Enhanced Neoplastic Transformation by Mammography X Rays Relative to 200 kVp X Rays: Indication for a Strong Dependence on Photon Energy of the RBEMfor Various End Points

Abstract

Frankenberg, D., Kelnhofer, K., Bär, K. and Frankenberg-Schwager, M. Enhanced Neoplastic Transformation by Mammography X Rays Relative to 200 kVp X Rays: Indication for a Strong Dependence on Photon Energy of the RBE_M for Various End Points. Radiat. Res. 157, 99–105 (2002). The fundamental assumption implicit in the use of the atomic bomb survivor data to derive risk estimates is that the γ rays of Hiroshima and Nagasaki are considered to have biological efficiencies equal to those of other low-LET radiations up to 10 keV/μm, including mammography X rays. Microdosimetric and radiobiological data contradict this assumption. It is therefore of scientific and public interest to evaluate the efficiency of mammography X rays (25–30 kVp) to induce cancer. In this study, the efficiency of mammography X rays relative to 200 kVp X rays to induce neoplastic cell transformation was evaluated using cells of a human hybrid cell line (CGL1). For both radiations, a linear-quadratic dose–effect relationship was observed for neoplastic transformation of CGL1 cells; there was a strong linear component for the 29 kVp X rays. The RBE_M of mammography X rays relative to 200 kVp X rays was determined to be about 4 for doses ≤0.5 Gy. A comparison of the electron fluences for both X rays provides strong evidence that electrons with energies of ≤15 keV can induce neoplastic transformation of CGL1 cells. Both the data available in the literature and the results of the present study strongly suggest an increase of RBE_M for carcinogenesis in animals, neoplastic cell transformation, and clastogenic effects with decreasing photon energy or increasing LET to an RBE_M approximately 8 for mammography X rays relative to ⁶⁰Co γ rays.