Biological Effectiveness of Photons and Electrons as a Function of Energy

Introduction

An unresolved question in evaluating the risk of cancer in humans from exposure to low linear-energy, transfer (LET) , radiation, (i.e. photons, and electrons) is the dependence of the biological effectiveness on energy. This dependence is relevant for estimating the level of cancer risk from exposure to low-LET radiation at lower energies in mammography and other medical imaging procedures, as well as certain sources of occupational and public radiation exposure (NCRP 2012) . Because of the broad importance of this topic to the basic responsibilities and interests of NCRP, the Council created a Scientific Community to evaluate this question. Numerous other expert groups have also considered this question, and several have concluded that the biological effectiveness of lower-energy, low-LET radiation based on chromosomal aberration data and biophysical considerations may be two or more times greater than for higher-energy, low-LET radiation (NCRP 1990; QCRP 2003). However, biological systems used in the experiments and biophysical analysis provide only indirect evidence and may not be strictly applicable to cancer in humans. Moreover, the true values likely vary according to disease endpoint and type of biological system studied. Because of the enormous complexity of the phenomena that are involved in the development of cancer following exposure to ionizing radiation and the inherent difficulty in observing small differences in biological effectiveness in epidemiologic studies, it is unlikely that any single area of study can provide a clear understanding of the relative biological effectiveness of different energies. For these reasons, an important aspect of the evaluation of SC 1-20 is the combined assessment of multiple lines of evidence and the related uncertainties.