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

Area of Science:

• Medical Physics
• Radiation Oncology
• Biophysics

Background:

• Current proton therapy treatment planning systems (TPS) utilize a constant relative biological effectiveness (RBE), neglecting its dependence on linear energy transfer (LET).
• Experimental data indicate that proton RBE varies with physical and biological parameters, necessitating more accurate models for clinical evaluation.

Purpose of the Study:

• To compare the accuracy of dosimetric parameters derived from planning target volume (PTV)-based plans versus robust plans.
• To assess the closeness of TPS-derived values (Dose_TPS) to true values (Dose_variable RBE) using variable RBE models.

Main Methods:

• Intensity-modulated proton therapy (IMPT) plans were generated for phantom and nasopharyngeal cases using PTV-based and robust optimizations.
• Physical dose and dose-averaged LET (LETd) distributions were calculated using an analytical pencil beam algorithm.
• Deviations between Dose_TPS and Dose_variable RBE were evaluated using D99 and Dmax, considering organ at risk (OAR) proximity to the clinical target volume (CTV).

Main Results:

• Robust plans showed smaller deviations between Dose_TPS and Dose_variable RBE for OARs compared to PTV-based plans across all RBE models.
• Deviations increased as OARs were closer to the CTV for both optimization methods.
• For CTVs, deviations were comparable between PTV-based and robust plans, irrespective of OAR proximity.

Conclusions:

• Robust optimization is superior to PTV-based optimization for proton therapy.
• Robust plans result in TPS-derived dosimetric parameters closer to true values derived from variable RBE models.
• Clinical evaluations based on TPS are more reliable with robust optimization, especially considering variable RBE.