Microdosimetric measurements in the build-up region of very pure high-energy photon and electron beams

Abstract

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.