Biomolecular radiation damage
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Mechanism of tissue radiation damage
occurs when energy is directly deposited to the biological macromolecule (e.g. DNA, RNA, protein)
occurs when energy is deposited to the surrounding molecules (e.g. water, oxygen), creating reactive reactive chemical species (radicals) that in turn interact with the target molecules
The energetic photons of x-rays and γ-rays are transferred by collision to orbital electrons in the absorbing tissue. These orbital electrons may be excited (excitation) or ejected (ionization), and radicals (a.k.a. free radicals) form as a result. Radicals are uncharged atoms/molecules that possess an unpaired valence electron.
An unpaired electron lends radicals a significant chemical reactivity and can bind efficiently to other molecules' electrons. Radicals produced via radiation are most commonly seen in water as hydrogen and hydroxyl radicals.
In some cases, radicals binding with other molecules can cause more radicals that again bind with other molecules. This chain reaction effect can result in significant alterations to organic material.
If this occurs amidst molecules that are critical to cellular metabolism, the fundamental functionality of the cell is at risk. Furthermore, radicals can affect nucleic acid molecules leading to cell mutation or cell death (cell death is most likely the result of DNA double-strand breaks).
The vast majority of radiation-induced damage is mediated by this indirect action on water molecules (the body is 70-85% water). In this regard, it is good to remember that the probability that radiation has of interacting with the molecules constituting a given tissue is greater the more the latter are represented in the irradiated tissue 5.
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