INFLUENCE OF RADIATION QUALITY CHANGES ON MICRODOSIMETRIC VARIANCE AND DOSE-RESPONSE RELATIONS

Dose level and radiation quality variations in therapeutic low LET bea ms combined with departures from charge particle equilibrium affects t he microdosimetric variance and consequently the normalised gradient o f the dose-response relation. These effects were analysed on microdosi metric single event distributions measured in the build-up region and close to interfaces of high Z backscatter materials in therapeutic Co- 60 and 15 MV beams free of contaminating electrons and in 21 MeV almos t monoenergetic electron beams. The normalised dose-response gradient decreases towards the surface in gamma ray and bremsstrahlung beams du e to the increasing relative variance while for the electron beams it is rather constant since the decrease in the absorbed dose is compensa ted by the increase z(D)BAR. In therapeutic photon beams the biologica l response is mainly affected by the quality at a specific depth as is shown from the survival of M-5 cells irradiated in a Co-60 beam at th e surface and at the dose maximum. The large microdosimetric variance in heavy ion beams decreases the normalised dose-response gradient eve n more and reduces the therapeutic window between tumour control and n ormal tissue damage.