Cytogenetic effects of low-energy X-rays

Mammography, using 26-30 kV X-rays, is a method routinely used to screen early stages of breast cancer, but some discussion about radiation-related risk associated with this methodology exists. For radioprotection purposes, it has been assigned a quality factor of 1 for all photon energies. However, the relative biological effectiveness (RBE) could increase as photon energy decreases. Analysing different biological parameters, for 30 kV X-rays RBE values from 1 to 8 has been estimated. In the present study a cytogenetic evaluation of the effect of X-rays of 30, 80 and 120 kV using three fluorescence in situ hybridisation (FISH) techniques is presented. The yields of apparently simple translocations (AST) and dicentrics (dic), by using whole chromosome probes for chromosomes 1, 4 and 11 together with a pancentromeric probe, have been analyzed to estimate the RBEM. The ratio between complete and incomplete chromosome aberrations was evaluated using a pan-centromeric and a pan-telomeric probes; and the percentage of complex chromosome aberrations was evaluated using mFISH. Using as reference the curves obtained for 120 kV, the estimated RBEM for dicentrics were 1.08±0.43 and 1.73±0.59 for 80 and 30 kV X-rays respectively, and for AST were 1.26±0.40 and 1.51±0.47. The ratio between incomplete and complete aberrations for 80 and 120 kV were 0.37 and 0.38, lower than the 0.59 observed for 30 kV. The percentage of complex aberrations increased as X-ray energy decreased (0.08, 0.09 and 0.16 at 120, 80 and 30 kV respectively). The results obtained in the present study, indicate that X-rays used in screening mammography (30 kV) are 1.5 times more efficient in inducing dicentrics, translocations and incomplete chromosome aberrations, and two times more efficient in inducing complex aberrations than X-rays of higher energies (120 kV).