The authors present a comparison of the frequency of dicentrics in human leukocytes irradiated in whole blood in vitro and in vivo. The results indicate that the frequency induced in vivo can be a factor of two higher or lower than the frequency in vitro for the same radiation dose. Whole blood was heparinized and irradiated in plastic bottles at 37° C. immediately after drawing. The oxygen tension in the blood during irradiation was essentially that of normal venous blood. Irradiations were carried out with the x-ray beam from a 6 Mev Linac (effective photon energy about 1.9 Mev) at dose rates of 200 rad/min. and 10 rad/min. The leukocytes were separated from the whole blood and incubated for seventy-two or ninety hours; chromosome preparations were then made in standard fashion. Every metaphase figure was scanned for chromosome aberrations, but tetraploid cells were excluded from the scoring. Data are presented herein for the class of aberrations known as dicentrics only, because these can be readily detected and can be scored unambiguously. Figure 1 illustrates, as an example, a dicentric in a leukocyte from a radiation worker. At the bottom of the figure is the cell as it appears under the microscope. At the top, the chromosomes are arranged in pairs, revealing that the dicentric was formed from the fusion of chromosomes numbered one and eleven. Figure 2 shows the results obtained in vitro. Data procured at the two dose rates and the two incubation times were not significantly different and therefore are not separately indicated on the graph. Also included are two points obtained with 100-kvp x-ray and one point obtained with 250-kvp x-ray. None of the three points is significantly different from the results obtained with the Linac, but effects of x-ray energy of as much as 20 per cent are not excluded. The figure clearly reveals that from a dose of 25 rads to one of 1,200 rads the yield of dicentrics is directly proportional to the square of the dose. The proportionality constant is 2.7 ± 0.14 × 10−6 dicentrics per cell per rad-squared. This figure is almost exactly half that reported by Bender and Gooch (1); we assume that one roentgen in their work corresponds to 0.93 rads. The reasons for the difference in results are not known, but do not appear to be a matter of x-ray energy, dose rate, or oxygen tension. They may be due to an accumulation of small errors; a discrepancy in absorbed dose of 10 per cent, for example, leads to a discrepancy of 20 per cent in dicentric frequency. Or they may be due to differences in the incubation conditions during the short-term culture of the leukocytes. At the highest dose, 1,200 rads, the yield is 4.2 dicentrics per cell. The failure of the yield to depart from a dose-squared dependence at this dose shows that the number of sites per cell at which chromosome exchanges can occur (2) must be large, probably in excess of ten sites per cell.
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Norman et al. (1964) studied this question.