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A 0.8 mM concentration of misonidazole was added to human blood samples before exposure to graded X-ray single doses, in order to investigate the dependence of the frequency ratio of acentrics to dicentrics, produced in lymphocytes, on treatment with radiation, the substance and the combination of the 2 agents. The results confirm the findings of a previous experiment carried out using sodium iothalamate, showing that this ratio is markedly influenced by the relative action of the physical and the chemical agents, especially at low radiation doses, because of the enhancement of the frequency of acentrics, and not of dicentrics, caused by the presence of the drug compared to the spontaneous level.

The yields of unstable chromosome aberrations induced in human lymphocytes by beta particles from tritiated water ( HTO ) have been measured. HTO was mixed with heparinized blood in various amounts so that doses of 0.25 to 7.0 Gy were delivered in 20 min and 1.5 and 2.5 hr. After culturing for 48 hr, the dicentric yield was measured as a function of dose to the blood and compared with data previously reported for X rays. Using a linear-quadratic dose-effect relation to fit the experimental data, a significant linear contribution was found. The main difference between the coefficients for beta and X radiation is in the alpha values, indicating that HTO beta rays are more efficient, particularly at lower doses, in producing two lesions with a single ionizing track. In accord with the theory of dual radiation, the RBE of HTO beta particles relative to X rays is 2.6 at 0.25 Gy and decreases with increasing dose. When the number of cells without aberrations is plotted against radiation dose, a curve is obtained which is similar in shape to those from human cell survival experiments.