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3.1 The Biological Effects of Radiation Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Develo­pment of Radiation Injury

Develo­pment of Radiation Injury

When they interact with living tissue, x-rays, gamma rays and other ionising particles eject fast electrons from atoms within the cell. These electrons in turn lose their energy by intera­ctions with other large molecules causing ionisation and excita­tion.
Energy deposition in irradiated cells thus occurs in the form of ionised and excited atoms or molecules distri­buted at random throughout the cells. Although much of the actual energy absorbed by irradiated cells goes into producing excited molecules, it appears that most of this energy does not produce chemical reaction and is dissipated in the form of heat.
It is the ionisation that causes most of the immediate chemical changes in the vicinity of the event.
Ionisation results from the ejection of an orbital electron from a molecule, producing a positively charged or ‘ionised’ molecule. Such molecules are highly unstable and will rapidly undergo chemical change. This damage results in the production of ‘free radicals’, which are atoms or molecules containing unpaired electrons.
Free radicals are extremely reactive and may lead to permanent damage of the affected molecule, or the energy may be transf­erred to another molecule.
Because 80 percent of the cell is water, most energy deposited within the cell by radiation results in the production of aqueous free radicals, for example, OH and H.
Chemical damage may be repaired before it is irreve­rsible by recomb­ination of the radicals, and the energy dissipated as fluore­scence, phosph­ore­scence, or vibrat­ional energy. If unrepa­ired, chemical change will lead to biological damage.
The radios­ens­itive site of the cell has been identified as the deoxyr­ibo­nucleic acid (DNA) molecule within the nucleus.

Intera­ction of Radiation with Tissue

The process by which x-ray or γ-ray photons are absorbed depends on their energy and the chemical compos­ition of the absorbing material.
At high energies, the Compton process dominates. In this process the photon interacts with a loosely bound electron of an atom of the absorbing material. Part of the photon energy is given to the electron as kinetic energy. The photon, deflected from its original path, proceeds with reduced energy and may undergo further intera­ctions. The net result is the production of fast electrons, many of which can ionise other atoms of the absorber, break vital chemical bonds, and initiate biological damage.
In the photoe­lectric effect, the photon gives up all its energy to the bound electron; some of which is used to overcome the binding energy of the electron and release it from its orbit, while the remainder is given to the electron as kinetic energy.
In radiot­herapy, for example, with a cobalt-60 unit or a linear accele­rator, the Compton process, which is indepe­ndent of Z, is overwh­elm­ingly important. As a conseq­uence, the absorbed dose is approx­imately the same in soft tissue, muscle and bone.
In radiob­iology, it doesn't matter which process dominates because most of the energy of the absorbed photons is converted into the kinetic energy of fast electrons. This energy is dissipated as the electrons move through the medium where they ionise and excite atoms with which they interact.

Linear Energy Transfer

Linear Energy Transfer (LET) accounts for all the energy liberated along the path of an ionising particle
Usually expressed as keV per microm­eter, keV/μm (ie. Energy per Length).
LET determins if a type of ionising radiation is sparsely (x or γ rays), moderately (neutrons) or densely (α-par­ticles) ionising
In general, particles of high-LET radiations (densely ionising) are more likely to produce change in a given volume of living matter, because their intera­ctions are produced more closely together.

Direct action of Radiation

When radiation (x or γ rays, charged or uncharged particles) is absorbed in biological material, it may interact directly with the sensitive targets in the cells initiating the chain of events that lead to a biological change.
This is the dominant process for medium LET radiation (for example, neutrons) and is essent­ially the sole method of absorption for high LET radiation (for example, α-part­icles).

Indirect action of Radiation

Radiation may interact with other atoms or molecules in the cell (parti­cularly water) to produce free radicals. If the radicals are formed within a critical distance of the target, they are able to diffuse far enough to reach and damage it
This is dominant for sparsely ionising radiation, for example, x-rays or γ rays
Radiolysis of water occurs when water is irradi­ated, it dissoc­iates into other molecular products
When an atom of water (H2O) is irradi­ated, it is ionised and dissoc­iates into two ions - an ion pair:
Ionisa­tion: H2O + radiation → HOH+ + e-
After this initial ionisa­tion, a number of reactions can happen:
1. the ion pair may rejoin into a stable water molecule (and no damage occurs);
2. if the ions do not rejoin, it is possible for the negative ion (electron) to attach to another water molecule, producing a third type of ion:
Additional Ionisa­tion: H2O + e- → HOH-
The HOH+ and HOH- are relatively unstable and can dissociate into still smaller molecules:
Dissoc­iation: HOH+ → H+ + OH
HOH- → OH- + H
The final result of the radiolysis of water is the formation of an ion pair (H+ and OH-) and two free radicals (H and OH).
The ions can recombine (H+ and OH-), and hence no biological damage would occur. However, the free radicals are another story. They are highly reactive, unstable and exist with a lifetime of less than 1 ms. During this time, they are capable of diffusion through the cell and intera­ction at a distant site. Free radicals contain excess energy that can be transf­erred to other molecules to disrupt bonds and produce point lesions at some distance from the initial ionising event. It is estimated that about two thirds of x- ray damage to mammalian cells is due to the OH*.