Which of the following occur within fractions of a second after exposure: absorption of radiation energy; formation of active radicals; biochemical changes in cells?

• A. 1 and 2
• B. 1 and 3
• C. 2 and 3
• D. 1, 2, and 3

Answer: (D)

Immediately after ionizing radiation hits tissue, energy is deposited at the molecular level. This energy deposition happens essentially instantly and mainly causes ionization and excitation of molecules, with water being the most affected due to its abundance. That rapid energy interaction drives radiolysis of water and the immediate creation of reactive free radicals, such as hydroxyl radicals and other reactive species. These radicals then rapidly interact with cellular components, producing chemical changes in DNA, lipids, and proteins. These initial biochemical changes in cells begin within fractions of a second and can trigger downstream damage and cellular responses. Because energy absorption occurs right away, radicals form almost instantly, and the first biochemical alterations begin in the same brief time frame, all three occur within fractions of a second. This is why including all three events reflects the true immediate sequence after exposure.