Ionizing Radiation Carcinogenesis

Abstract

Human exposure to ionizing radiation is a natural part of life on earth. These exposures occur every day from radiation associated with naturally occurring radionuclides in soil, air, and food, and also from cosmic rays. In addition, many people are exposed to dental and medical diagnostic procedures and therapy involving: x rays, gamma rays, charged particles, radionuclides, or other ionizing radiation sources. Others may be exposed in their workplace such as in laboratories, hospitals, underground mines, or nuclear power plants. Excessive exposures may lead to the development of cancer by promotion of ongoing carcinogenic biological processes or by independent cancer induction. Ionization converts a neutral atom to a charged atom. The γ rays and x rays are ionizing electromagnetic radiation with energies above ultraviolet in the electromagnetic spectrum. The difference between x rays and γ rays is a matter of nomenclature. The γ rays are emitted from the nucleus of a radioactive atom while x rays originate in the orbital electrons of a energized atom. The α2+ particle has been identified as a positively charged helium nucleus with 2 protons and two neutrons emitted from the nucleus of certain radionuclides. The βradiation was identified as negatively charged electrons emitted from the nucleus of certain radionuclides. There are other types of ionizing radiation including most prominently uncharged neutrons emitted from the nucleus of atoms that ionize atoms via nuclear interactions, and positrons and that are positive beta particles, β+. The measure of absorbed radiation dose in matter or tissue is the gray (Gy) equal to one joule of energy per kilogram of matter or tissue. To account for different types of ionizing radiation, such as gamma, alpha, and beta radiations, the dose from ionizing radiation is corrected for the theoretical relative biological effectiveness of the different radiations in causing cellular damage using a radiation weighting factor, wR (Sv/Gy), and this is reported as the equivalent dose in sieverts (Sv). Hence, the measure of biological dose is the sievert (Sv), equal to the absorbed dose in Gy times the radiation weighting factor. The γ rays, x rays, and β radiation typically have a radiation weighting factor of 1, while α radiation has a radiation weighting factor of 20 and neutrons may have a radiation weighting factor from 5 to 20 depending on energy. The quantity of radioactivity is measured in becquerel (Bq), equal to one radioactive atomic event or nuclear disintegration per second. Radiation induced cancer is a complex and not completely understood process involving multiple events including but not limited to cellular DNA damage, up and down regulation of genes, intercellular communication, tissue and organ responses, clonal expansion of