Damage Induction and Repair Processes in Chinese Hamster Cells and Normal Human Skin Fibroblasts Irradiated by Light Ions of Different Energies

For effective application of protons and ions in tumour radiotherapy is necessary understanding of the basic characteristics of radiobiological mechanism in individual cell. To study the cell inactivation mechanisms of various ions at different energies, damage induction and repair processes in hamster and human cell lines have been analyzed. Published survival data for Chinese hamster CHO-K1 cells and their radiosensitive mutants xrs5 irradiated by carbon ions and for normal human fibroblasts irradiated by various ions have been analyzed using the probabilistic twostage model [Kundrat et al, 2005]. This model enables to reflect different types of lesions caused by ionizing particles and also the consequent repair processes in cells. The characteristics of damage induction and repair processes in cells in dependence on ion kinds and their LET will be presented. In the case of Chinese hamster cells, the comparison of a normal cell line and its radiosensitive mutant emphasizes the necessity to explicitly represent the cell repair processes in radiobiological models. Introduction Radiotherapy represents one of the main methods used in the treatment of tumours. The purpose is to inactivate all tumour cells by ionising radiation with the smallest possible damage to normal tissues. Conventional radiotherapy is based on electron, X or gamma rays characterised with exponentially decreasing depth-dose curves [Hall 2000]. Hence cells in front of target volume are irradiated with much higher dose than the tumour cells and this fact must be compensated for using multiple-field irradiation. Hadrontherapy, which is based on using light ions for irradiating tumours is in present time one of promising methods used in oncologic treatment [Debus et al 1998, Mayer et al, 2004]. In the case of proton and light ions, tumour cells are irradiated by the area of Bragg peak at the end of particle range, where energy deposition sharply grows. Cells in front of tumour volume are irradiated by much smaller dose and absorbed dose behind this area is practically negligible as compared with conventional methods. For light ions highly conformal dose distribution and high relative biological effectiveness (RBE) induced by high linear energy transfer (LET) in area of the Bragg peak are typical [Judas et al 2001, Kraft 1999]. Biological effect of proton and ion irradiation is dependent on transferred dose, type of particle, energy and LET. Realistic mathematical model of biological effect such as damage induction and cellular repair is necessary for the optimal use of protons and light ions in clinical radiotherapy. Radiobiological Model To represent the damage induction and repair processes, the probabilistic two-stage model [Kundrat et al 2005] has been used. The model is based on distinguishing the single-particle and collective effects of ionizing irradiation and enables to take into account lesions caused by ionizing particles and also subsequent repair processes. We assume that radiation damage to DNA can be divided into categories of negligible damage (no effect for survival of the cell), damages of lower severity (can be lethal if two or more events of this type are combined), and severe damages caused by single particle [Kundrát 2006, Kundrát et al 2005]. The cell survival curve can be described as [Kundrát et al 2005] ( ) ( ) k k k s D P D = ∑ q , (1) WDS'06 Proceedings of Contributed Papers, Part III, 101–106, 2006. ISBN 80-86732-86-X © MATFYZPRESS