ESR dosimetry of human cortical bone irradiated by a therapeutic proton beam

Dosimetry based on the electron spin resonance (ESR) technique was used to study human cortical bone irradiated by a therapeutic proton beam. Bone samples were irradiated with 160-MeV protons and, for comparison, 6-MV photons, Additional samples were held aside as controls. The ESR peak-to-peak heights (PPH) for various doses were studied. A Bragg curve was generated by irradiating bone samples at various depths and measuring the ESR response. A companion Bragg curve was also generated via parallel plate ion chamber (PPIC) measurements. The proton-to-photon dose-response was 0.806±0.034 (+1 s.d.) in the Bragg plateau. This ratio is lower than previously reported and is likely attributable to an intermediate-length (year,) decay component. In the Bragg curve analysis, an iterative technique for sub-millimeter sample depth-correction was employed. The Bragg peak position and height of the ESR data differed from the PPIC data. The ESR Bragg peak shift was -0.6 mm. The ratio of the ESR-to-PPIC Bragg peak height was 0.804±0.029 (+1 s.d.), for a normalization in the Bragg plateau region. In conclusion, the difference in the peak position and the peak value of the Bragg curve may, in part, be from detector size effects, as noted in the very interesting work of Bichsel (1995). A direct comparison, however, can not be made because of differences in the two studies. The role of differential proton stopping-power effects in the bone response is discussed