Transitioning from Gamma Rays to X Rays for Comparable Biomedical Research Irradiations: Energy Matters.

Many studies in biomedical research and various allied fields, in which cells or laboratory animals are exposed to radiation, rely on adequate radiation dose standardization for reproducibility and comparability of biological data. Due to increasing concerns regarding international terrorism, the use of radioactive isotopes has recently been met with enhanced security measures. Thus, a growing number of researchers have considered transferring their studies from gamma-ray to kilovoltage X-ray irradiators. Current commercially-available X-ray biological irradiators produce radiation beams with reasonable field geometry and overall dose-homogeneity; however, they operate over a wide range of different energies, both between different models and for a specific unit as well. As a result, the contribution from Compton scattering and the photoelectric effect also varies widely between different irradiators and different beam qualities. The photoelectric effect significantly predominates at the relatively low X-ray energies in which these irradiators operate. Consequently, a higher dose is delivered to bony tissues and the adjacent hematopoietic cells of the bone marrow. The increase in average radiation absorbed dose to the bone marrow compartment of the mouse can be as high as 30%, causing higher hematological sensitivity of animals when exposed to kilovoltage X rays. Adjusting the radiation dose to simply provide biological equivalency is complicated due to steep dose gradients within the marrow tissue and the qualitatively different outcomes depending on the spatial location of critical stem and progenitor populations in relationship to bone. These concerns may be practically addressed by efforts to implement X rays of the highest possible beam energy and penetration and increased awareness that radiation damage to hematopoietic cells will not be identical to data obtained from standard 137Cs gamma rays. ©2020 by Radiation Research Society. All rights of reproduction in any form reserved.