Mixed radiation fields, where different ionizing particles act together, are very important in radiobiology and in radiation protection. Mixed beams are not only the most common form of radiation exposure, but the prediction of their biological effect is also full of uncertainties. Currently, prediction of the biological damage of exposure to mixed radiation fields is based on the default assumption of simple additivity between the effects of all the radiation in the field. This assumption has been proven to be incorrect. Indeed, the simultaneous effect of different radiation qualities has been shown to be greater than additive, namely synergistic. This implicates that, for instance, the predicted cancer risk for astronauts, that remain a prolonged time in space, is currently underestimated as well as the risk of developing secondary cancer for radiotherapy patients.

This thesis aims at understanding the mechanisms behind the cellular response to simultaneous exposure to alpha particles and X-rays (that is referred as mixed beam).

Paper I describes the cell killing and the mutagenic effect of mixed beam exposure in human lymphoblastoid wild type and in cells with impaired capacity to repair oxidative DNA damage .We found that oxidative DNA damage plays an important role in the lethal, synergistic effect of mixed beams.

Paper II and III investigates whether mixed beams exposure leads to an augmented DNA double strand breaks (DSB) induction or to an altered response of the cellular DSB repair machinery. We found that mixed irradiation resulted in synergistic induction of DSB, and that those lesions were repaired with slow kinetics.

Paper IV focuses on the effect of mixed beams at the level of DNA damage in normal cells. Induction and repair of DNA lesions such as DSB, single strand breaks and apurinic sites was quantified using the alkaline comet assay. We found that alpha particles and X-rays interacted in inducing DNA damage. Moreover, although mixed beam exposure resulted in strong activation of the DNA damage response, it resulted in delayed repair.

Although more research is needed to fully elucidate the mechanisms behind the detected synergistic effects, our results strongly suggest that an overwhelmed DNA-repair system causes delay in repair of damage.

Many physical factors influence the biological effect of exposure to ionizing radiation, including radiation quality, dose rate and temperature. This thesis focuses on how these factors influence the outcome of exposure and the mechanisms behind the cellular response. 

Mixed beam exposure, which is the combination of different ionizing radiations, occurs in many situations and the effects are important to understand for radiation protection and effect prediction. Recently, studies show that the effect of simultaneous irradiation with different qualities is greater than simple additivity of single radiation types, which is called a synergistic effect. But its mechanism is unclear. In Paper I, II and III, alpha particles and X-rays were used to study the effect of mixed beams. Paper I shows that mixed exposure induced a synergistic effect in generating double strand breaks (DSB), and these DSB were repaired by slow kinetics in U2OS cells. In Paper II, alkaline comet assay was applied to investigate the induction and repair of DNA lesions including DSB, single strand breaks and alkali labile sites in peripheral blood lymphocytes (PBL). We demonstrate that mixed beams interact in inducing DNA damage and influencing DNA damage response (DDR), which result in a delay of DNA repair. Both in Paper I and II, mixed beams showed a capability in inducing higher activity of DDR proteins than expected from additivity. Paper III investigates selected DDR-related gene expression levels after exposure to mixed beams in PBL from 4 donors. Synergy was present for all donors but the results suggested individual variability in the response to mixed beams, most likely due to life style changes.

Low temperature at exposure is radioprotective at the level of cytogenetic damage. In Paper IV, data indicate that this effect is through promotion of DNA repair, which leads to reduced transformation of DNA damage into chromosomal aberrations.  

Paper V aims to compare the biological effectiveness of gamma radiation delivered at a very high dose rate (VHDR) with that of a high dose rate (HDR) in order to optimize chronic exposure risk prediction based on the data of atomic bomb survivors. The results suggest that VHDR gamma radiation is more effective in inducing DNA damage than HDR.