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

Area of Science:

• Radiation Biology
• Medical Physics
• Preclinical Research

Background:

• Radionuclide irradiators (cesium-137, cobalt-60) are standard in preclinical research.
• Concerns about radiological terrorism drive a shift towards lower-energy X-ray sources.
• The biological equivalence of X-rays versus gamma-rays remains a critical question due to differing energy profiles.

Purpose of the Study:

• To compare the biological effects of orthovoltage X-rays with varying filtration to 137Cs gamma-rays.
• To assess the impact on acute radiation syndrome and overall survival in mouse models.
• To determine if physical dose accurately predicts biological outcomes across different radiation types.

Main Methods:

• Mouse models of acute radiation syndrome were whole-body irradiated.
• Orthovoltage X-rays (1-mm Cu or Thoraeus filtered) and 137Cs gamma-rays were used.
• Thirty-day overall survival and LD50 (lethal dose for 50% mortality) were calculated.
• Histological analysis of bone marrow, spleen, and intestinal tissues was performed.

Main Results:

• LD50 doses were 6.7 Gy (Cu-filtered X-rays), 7.4 Gy (Thoraeus X-rays), and 8.1 Gy (137Cs).
• 1-mm Cu-filtered X-rays induced the most severe injury, with significant reductions in bone marrow cellularity, hematopoietic stem cells, intestinal crypts, and OLFM4+ cells.
• Thoraeus-filtered X-rays showed intermediate damage, while 137Cs gamma-rays caused the least damage at equivalent doses.

Conclusions:

• A dichotomy exists between physical radiation dose and biological effect when switching to orthovoltage X-rays.
• Decreasing radiation energy correlates with increased hematopoietic and intestinal injury.
• Dose adjustments are necessary when using lower-energy X-ray sources to achieve biological effects comparable to gamma-rays.