An estimation of the relative biological effectiveness of 50 MV bremsstrahlung beams by microdosimetric techniques.

An efficient algorithm has been developed to estimate the relative biological effectiveness (RBE) from experimental RBE data and measured single-event energy deposition spectra. As benchmarks for the calculations well established RBE data for crypt cell survival and associated microdosimetric distributions from a number of therapeutic high- and low-LET beams was used. As a by-product of the calculations the RBE for this system was determined for the lineal energy range 0.1-5000 keV microns-1 in very good agreement with experimental data. These data have been applied to estimate the biological effectiveness of therapeutic high-energy bremsstrahlung beams characterized by microdosimetric measurements with a wall-less proportional counter. The absorbed dose component due to photoneutrons and charged particles from photonuclear reactions in scanned 50 MV bremsstrahlung beams was measured to be about 2% of the total absorbed dose. The RBE of 50 MV scanned bremsstrahlung beams was estimated to be 1.09 for jejunum crypt cell survival as the biological endpoint at absorbed doses of the order of 10 Gy, in fair agreement with reported results based on radiobiological experiments. The RBE is fairly independent of the bremsstrahlung target used. Using the estimated increase in the RBE of neutrons compared to photons for absorbed dose levels applied in clinical fractionation schedules increases the RBE further to 1.13 at absorbed doses per fraction of the order of 2 Gy.