The why and how radiation performs it’s role as a treatment for oncologists is as follows.When the photon hits the electron that ultimately damages the DNA, radiation has acted directly. However, when the photon hits an electron, that ionizes water forming hydroxyl radicals that float around and hit the DNA, genetic damage ensues to the DNA strand. In this manner, the radiation is said to be acting indirectly. This indirect action of the DNA is more common. Double-strand damage to the DNA proves lethal to the cell and apoptosis occurs. The repair of DNA damage may be by non-homologous end joining (NHEJ) or by homologous recombination (HR). In non-homologous end-joining the ends are fixed-up enough so that they can aonnect back together. Homologous recombination used the template strand from the sister chromatid, and can repair the DNA strand with no loss of information.
Non-homologous end-joining occurs in the G1, and early S phases of the cell cycle, when you don’t have a second copy of DNA. Whereas. homologous recombination occurs in S and G2. The cell phase that is most resistant to radiation is late S, and early G2. When you get to late G2, you’re getting near mitosis, where the cell is getting ready to divide. If you damage the DNA it’s already a double-strand break. The photon interactions with tissue are as follows. Photons are indirectly ionizing. To damage DNA they must first create a free radical to ionize and cause damage.
Coherent scattering, where the photon makes slight deviation, causes no significant energy deposited in the tissue. In the photoelectric effect, a photon kicks out a shell electron; and other electrons fall into a hole which then releases “characteristic x-rays,” used for diagnostic imaging. In compton interactions, the photon knocks an electron out of the atom creating a free radical. This then interacts with the DNA causing damage. Pair production occurs when photons spontaneously produce an electron and a positron which can then cause other reactions.