Relative biological effectiveness of mammography X-rays at the level of DNA and chromosomes in lymphocytes

Purpose: In many countries, breast cancer screening programs based on periodic mammography exist, giving a large group of women regularly a small dose of ionizing radiation. In order to assess the benefit/risk ratio of those programs the relative biological effectiveness (RBE) of mammography X-rays needs to be determined. Materials and methods: Blood of five healthy donors was irradiated in vitro with 30 kV X-rays and Co-60 gamma-rays with doses between 5 and 2000 mGy. The phosphorylated histone subtype H2A isoform X-foci (gamma H2AX-foci) technique was used to quantify the number of DNA double-strand breaks (DSB) after irradiation. Chromosomal damage resulting from non-or misrepaired DNA DSB was quantified with the micronucleus (MN)-assay and the sensitivity was improved by counting only centromere negative micronuclei (MNCM.). Results: The threshold detection dose obtained with the gamma H2AX-foci test was 10 mGy for mammography X-rays compared to 50 mGy for gamma-rays. With the MN-assay respectively MN-centromere-assay threshold detection doses of 100, respectively, 50 mGy were obtained for mammography X-rays compared to 200 respectively 100 mGy for gamma-rays. An RBE of 1.4 was obtained with the gamma H2AX-foci assay. With the MN-assays low-dose RBE values between 3 and 4 were determined. Conclusion: Our results indicate that exposure to mammography X-rays resulted in a modest increase in the induction of DSB compared to gamma-rays. However, due to the higher linear energy transfer (LET) of mammography X-rays more clustered DNA damage is produced that is more difficult to repair and results in a more pronounced increase in micronucleus formation