ionizing radiation
Ionizing radiation is a type of particle radiation in which an individual particle (for example, a photon, electron, or helium nucleus) carries enough energy to ionize an atom or molecule (that is, to completely remove an electron from its orbit). If the individual particles do not carry this amount of energy, it is essentially impossible for even a large flood of particles to cause ionization. These ionizations, if enough occur, can be very destructive to living tissue.
The composition of ionizing radiation can vary. Electromagnetic radiation can cause ionization if the energy per photon is high enough (that is, the wavelength is short enough).
Far ultraviolet, X-rays, and gamma rays are all ionizing radiation, while visible light, microwaves, and radio waves are non-ionizing radiation. Ionizing radiation may also consist of fast-moving particles such as electrons, positrons, or small atomic nuclei.
Pathology
Radiant energy, whether in the form of the UV rays of sunlight or as ionizing electromagnetic and particulate radiation, can transform virtually all cell types in vitro and induce neoplasms in vivo in both humans and experimental animals.
UV light is clearly implicated in the causation of skin cancers, and ionizing radiation exposure from medical or occupational exposure, nuclear plant accidents, and atomic bomb detonations have produced a variety of forms of malignant neoplasia.
Although the contribution of radiation to the total human burden of cancer is probably small, the well-known latency of radiant energy and its cumulative effect require extremely long periods of observation and make it difficult to ascertain its full significance.
An increased incidence of breast cancer has become apparent decades later among women exposed during childhood to the atomic bomb. The incidence peaked during 1988-1992 and then declined during the period 1993-1997.
Moreover, radiation’s possible additive or synergistic effects with other potential carcinogenic influences add another dimension to the picture. The effects of UV light on DNA differ from those of ionizing radiation.
Ionizing radiation can produce a variety of lesions in DNA, including DNA-protein cross-links, cross-linking of DNA strands, oxidation and degradation of bases, cleavage of sugar-phosphate bonds, and single-stranded or double-stranded DNA breaks.
This damage may be produced directly by particulate radiation, x-rays, or gamma rays or indirectly by oxygen-derived free radicals or soluble products derived from peroxidized lipids.
Even relatively low doses of ionizing radiation (less than 0.5 Gy) induce alterations in gene expression in some target cell populations. Free radicals generated directly or indirectly by exposure to ionizing radiation may produce oxidant stress that activates transcription factors (such as NF-κB) that increase gene expression.
DNA damage itself stimulates the expression of several genes involved in DNA repair, cell-cycle arrest, and apoptosis. The tumor-suppressor gene p53 is activated after many different forms of DNA damage. The end-points result from activation of this p53-mediated DNA damage response. Activation of p53 induces cell-cycle arrest, DNA repair and, in some cases, apoptosis. Apoptosis of microvascular endothelial cells may be the primary target of acute radiation in the GI tract, resulting in secondary damage to intestinal crypt stem cells56 and the GI syndrome.
See also