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 equili brium, 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-equivalen t phantom and a wall-less chamber surrounded by a low-pressure tissue- equivalent gas minimizes the fluence perturbation caused by the detect or. Along with microdosimetric beam quality data, ordinary dosimetric information can be obtained, together allowing estimation of the prope rties of the beam with regard to radiation response in tissue. By appl ying an inversion algorithm on the measured microdosimetric distributi ons to correct for plural events, it is shown that already at a probab ility of coinciding pulses as low as 1% the dose mean value of the sin gle-event distributions increases by 3%. The dose mean lineal energy f or the pure bremsstrahlung beam is strongly affected by both photon co ntamination and in-phantom scattering, thus being 55% higher close to the surface than near the dose maximum. The corresponding values for t he electron beam increase slowly with depth in the build-up region due to the influence of multiple scattering and production of secondary e lectrons. Microdosimetric distributions in tissue are obtained from th e distributions measured in gas by applying a correction for the densi ty effect. The dose mean value of the lineal energy in a tissue-equiva lent material is 2-3% lower than the corresponding value in tissue-equ ivalent gas. Multiple-event distributions in tissue, calculated for th e bremsstrahlung beam at doses common in daily fractionated treatments , show that the biological response at very shallow depths is mainly a ffected 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.