Indirect effect in radiobiology is primarily caused by radiation interactions with which cellular component?

• A. Direct effect
• B. Indirect effect
• C. Radiolysis
• D. Photoelectric effect

Answer: (B)

Indirect effect arises because ionizing radiation mostly interacts with water in the cell, not directly with DNA. This interaction causes radiolysis of water, producing reactive free radicals such as hydroxyl radicals and solvated electrons. These radicals diffuse a short distance and react with DNA and other critical biomolecules, causing strand breaks and base damage that lead to cell injury or death. So the initiating cellular component is water, and the key process is radiolysis that creates damaging radicals. Direct effect would involve the radiation hitting DNA directly, while the photoelectric effect is a photon interaction not specifically responsible for the radical-mediated indirect damage.