Orthovoltage X-Rays Exhibit Increased Efficacy Compared with g-Rays in Preclinical Irradiation

Radionuclide irradiators (¹³⁷Cs and ⁶⁰Co) are commonly used in preclinical studies ranging from cancer therapy to stem cell biology. Amidst concerns of radiological terrorism, there are institutional initiatives to replace radionuclide sources with lower energy X-ray sources. As researchers transition, questions remain regarding whether the biological effects of γ-rays may be recapitulated with orthovoltage X-rays because different energies may induce divergent biological effects. We therefore sought to compare the effects of orthovoltage X-rays with 1-mm Cu or Thoraeus filtration and 137Cs g-rays using mouse models of acute radiation syndrome. Following whole-body irradiation, 30-day overall survival was assessed, and the lethal dose to provoke 50% mortality within 30-days (LD₅₀) was calculated by logistic regression. LD50 doses were 6.7 Gy, 7.4 Gy, and 8.1 Gy with 1-mm Cufiltered X-rays, Thoraeus-filtered X-rays, and ¹³⁷Cs γ-rays, respectively. Comparison of bone marrow, spleen, and intestinal tissue from mice irradiated with equivalent doses indicated that injury was most severe with 1-mm Cu-filtered X-rays, which resulted in the greatest reduction in bone marrow cellularity, hematopoietic stem and progenitor populations, intestinal crypts, and OLFM4⁺intestinal stem cells. Thoraeus-filtered X-rays provoked an intermediate phenotype, with ¹³⁷Cs showing the least damage. This study reveals a dichotomy between physical dose and biological effect as researchers transition to orthovoltage X-rays. With decreasing energy, there is increasing hematopoietic and intestinal injury, necessitating dose reduction to achieve comparable biological effects. 2022 American Association for Cancer Research.