MICRODOSIMETRIC MEASUREMENTS IN THE BUILDUP REGION OF VERY PURE HIGH-ENERGY PHOTON AND ELECTRON-BEAMS

A microdosimetric system, specially designed to allow measurements in both the presence and the absence of secondary charged particle equilibrium, has been used to investigate the build-up region of a clean 15 MV bremsstrahlung beam and a 21 MeV almost monoenergetic electron beam. The use of quasi-broad-beam conditions with a split tissue-equivalent phantom and a wall-less chamber surrounded by a low-pressure tissue-equivalent gas minimizes the fluence perturbation caused by the detector. Along with microdosimetric beam quality data, ordinary dosimetric information can be obtained, together allowing estimation of the properties of the beam with regard to radiation response in tissue. By applying an inversion algorithm on the measured microdosimetric distributions to correct for plural events, it is shown that already at a probability of coinciding pulses as low as 1% the dose mean value of the single-event distributions increases by 3%. The dose mean lineal energy for the pure bremsstrahlung beam is strongly affected by both photon contamination and in-phantom scattering, thus being 55% higher close to the surface than near the dose maximum. The corresponding values for the electron beam increase slowly with depth in the build-up region due to the influence of multiple scattering and production of secondary electrons. Microdosimetric distributions in tissue are obtained from the distributions measured in gas by applying a correction for the density effect. The dose mean value of the lineal energy in a tissue-equivalent material is 2-3% lower than the corresponding value in tissue-equivalent gas. Multiple-event distributions in tissue, calculated for the bremsstrahlung beam at doses common in daily fractionated treatments, show that the biological response at very shallow depths is mainly affected by the microdosimetric variance which is much larger than both the macrodosimetric and biological variances. For electron beams the influence of the microdosimetric variance is negligible.