A rapid method of biological dosimetry to assist in the event of a nuclear emergency

Abstract

There are concerns that a combination of human and mechanical errors at a nuclear facility could result in plant failure and lead to a significant level of harm to people and the environment. During emergencies where a large number of individuals have been exposed, dose assessment results are needed as soon as possible to help physicians develop a treatment strategy within a few hours of the catastrophe. Hence, the purpose of this research is to test an effective rapid technique for biological dosimetry that will enable the quantification of the amount of the radiation dose absorbed in the event of a nuclear emergency. Blood samples collected from 6 donors were exposed in vitro to different doses of gamma rays (i.e. 0, 10, 20, 50, 100, 200, 500, and 1000 mGy). The induction of double-strand breaks (DSBs) by ionizing radiation immediately results in the phosphorylation of the H2AX protein, a variant of the nucleosomal H2A histone core protein. H2AX foci formation is considered a consistent and quantitative marker of radiation-induced DNA DSBs. The detection and analysis of γ-H2AX foci was performed using the Metacyte software of Metafer 4 image analysis system of an automated microscope recently obtained by iThemba LABS. To validate the automated foci scoring, the images captured by the automated image analysis system were manually analysed. From the constructed dose response curves, it was noticed that data obtained manually were not considerably different from automated counts. The validation method of visual inspections of the captured images suggested reliable results compared to automated results. A fit of a second order polynomial to the foci as a function of the applied radiation dose created the impression that the foci reached a peak before a dose of 1000 mGy and then declined. When a better function was fitted to the data, it kept increasing monotonously with dose all the way up to 1000 mGy, although it started to flatten out gradually as it approached 1000 mGy. Unfortunately no measurements were taken between 500 mGy and 1000 mGy and therefore it is not possible to see whether the actual data did or did not reach a plateau in this region. Furthermore, for the α-values testing the sensitivity of individuals, the values for all 6 donors range from 0.015 to 0.018. For the pooled data, the α-values are 0.018 12 and 0.017 98 with the standard error 0.001 427 and 0.001 033 for manual and automated analysis respectively. A very strong correlation was noted, and an almost one-to-one relationship emerged between the two scoring methods. From the accuracy chart, an average dose percentage error calculated is 0.25% and 2.14% for auto and manual analysis respectively. The automated method can produce results of about 100 exposed individuals within 2 days. For the calibration curve a compound fit function, consisting of separate fits for the low-dose and high-dose regions were created: If Foci  2.02 : 2 Dose  09.2828Foci 19.43Foci If Foci  2.02 : 2 3.674 0.5035Foci–0.01473Foci Dose e   This compound function produced dose estimates that gave a reasonably accurate fit to the average data for the six donor samples, with fractional errors of +21% at 10 mGy and -24% at 25 mGy. At higher doses the absolute value of this error dropped to less than 5%. However statistical fluctuations in the foci counts for the individual donors introduced much larger Root Mean Square (RMS) errors in the individual dose estimates. This error dropped monotonously from 104% at 10 mGy to 20% at 100 mGy and then fluctuates between 16% and 19% for higher doses. Therefore, it can be concluded that the automated scoring system may be used as a reasonable reliable tool for assessing the average frequency of ionising radiation-induced γ-H2AX foci in groups of exposed individuals and from this to deduce the average radiation dose received by the group. These results confirm the efficiency of the automated γ-H2AX foci assay for fast population triage in South Africa in the case of large radiation accidents