Orthovoltage X-rays exhibit increased efficacy compared to γ-rays in preclinical irradiation

Radionuclide irradiators ( 137 Cs and 60 Co) 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 since 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 137 Cs γ-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 (LD 50 ) was calculated by logistic regression. LD 50 doses were 6.7 Gy, 7.4 Gy, and 8.1 Gy with 1 mm Cu-filtered X-rays, Thoraeus-filtered X-rays, and 137 Cs 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 137 Cs 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.