Mechanistic and phenomenological models of radiation induced biological damages,

During the last few years mechanistic approaches in studying biological effects of ionizing radiation have gained increasing favour. Average quantities such as absorbed dose or LET have proven to be inadequate to explain many features of radiation action. Much emphasis has been devoted to the necessity of understanding the intermediate steps linking the initial depositions of energy, to cellular end-points. through the biochemical processes of DNA damage and repair. A critical comparison is presented between the information achievable from phenomenological and mechanistic models, and the fundamental needs for the development of mechanistic models are discussed. Results are presented, based on track structure and DNA damage simulations for protons and alpha-particles of various energies, on the first steps of radiation action, i.e. from initial energy depositions to DNA damage. The importance of ionization density along the primary ion in determining the greater efficency of protons with respect to alpha-particles of equal LET is highlighted, and the relationship with similar conclusions obtained with more phenomenological models based on average quantities such as restricted LET or mean free path are discussed.