Damage that occurs when ionizing radiation interacts directly with a radiosensitive molecule is called what?

• A. Direct effect
• B. Indirect effect
• C. Scattering
• D. Absorption

Answer: (A)

Direct effect refers to damage that happens when radiation hits a radiosensitive molecule itself and ionizes or excites it. When a photon directly interacts with something like DNA, the energy deposit causes chemical changes in that molecule, potentially producing breaks in the DNA strands. This is different from the indirect effect, where radiation first interacts with water in the cell to form free radicals that then damage molecules nearby. Scattering describes photons changing direction, and absorption is simply energy being taken up by matter, not the specific mechanism of molecular damage. So the direct interaction with the target molecule is the concept described here.