ESR dosimetry study of human cortical bone irradiated by proton therapeutic beams: Monte Carlo modeling of experimental results

The purpose of this work was to improve the accuracy and precision of electron spin resonance (ESR) bone dosimetry as applied to therapeutic proton beams, using Monte Carlo (MC) tools and experimental work. The radiation dose produced in bone was measured by ESR spectroscopy, which was done in previous work. Methods. The irradiation was performed with 160 MeV proton beams at the Harvard Cyclotron Laboratory. Monte Carlo tools were used to investigate the ESR bone response in comparison with parallel plate ionization chamber measurements. The PTRAN MC code was used to estimate the stopping power ratio of cortical bone and water, rectify the beam divergence/geometry, and estimate the detector size effect, the iterative depth correction, and the stopping power correction. The GEANT4 MC code was implemented to determine inelastic nuclear interactions between cortical bone and water, and the detector size effect in water. Results. It was determined from GEANT4 simulations that the relative probabilities of inelastic nuclear interactions in bone and water are similar. In the detector size effect, between 0 mm and 2.38 mm (mean water equivalent bone sample size) of detector thickness, there were peak-plateau ratio changes without major peak shifts. For the iterative depth correction (sample position), the maximum change was 0.12 mm in the Bragg peak region. The ratios of the slopes for proton (at 107.7 mm) and photon (6 MV) dose-responses from previous work was 0.850 ± 0.046 (±1 s.d.), and the ratio with the revised stopping power correction we 0.895 ± 0.052 (±1 s.d.). There was a stopping power dependent loss of ESR signal bone. A correction curve was determined for the ESR response as a function of the proton stopping-power in bone. To normalize these data to a photon response, the ordinate values should be divided by 0.895. In this work, bone has been characterized as an accurate solid-state dosimeter for proton dosimetry, and a methodology has been presented to characterize other solid state detectors for proton dosimetry.