variable RBE optimization is particularly suitable for existing robust optimization methods developed for IMPT planning. One only needs to replace constant RBE with variable RBE in the optimization criteria.…”

A Critical Review of LET-Based Intensity-Modulated Proton Therapy Plan Evaluation and Optimization for Head and Neck Cancer Management

“…Multiple studies have been carried out to evaluate the dosimetric effects of different RBE models in passive scattering proton therapy [ 80 ], and IMPT plans for numerous tumor sites including H&N [ 81 – 85 ], thorax [ 84 , 86 ], liver [ 81 ], prostate [ 84 , 87 , 88 ], and craniospinal irradiations [ 89 ]. Tilly et al [ 68 ] first reported the influence on the dose volume histograms after RBE correction for a patient with hypopharynx cancer.…”

“…Both studies indicated increased RBE doses at nearby OARs, compared to that using fixed RBE of 1.1. In addition, compared to planning target volume (PTV)–based optimization, robust optimization plans showed that dose distributions calculated with fixed-RBE of 1.1 were closer to the biological doses calculated with variant RBE models [ 85 ]. All these studies revealed dosimetric uncertainties of using different RBE models, as well as possible interpatient variations when applying RBE models.…”