ABSTRACT
Energy from radiations of relatively long wavelength, e.g., in the ultraviolet range, may be dissipated in matter largely in the process called excitation in which an electron is raised to a higher energy level. X-rays and gamma rays are electromagnetic radiations consisting of streams of energetic photons which can cause ionization. In general, the greater the time before mitosis that irradiation occurs, the greater the opportunity for repair of the radiation damage before mitosis and the less the delay in division. Fractionation of low linear energy transfer (LET) radiation doses to the proliferating cell populations over considerable periods of time tends to reduce the effectiveness of the radiation doses, presumably because of greater recovery and repopulation of cells. The relationship between radiation damage of DNA and the responses of the irradiated cell is one of the major problems in molecular radiobiology. The effectiveness of high-LET radiation is considerably less dependent on fractionation or reduced dose rate than low LET radiation.
