Mechanism of action of radiotherapy is-

Ans. is 'a' i.e., DNA damage o Radiation therapy works by damaging the DNA of cells. Radiation deposition results in DNA damage manifested by single and double strand breaks in the sugar phosphate backbone of the DNA molecules (charring of nucleoprotein)The damage is caused by ionization of atoms which make up the DNA chain. However, the mechanism of ionization differs between the various radiation type:-1) Electromagnetic radiations (x-ray and gamma rays) cause indirect ionization via short lived, free hydroxyl radicals produced primarily by the ionization of cellular H2O.2) Particulate radiation (Photon, electron, proton, neutron and a-particle) are directly ionizing and damage directly.