Which type of radiation causes cells to demonstrate the greatest increase in radiosensitivity when in an highly oxygenated state?

• A. Diagnostic x-rays.
• B. Alpha particles.
• C. Ions of heavy nuclei.
• D. Fast neutrons.

Answer: (A)

Oxygen greatly enhances the effect of radiation that damages cells through indirect chemical reactions. When low-LET radiation, like diagnostic X-rays, passes through tissue, it mostly creates free radicals in water. Oxygen then reacts with these radicals to form reactive species that fix DNA damage, making it permanent and harder for the cell to repair. This oxygen-driven “fixation” boosts cell killing the most for low-LET radiation. High-LET radiations, such as alpha particles, heavy ions, and fast neutrons, already cause dense, direct DNA damage that doesn’t rely as much on the indirect chemical effects mediated by oxygen, so adding oxygen doesn’t increase their radiosensitivity as much. Since diagnostic X-rays show the largest boost in effectiveness with oxygen, they are the best answer.