Effects of Ionization Density and Volume
This chapter describes the effects of ionization density and ionization volume on cell inactivation by the α-and βo-inactivation mechanisms of the linearquadratic (LQ) model. Radiation events of very high ionization densities (for example, charged-particle Bragg-peak radiation and Auger cascades) produce cell killing by mainly single-hit, non-reparable lesions, which are not readily modified by chemical environments. High-energy charged-particle beams designed for cancer treatment produce cell killing by both LQ mechanisms in proportions defined by their average linear energy transfer (LET). Sparsely ionizing (low-LET) radiations produce tumor cell killing by both LQ mechanisms and their single-hit killing correlates with the dose deposited by electron trackends. Since the majority of radiotherapy delivered today is by photon beams from linear accelerators that produce mainly sparse ionizations, tumor cell killing and tumor treatment response will result from both α-and βo-inactivation mechanisms and in a proportion that can be predicted from the size of the dose fraction and the intrinsic radiosensitivity of the clonogens of each specific tumor. For proton and carbon-ion radiations, their superior dose-depth distributions will allow for significantly higher doses to planned treatment volumes (PTVs) and increased biological effectiveness might be exploited for carbon ions.