Radiation Dose-Rate and DNA Damage

In their article, Olipitz et al. (2012) examined signs of DNA damage after chronic exposure of C57Bl6 mice to low-level ionizing radiation (3 mGy/day). For 5 weeks, mice were irradiated continuously with 35.5 keV X-rays produced by the decay of iodine-125, yielding an accumulated dose of 105 mGy. The observed effects were compared with those from acute irradiation by X-ray machine at 1,700 mGy/day up to the same accumulated dose. Olipitz et al. (2012) investigated signs of DNA damage using four histological methods. Most prominent of the methods was the expression of functional fluorescent protein as a result of recombination by homology-directed repair in pancreatic cells of transgenic FYDR (fluorescent yellow direct repeat) mice derived from the C57Bl6 strain. The other three methods were carried out using genetically unaltered C57Bl6 mice. The authors investigated DNA base damage in spleno cytes; DNA double strand breaks in bone marrow erythro cytes; and the expression of select genes implicated in cell cycle arrest, tumor suppression, and apoptosis in white blood cells from blood samples. Olipitz et al. (2012) used equal numbers of unirradiated mice as controls. However, sample sizes across the study ranged from 6 to 60 animals. Because of the wide range in animal numbers, non parametric methods should have been used in statistical analyses. A multi variate analysis of variance comprising all observations in the study should have preceded any pairwise comparisons to allow the authors to evaluate the variability of observations within samples compared with the variability among samples (Mickey and Dunn 2009). Furthermore, the use of trans genic mice with one method and unaltered mice with the other three might have increased the variability in observation, reducing the chance of detecting statistically significant differences. The above weaknesses in experi mental design and statistical analysis may have profoundly compromised the authors' ability to discover statistically signifi cant effects of chronic exposure to low-level ionizing radiation. In the " Discussion " of their paper, Olipitz et al. (2012) stated that Chromosome aberrations offer an alternative approach for detecting chromosome breaks, and using this approach, others have shown that low dose-rate radiation indeed induces aberrations in vitro (although the dose-rate was approximately 10-fold higher than that used in the present study) (Tanaka et al. 2009). However, in the paper cited by Olipitz et al., Tanaka et al. (2009) noted that Significant changes in regression coefficients (b3, b4 and b6) obtained by multiple …