Mouse embryos in the pronuclear-zygote stage (day 0 of gestation) were irradiated with 13.5 rad of 140kV X-rays or negative pions. On day 13 the fetuses were examined for developmental anomalies such as intrauterine death, growth retardation and malformations. Significant decreases in the percentage of normal implantations were obtained with peak pions (high LET) and X-rays, whereas the effect of plateau pions was less obvious. Irradiation with peak pions was more effective than with X-rays by a factor of about 1.7.
Mouse embryos on day 8 of gestation were irradiated with negative pions (12.5-100 rad) or 200 kV X-rays (12.5-150 rad). Misonidazole (MISO), a hypoxic cell radiosensitizer, was applied 30 min before exposure. On day 13 the fetuses were examined for lethality, growth retardation and malformation. No significant embryolethal effects were observed after irradiation alone in the dose range of 12.5-100 rad (X-rays or pions). However, MISO alone and in combination with radiation led to high rates of lethality. The frequency of growth retardation was significantly increased at 100 rad and in combined treatments at low radiation doses. MISO and irradiation with 50 rad and more induced complex damages consisting of multiple and severe malformations and growth retardation. The relative biological effectiveness (RBE) for teratogenic effects was 1.6. In conclusion, the combined application of MISO and radiation of different LET revealed a strong enhancing action compared to single treatments. The extent of enhancement depends on both radiation quality and dose.
The quantification of the physical effects of ionizing radiation in human tissue is the basis of risk assessment. This quantification results from determination of kerma or absorbed dose. The procedure for the absolute determination of absorbed dose with an ionization chamber is discussed. The biological effects of ionizing radiation are dependent, not only on the absorbed dose but also on a second physical parameter, the linear energy transfer.
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