A Clinical Dose Prescription System for Proton Radiotherapy Based on Microdosimetric Spectral Information
Abstract
Purpose
Clinical proton dose prescription uses a fixed relative biological effectiveness (RBE) factor of 1.1 despite preclinical and clinical evidence that RBE varies with dose, radiation quality and tissue radiosensitivity, particularly near the distal edge of treatments. We present a proton clinical dose-prescription system that: 1) is consistent with the clinical RBE = 1.1 for target dose definition, 2) uses full microdosimetric spectral distributions including primary and secondary ions, and 3) accounts for RBE variations in a framework compatible with clinical practice in carbon ion radiotherapy.
Methods
Monte Carlo simulations were performed using OpenTOPAS. The clinical dose was defined as the product of the absorbed dose, a clinical scaling factor, and the biological RBE calculated relative to the center of a reference Spread-Out Bragg Peak (SOBP). The clinical dose system was evaluated across various dose levels, target depths, and SOBP widths. The results were compared with calculations based on the fixed RBE factor and established approaches based on dose-averaged linear energy transfer (LET) of protons only (Beltran and McNamara models).
Results
Across the investigated scenarios, the target-averaged RBE ranged from 1.08 to 1.15. The corresponding target-average RBE values obtained with the McNamara and the Beltran model were typically 2% lower and 10% higher, respectively. For all models, substantially higher RBE values were observed at the distal edge and in the out-of-field penumbra.
Conclusion
This work establishes, for the first time, a proton clinical dose system that harmonizes proton and carbon ion dose prescription methodologies while maintaining consistency with historical proton practice based on a fixed RBE of 1.1. Unlike approaches relying on proton dose-averaged LET alone, this system accounts for the spatial variations of the radiation quality using microdosimetric distributions that include all primary and secondary ions. The resulting formalism enables a unified framework for multi-ion clinical implementation.