Influence of Radiation Quality Changes on Microdosimetric Variance and Dose-Response Relations

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Abstract
Dose level and radiation quality variations in therapeutic low LET beams combined with departures from charge particle equilibrium affects the microdosimetric variance and consequently the normalised gradient of the dose-response relations. These effects were analysed on microdosimetric single event distributions measured in the build-up region and close to interfaces of high Z backscatter materials in therapeutic 60Co and 15 MV beams free of contaminated electrons and in 21 MeV almost monoenergetic electron beams. The normalised dose-response gradient decreases towards the surface in gamma ray and bremsstrahlung beams due to the increasing relative variance while for the electron beams it is rather constant since the decrease in the absorbed dose is compensated by the increase in zD. In therapeutic photon beams the biological response is mainly affected by the quality at a specific depth as is shown form the survival of M-5 cells irradiated in a 60Co beam at the surface and at the dose maximum. The large microdosimetric variance in heavy ion beams decreases the normalised dose-response gradient even more and reduces the therapeutic window between tumour control and normal tissue damage.

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