Influence of x-ray beam quality on retrospective bone dose estimation in a murine model: experimental investigation of EPR signal kinetics and computational modeling.

In the context of radiological accidents, retrospective dosimetry methods such as electron paramagnetic resonance (EPR) spectroscopy and Monte Carlo simulation are used for dose reconstruction. EPR analysis of bone biopsies allows experimental assessment of absorbed dose. However, several factors may affect EPR dose estimation. The time delay between irradiation and sampling can lead to underestimation of absorbed dose to bone, and the influence of beam quality needs investigation. Therefore, this study aims to evaluate the influence of X-ray beam quality on bone EPR signal kinetics and the absorbed dose distribution in bone.
Approach: A murine model of localized hind limb irradiation was developed using three beam qualities (50 kVp, 220 kVp, 10 MV). EPR measurements were performed to characterize radio-induced free radical kinetics in bone collected at different time points, from irradiation day to six months post-irradiation. A computational model of the irradiation setup was implemented in GEANT4 to study the dose distribution and complement the experimental observations.
Main results: High-energy X-rays (10 MV) led to a rapid decrease in EPR signal, with 75% signal loss after 21 days and no detectable signal under our conditions after 42 days. For low-energy X-rays, the signal decreased more rapidly at 220 kVp than at 50 kVp, with signal loss of 51% and 31% respectively at 21 days, and a residual EPR signal remains at later time points (70-80% signal loss at six months). Monte Carlo simulations highlighted heterogeneous dose deposition between tissues with low-energy X-rays, as well as significant beam attenuation within mouse tibia for 50 kVp X-rays, leading to inhomogeneous dose deposition in bone.
Significance: Using a preclinical model, we have demonstrated that X-ray beam quality and time affect bone dose estimation. These findings improve our understanding of how radiation quality and bone physiology affect outcomes over time after irradiation.