Linear energy transfer

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Linear_energy_transfer 

 

Diffusion cloud chamber with tracks of ionizing radiation (alpha particles) that are made visible as strings of droplets

 

In dosimetry, linear energy transfer (LET) is the amount of energy that an ionizing particle transfers to the material traversed per unit distance. It describes the action of radiation into matter.

It is identical to the retarding force acting on a charged ionizing particle travelling through the matter. By definition, LET is a positive quantity. LET depends on the nature of the radiation as well as on the material traversed. 

A high LET will attenuate the radiation more quickly, generally making shielding more effective and preventing deep penetration. On the other hand, the higher concentration of deposited energy can cause more severe damage to any microscopic structures near the particle track. If a microscopic defect can cause larger-scale failure, as is the case in biological cells and microelectronics, the LET helps explain why radiation damage is sometimes disproportionate to the absorbed dose. Dosimetry attempts to factor in this effect with radiation weighting factors.

Linear energy transfer is closely related to stopping power, since both equal the retarding force. The unrestricted linear energy transfer is identical to linear electronic stopping power, as discussed below. But the stopping power and LET concepts are different in the respect that total stopping power has the nuclear stopping power component, and this component does not cause electronic excitations. Hence nuclear stopping power is not contained in LET.

The appropriate SI unit for LET is the newton, but it is most typically expressed in units of kiloelectronvolts per micrometre (keV/μm) or megaelectronvolts per centimetre (MeV/cm). While medical physicists and radiobiologists usually speak of linear energy transfer, most non-medical physicists talk about stopping power.

Restricted and unrestricted LET

The secondary electrons produced during the process of ionization by the primary charged particle are conventionally called delta rays, if their energy is large enough so that they themselves can ionize. Many studies focus upon the energy transferred in the vicinity of the primary particle track and therefore exclude interactions that produce delta rays with energies larger than a certain value Δ. This energy limit is meant to exclude secondary electrons that carry energy far from the primary particle track, since a larger energy implies a larger range. This approximation neglects the directional distribution of secondary radiation and the non-linear path of delta rays, but simplifies analytic evaluation.

In mathematical terms, Restricted linear energy transfer is defined by

${\displaystyle L_{\Delta }={\frac {{\text{d}}E_{\Delta }}{{\text{d}}x}},}$

where ${\displaystyle {\text{d}}E_{\Delta }}$ is the energy loss of the charged particle due to electronic collisions while traversing a distance ${\displaystyle {{\text{d}}x}}$, excluding all secondary electrons with kinetic energies larger than Δ. If Δ tends toward infinity, then there are no electrons with larger energy, and the linear energy transfer becomes the unrestricted linear energy transfer which is identical to the linear electronic stopping power. Here, the use of the term "infinity" is not to be taken literally; it simply means that no energy transfers, however large, are excluded.

Application to radiation types

During his investigations of radioactivity, Ernest Rutherford coined the terms alpha rays, beta rays and gamma rays for the three types of emissions that occur during radioactive decay.

Alpha particles and other positive ions

Bragg curve of 5.49 MeV alpha particles in air. This radiation is produced by the decay of radon (²²²Rn); its range is 4.14 cm. Stopping power (which is essentially identical to LET) is plotted here versus path length; its peak is the "Bragg peak"

 

Linear energy transfer is best defined for monoenergetic ions, i.e. protons, alpha particles, and the heavier nuclei called HZE ions found in cosmic rays or produced by particle accelerators. These particles cause frequent direct ionizations within a narrow diameter around a relatively straight track, thus approximating continuous deceleration. As they slow down, the changing particle cross section modifies their LET, generally increasing it to a Bragg peak just before achieving thermal equilibrium with the absorber, i.e., before the end of range. At equilibrium, the incident particle essentially comes to rest or is absorbed, at which point LET is undefined.

Since the LET varies over the particle track, an average value is often used to represent the spread. Averages weighted by track length or weighted by absorbed dose are present in the literature, with the latter being more common in dosimetry. These averages are not widely separated for heavy particles with high LET, but the difference becomes more important in the other type of radiations discussed below.

Beta particles

Electrons produced in nuclear decay are called beta particles. Because of their low mass relative to atoms, they are strongly scattered by nuclei (Coulomb or Rutherford scattering), much more so than heavier particles. Beta particle tracks are therefore crooked. In addition to producing secondary electrons (delta rays) while ionizing atoms, they also produce bremsstrahlung photons. A maximum range of beta radiation can be defined experimentally which is smaller than the range that would be measured along the particle path.

Gamma rays

Gamma rays are photons, whose absorption cannot be described by LET. When a gamma quantum passes through matter, it may be absorbed in a single process (photoelectric effect, Compton effect or pair production), or it continues unchanged on its path. (Only in the case of the Compton effect, another gamma quantum of lower energy proceeds). Gamma ray absorption therefore obeys an exponential law; the absorption is described by the absorption coefficient or by the half-value thickness. 

LET has therefore no meaning when applied to photons. However, many authors speak of "gamma LET" anyway, where they are actually referring to the LET of the secondary electrons, i.e., mainly Compton electrons, produced by the gamma radiation. The secondary electrons will ionize far more atoms than the primary photon. This gamma LET has little relation to the attenuation rate of the beam, but it may have some correlation to the microscopic defects produced in the absorber. It must be noted that even a monoenergetic gamma beam will produce a spectrum of electrons, and each secondary electron will have a variable LET as it slows down, as discussed above. The "gamma LET" is therefore an average. 

The transfer of energy from an uncharged primary particle to charged secondary particles can also be described by using the mass energy-transfer coefficient.

Biological effects

The ICRP used to recommend quality factors as a generalized approximation of RBE based on LET.

 

Many studies have attempted to relate linear energy transfer to the relative biological effectiveness (RBE) of radiation, with inconsistent results. The relationship varies widely depending on the nature of the biological material, and the choice of endpoint to define effectiveness. Even when these are held constant, different radiation spectra that shared the same LET have significantly different RBE.

Despite these variations, some overall trends are commonly seen. The RBE is generally independent of LET for any LET less than 10 keV/µm, so a low LET is normally chosen as the reference condition where RBE is set to unity. Above 10 keV/µm, some systems show a decline in RBE with increasing LET, while others show an initial increase to a peak before declining. Mammalian cells usually experience a peak RBE for LET's around 100 keV/µm. These are very rough numbers; for example, one set of experiments found a peak at 30 keV/µm. 

The International Commission on Radiation Protection (ICRP) proposed a simplified model of RBE-LET relationships for use in dosimetry. They defined a quality factor of radiation as a function of dose-averaged unrestricted LET in water, and intended it as a highly uncertain, but generally conservative, approximation of RBE. Different iterations of their model are shown in the graph to the right. The 1966 model was integrated into their 1977 recommendations for radiation protection in ICRP 26. This model was largely replaced in the 1991 recommendations of ICRP 60 by radiation weighting factors that were tied to the particle type and independent of LET. ICRP 60 revised the quality factor function and reserved it for use with unusual radiation types that did not have radiation weighting factors assigned to them.

Application fields

When used to describe the dosimetry of ionizing radiation in the biological or biomedical setting, the LET (like linear stopping power) is usually expressed in units of keV/µm.

In space applications, electronic devices can be disturbed by the passage of energetic electrons, protons or heavier ions that may alter the state of a circuit, producing "single event effects". The effect of the radiation is described by the LET (which is here taken as synonymous with stopping power), typically expressed in units of MeV·cm²/mg of material, the units used for mass stopping power (the material in question is usually Si for MOS devices). The units of measurement arise from a combination of the energy lost by the particle to the material per unit path length (MeV/cm) divided by the density of the material (mg/cm³).

"Soft errors" of electronic devices due to cosmic rays on earth are, however, mostly due to neutrons which do not directly interact with the material and whose passage can therefore not be described by LET. Rather, one measures their effect in terms of neutrons per cm² per hour, see Soft error.

Relative biological effectiveness

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Relative_biological_effectiveness
In radiobiology, the relative biological effectiveness (often abbreviated as RBE) is the ratio of biological effectiveness of one type of ionizing radiation relative to another, given the same amount of absorbed energy. The RBE is an empirical value that varies depending on the type of ionizing radiation, the energies involved, the biological effects being considered such as cell death, and the oxygen tension of the tissues or so-called oxygen effect.

Application

The absorbed dose can be a poor indicator of the biological effect of radiation, as the biological effect can depend on many other factors, including the type of radiation, energy, and type of tissue. The relative biological effectiveness can help give a better measure of the biological effect of radiation. The relative biological effectiveness for radiation of type R on a tissue is defined as the ratio

${\displaystyle RBE={\frac {D_{X}}{D_{R}}}}$

where D_X is a reference absorbed dose of radiation of a standard type X, and D_R is the absorbed dose of radiation of type R that causes the same amount of biological damage. Both doses are quantified by the amount of energy absorbed in the cells.

Different types of radiation have different biological effectiveness mainly because they transfer their energy to the tissue in different ways. Photons and beta particles have a low linear energy transfer (LET) coefficient, meaning that they ionize atoms in the tissue that are spaced by several hundred nanometers (several tenths of a micrometer) apart, along their path. In contrast, the much more massive alpha particles and neutrons leave a denser trail of ionized atoms in their wake, spaced about one tenth of a nanometer apart (i.e., less than one-thousandth of the typical distance between ionizations for photons and beta particles).

RBEs can be used for either cancer/hereditary risks (stochastic) or for harmful tissue reactions (deterministic) effects. Tissues have different RBEs depending on the type of effect. For high LET radiation (i.e., alphas and neutrons), the RBEs for deterministic effects tend to be lower than those for stochastic effects.

The concept of RBE is relevant in medicine, such as in radiology and radiotherapy, and to the evaluation of risks and consequences of radioactive contamination in various contexts, such as nuclear power plant operation, nuclear fuel disposal and reprocessing, nuclear weapons, uranium mining, and ionizing radiation safety.

Relation to radiation weighting factors (W_R)

ICRP Protection Dose quantities in SI units

For the purposes of computing the equivalent dose to an organ or tissue, the International Commission on Radiological Protection (ICRP) has defined a standard set of radiation weighting factors (W_R), formerly termed the quality factor (Q). The radiation weighting factors convert absorbed dose (measured in SI units of grays or non-SI rads) into formal biological equivalent dose for radiation exposure (measured in units of sieverts or rem). However, ICRP states:

"The quantities equivalent dose and effective dose should not be used to quantify higher radiation doses or to make decisions on the need for any treatment related to tissue reactions [i.e., deterministic effects]. For such purposes, doses should be evaluated in terms of absorbed dose (in gray, Gy), and where high-LET radiations (e.g., neutrons or alpha particles) are involved, an absorbed dose, weighted with an appropriate RBE, should be used"

Radiation weighting factors are largely based on the RBE of radiation for stochastic health risks. However, for simplicity, the radiation weighting factors are not dependent on the type of tissue, and the values are conservatively chosen to be greater than the bulk of experimental values observed for the most sensitive cell types, with respect to external (external to the cell) sources. Radiation weighting factors have not been developed for internal sources of heavy ions, such as a recoil nucleus.

The ICRP 2007 standard values for relative effectiveness are given below. The higher radiation weighting factor for a type of radiation, the more damaging it is, and this is incorporated into the calculation to convert from gray to sievert units. 

The radiation weighting factor for neutrons has been revised over time and remains controversial.

 

Radiation	Energy WR (formerly Q)
x-rays, gamma rays, beta particles, muons	1
neutrons (< 1 MeV)	2.5 + 18.2e-[ln(E)]2/6
neutrons (1 - 50 MeV)	5.0 + 17.0e-[ln(2E)]2/6
neutrons (> 50 MeV)	2.5 + 3.25e-[ln(0.04E)]2/6
protons, charged pions	2
alpha particles, nuclear fission products, heavy nuclei	20

Radiation weighting factors that go from physical energy to biological effect must not be confused with tissue weighting factors. The tissue weighting factors are used to convert an equivalent dose to a given tissue in the body, to an effective dose, a number that provides an estimation of total danger to the whole organism, as a result of the radiation dose to part of the body.

Experimental methods

LD-30 limit for CHO-K1 cell line irradiated by photons (blue curve) and by carbon ions (red curve).

 

Typically the evaluation of relative biological effectiveness is done on various types of living cells grown in culture medium, including prokaryotic cells such as bacteria, simple eukaryotic cells such as single celled plants, and advanced eukaryotic cells derived from organisms such as rats. The doses are adjusted to the LD-30 point; that is, to the amount that will cause 30% of the cells to become unable to undergo mitotic division (or, for bacteria, binary fission), thus being effectively sterilized — even if they can still carry out other cellular functions. LD-50 is more commonly used, but whoever drew the plot did not realise that the grid line closest to halfway between factors of 10 on a log plot is actually 3, not 5. LD-50 values are actually 1 gray for Carbon ions and 3 grays for photons.

The types R of ionizing radiation most considered in RBE evaluation are X-rays and gamma radiation (both consisting of photons), alpha radiations (helium-4 nuclei), beta radiation (electrons and positrons), neutron radiation, and heavy nuclei, including the fragments of nuclear fission. For some kinds of radiation, the RBE is strongly dependent on the energy of the individual particles.

Dependence on tissue type

Early on it was found that X-rays, gamma rays, and beta radiation were essentially equivalent for all cell types. Therefore, the standard radiation type X is generally an X-ray beam with 250 keV photons or cobalt-60 gamma rays. As a result, the relative biological effectiveness of beta and photon radiation is essentially 1. 

For other radiation types, the RBE is not a well-defined physical quantity, since it varies somewhat with the type of tissue and with the precise place of absorption within the cell. Thus, for example, the RBE for alpha radiation is 2–3 when measured on bacteria, 4–6 for simple eukaryotic cells, and 6–8 for higher eukaryotic cells. According to one source it may be much higher (6500 with X rays as the reference) on ovocytes. The RBE of neutrons is 4–6 for bacteria, 8–12 for simple eukaryotic cells, and 12–16 for higher eukaryotic cells.

Dependence on source location

In the early experiments, the sources of radiation were all external to the cells that were irradiated. However, since alpha particles cannot traverse the outermost dead layer of human skin, they can do significant damage only if they come from the decay of atoms inside the body. Since the range of an alpha particle is typically about the diameter of a single eukaryotic cell, the precise location of the emitting atom in the tissue cells becomes significant.

For this reason, it has been suggested that the health impact of contamination by alpha emitters might have been substantially underestimated. Measurements of RBE with external sources also neglect the ionization caused by the recoil of the parent-nucleus due to the alpha decay. While the recoil of the parent-nucleus of the decaying atom typically carries only about 2% of the energy of the alpha-particle that is emitted by the decaying atom, its range is extremely short (about 2–3 angstroms), due to its high electric charge and high mass. The parent nucleus is required to recoil, upon emission of an alpha particle, with a discrete kinetic energy due to conservation of momentum. Thus, all of the ionization energy from the recoil-nucleus is deposited in an extremely small volume near its original location, typically in the cell nucleus on the chromosomes, which have an affinity for heavy metals. The bulk of studies, using sources that are external to the cell, have yielded RBEs between 10 and 20.

History

In 1931, Failla and Henshaw reported on determination of the relative biological effectiveness (RBE) of x rays and γ rays. This appears to be the first use of the term ‘RBE’. The authors noted that RBE was dependent on the experimental system being studied. Somewhat later, it was pointed out by Zirkle et al. (1952) that the biological effectiveness depends on the spatial distribution of the energy imparted and the density of ionisations per unit path length of the ionising particles. Zirkle et al. coined the term ‘linear energy transfer (LET)’ to be used in radiobiology for the stopping power, i.e. the energy loss per unit path length of a charged particle. The concept was introduced in the 1950s, at a time when the deployment of nuclear weapons and nuclear reactors spurred research on the biological effects of artificial radioactivity. It had been noticed that those effects depended both on the type and energy spectrum of the radiation, and on the kind of living tissue. The first systematic experiments to determine the RBE were conducted in that decade.

Radiobiology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Radiobiology 

 

Radiobiology (also known as radiation biology) is a field of clinical and basic medical sciences that involves the study of the action of ionizing radiation on living things, especially health effects of radiation. Ionizing radiation is generally harmful and potentially lethal to living things but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis. Its most common impact is the induction of cancer with a latent period of years or decades after exposure. High doses can cause visually dramatic radiation burns, and/or rapid fatality through acute radiation syndrome. Controlled doses are used for medical imaging and radiotherapy.

Health effects

In general, ionizing radiation is harmful and potentially lethal to living beings but can have health benefits in radiation therapy for the treatment of cancer and thyrotoxicosis.

Most adverse health effects of radiation exposure may be grouped in two general categories:

• deterministic effects (harmful tissue reactions) due in large part to the killing/ malfunction of cells following high doses; and
• stochastic effects, i.e., cancer and heritable effects involving either cancer development in exposed individuals owing to mutation of somatic cells or heritable disease in their offspring owing to mutation of reproductive (germ) cells.

Stochastic

Some effects of ionizing radiation on human health are stochastic, meaning that their probability of occurrence increases with dose, while the severity is independent of dose. Radiation-induced cancer, teratogenesis, cognitive decline, and heart disease are all examples of stochastic effects.

Its most common impact is the stochastic induction of cancer with a latent period of years or decades after exposure. The mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial. The most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. If this linear model is correct, then natural background radiation is the most hazardous source of radiation to general public health, followed by medical imaging as a close second. Other stochastic effects of ionizing radiation are teratogenesis, cognitive decline, and heart disease. 

Quantitative data on the effects of ionizing radiation on human health is relatively limited compared to other medical conditions because of the low number of cases to date, and because of the stochastic nature of some of the effects. Stochastic effects can only be measured through large epidemiological studies where enough data has been collected to remove confounding factors such as smoking habits and other lifestyle factors. The richest source of high-quality data comes from the study of Japanese atomic bomb survivors. In vitro and animal experiments are informative, but radioresistance varies greatly across species.

The added lifetime risk of developing cancer by a single abdominal CT of 8 mSv is estimated to be 0.05%, or 1 one in 2,000.

Deterministic

Deterministic effects are those that reliably occur above a threshold dose, and their severity increases with dose.

High radiation dose gives rise to deterministic effects which reliably occur above a threshold, and their severity increases with dose. Deterministic effects are not necessarily more or less serious than stochastic effects; either can ultimately lead to a temporary nuisance or a fatality. Examples of deterministic effects are:

• Acute radiation syndrome, by acute whole-body radiation
• Radiation burns, from radiation to a particular body surface
• Radiation-induced thyroiditis, a potential side effect from radiation treatment against hyperthyroidism
• Chronic radiation syndrome, from long-term radiation.
• Radiation-induced lung injury, from for example radiation therapy to the lungs
• Cataracts, and infertility.

The US National Academy of Sciences Biological Effects of Ionizing Radiation Committee "has concluded that there is no compelling evidence to indicate a dose threshold below which the risk of tumor induction is zero."

Phase	Symptom	Whole-body absorbed dose (Gy)
		1–2 Gy	2–6 Gy	6–8 Gy	8–30 Gy	> 30 Gy
Immediate	Nausea and vomiting	5–50%	50–100%	75–100%	90–100%	100%
	Time of onset	2–6 h	1–2 h	10–60 min	< 10 min	Minutes
	Duration	< 24 h	24–48 h	< 48 h	< 48 h	N/A (patients die in < 48 h)
	Diarrhea	None	None to mild (< 10%)	Heavy (> 10%)	Heavy (> 95%)	Heavy (100%)
	Time of onset	—	3–8 h	1–3 h	< 1 h	< 1 h
	Headache	Slight	Mild to moderate (50%)	Moderate (80%)	Severe (80–90%)	Severe (100%)
	Time of onset	—	4–24 h	3–4 h	1–2 h	< 1 h
	Fever	None	Moderate increase (10–100%)	Moderate to severe (100%)	Severe (100%)	Severe (100%)
	Time of onset	—	1–3 h	< 1 h	< 1 h	< 1 h
	CNS function	No impairment	Cognitive impairment 6–20 h	Cognitive impairment > 24 h	Rapid incapacitation	Seizures, tremor, ataxia, lethargy
Latent period		28–31 days	7–28 days	< 7 days	None	None
Illness		Mild to moderate Leukopenia Fatigue Weakness	Moderate to severe Leukopenia Purpura Hemorrhage Infections Alopecia after 3 Gy	Severe leukopenia High fever Diarrhea Vomiting Dizziness and disorientation Hypotension Electrolyte disturbance	Nausea Vomiting Severe diarrhea High fever Electrolyte disturbance Shock	N/A (patients die in < 48h)
Mortality	Without care	0–5%	5–95%	95–100%	100%	100%
	With care	0–5%	5–50%	50–100%	99–100%	100%
	Death	6–8 weeks	4–6 weeks	2–4 weeks	2 days – 2 weeks	1–2 days

By type of radiation

When alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest. This is due to the high relative biological effectiveness of alpha radiation to cause biological damage after alpha-emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes such as transuranics or actinides are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous than an equivalent activity of beta emitting or gamma emitting radioisotopes. If the radiation type is not known then it can be determined by differential measurements in the presence of electrical fields, magnetic fields, or varying amounts of shielding. 

External dose quantities used in radiation protection. See article on sievert on how these are calculated and used.

In pregnancy

The risk for developing radiation-induced cancer at some point in life is greater when exposing a fetus than an adult, both because the cells are more vulnerable when they are growing, and because there is much longer lifespan after the dose to develop cancer.

Possible deterministic effects include of radiation exposure in pregnancy include miscarriage, structural birth defects, Growth restriction and intellectual disability. The determinstistic effects have been studied at for example survivors of the atomic bombings of Hiroshima and Nagasaki and cases where radiation therapy has been necessary during pregnancy:

Gestational age	Embryonic age	Effects	Estimated threshold dose (mGy)
2 to 4 weeks	0 to 2 weeks	Miscarriage or none (all or nothing)	50 - 100
4 to 10 weeks	2 to 8 weeks	Structural birth defects	200
		Growth restriction	200 - 250
10 to 17 weeks	8 to 15 weeks	Severe intellectual disability	60 - 310
18 to 27 weeks	16 to 25 weeks	Severe intellectual disability (lower risk)	250 - 280

The intellectual deficit has been estimated to be about 25 IQ-points per 1,000 mGy at 10 to 17 weeks of gestational age.

These effects are sometimes relevant when deciding about medical imaging in pregnancy, since projectional radiography and CT scanning exposes the fetus to radiation.

Also, the risk for the mother of later acquiring radiation-induced breast cancer seems to be particularly high for radiation doses during pregnancy.

Measurement

The human body cannot sense ionizing radiation except in very high doses, but the effects of ionization can be used to characterize the radiation. Parameters of interest include disintegration rate, particle flux, particle type, beam energy, kerma, dose rate, and radiation dose.

The monitoring and calculation of doses to safeguard human health is called dosimetry and is undertaken within the science of health physics. Key measurement tools are the use of dosimeters to give the external effective dose uptake and the use of bio-assay for ingested dose. The article on the sievert summarises the recommendations of the ICRU and ICRP on the use of dose quantities and includes a guide to the effects of ionizing radiation as measured in sieverts, and gives examples of approximate figures of dose uptake in certain situations.

The committed dose is a measure of the stochastic health risk due to an intake of radioactive material into the human body. The ICRP states "For internal exposure, committed effective doses are generally determined from an assessment of the intakes of radionuclides from bioassay measurements or other quantities. The radiation dose is determined from the intake using recommended dose coefficients".

Absorbed, equivalent and effective dose

The Absorbed dose is a physical dose quantity D representing the mean energy imparted to matter per unit mass by ionizing radiation. In the SI system of units, the unit of measure is joules per kilogram, and its special name is gray (Gy). The non-SI CGS unit rad is sometimes also used, predominantly in the USA. 

To represent stochastic risk the equivalent dose H _T and effective dose E are used, and appropriate dose factors and coefficients are used to calculate these from the absorbed dose. Equivalent and effective dose quantities are expressed in units of the sievert or rem which implies that biological effects have been taken into account. These are usually in accordance with the recommendations of the International Committee on Radiation Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU). The coherent system of radiological protection quantities developed by them is shown in the accompanying diagram.

Organizations

The International Commission on Radiological Protection (ICRP) manages the International System of Radiological Protection, which sets recommended limits for dose uptake. Dose values may represent absorbed, equivalent, effective, or committed dose.

Other important organizations studying the topic include

• International Commission on Radiation Units and Measurements (ICRU)
• United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)
• US National Council on Radiation Protection and Measurements (NCRP)
• UK Public Health England
• US National Academy of Sciences (NAS through the BEIR studies)
• French Institut de radioprotection et de sûreté nucléaire (IRSN)
• European Committee on Radiation Risk (ECRR) the stage of radiation depends on the stage the body parts are affected

Exposure pathways

External

A schematic diagram showing a rectangle being irradiated by an external source (in red) of radiation (shown in yellow).

A schematic diagram showing a rectangle being irradiated by radioactive contamination (shown in red) which is present on an external surface such as the skin; this emits radiation (shown in yellow) which can enter the animal's body

 

External exposure is exposure which occurs when the radioactive source (or other radiation source) is outside (and remains outside) the organism which is exposed. Examples of external exposure include:

• A person who places a sealed radioactive source in his pocket
• A space traveller who is irradiated by cosmic rays
• A person who is treated for cancer by either teletherapy or brachytherapy. While in brachytherapy the source is inside the person it is still considered external exposure because it does not result in a committed dose.
• A nuclear worker whose hands have been dirtied with radioactive dust. Assuming that his hands are cleaned before any radioactive material can be absorbed, inhaled or ingested, skin contamination is considered external exposure.

External exposure is relatively easy to estimate, and the irradiated organism does not become radioactive, except for a case where the radiation is an intense neutron beam which causes activation.

By type of medical imaging

Effective dose by medical imaging type
Target organs	Exam type	Effective dose in adults	Equivalent time of background radiation
CT of the head	Single series	2 mSv	8 months
	With + without radiocontrast	4 mSv	16 months
Chest	CT of the chest	7 mSv	2 years
	CT of the chest, lung cancer screening protocol	1.5 mSv	6 months
	Chest X-ray	0.1 mSv	10 days
Heart	Coronary CT angiography	12 mSv	4 years
	Coronary CT calcium scan	3 mSv	1 year
Abdominal	CT of abdomen and pelvis	10 mSv	3 years
	CT of abdomen and pelvis, low dose protocol	3 mSv[13]	1 year
	CT of abdomen and pelvis, with + without radiocontrast	20 mSv	7 years
	CT Colonography	6 mSv	2 years
	Intravenous pyelogram	3 mSv	1 year
	Upper gastrointestinal series	6 mSv	2 years
	Lower gastrointestinal series	8 mSv	3 years
Spine	Spine X-ray	1.5 mSv	6 months
	CT of the spine	6 mSv	2 years
Extremities	X-ray of extremity	0.001 mSv	3 hours
	Lower extremity CT angiography	0.3 - 1.6 mSv	5 weeks - 6 months
Dental X-ray		0.005 mSv	1 day
DEXA (bone density)		0.001 mSv	3 hours
PET-CT combination		25 mSv	8 years
Mammography		0.4 mSv	7 weeks

Internal

Internal exposure occurs when the radioactive material enters the organism, and the radioactive atoms become incorporated into the organism. This can occur through inhalation, ingestion, or injection. Below are a series of examples of internal exposure.

• The exposure caused by potassium-40 present within a normal person.
• The exposure to the ingestion of a soluble radioactive substance, such as ⁸⁹Sr in cows' milk.
• A person who is being treated for cancer by means of a radiopharmaceutical where a radioisotope is used as a drug (usually a liquid or pill). A review of this topic was published in 1999. Because the radioactive material becomes intimately mixed with the affected object it is often difficult to decontaminate the object or person in a case where internal exposure is occurring. While some very insoluble materials such as fission products within a uranium dioxide matrix might never be able to truly become part of an organism, it is normal to consider such particles in the lungs and digestive tract as a form of internal contamination which results in internal exposure.
• Boron neutron capture therapy (BNCT) involves injecting a boron-10 tagged chemical that preferentially binds to tumor cells. Neutrons from a nuclear reactor are shaped by a neutron moderator to the neutron energy spectrum suitable for BNCT treatment. The tumor is selectively bombarded with these neutrons. The neutrons quickly slow down in the body to become low energy thermal neutrons. These thermal neutrons are captured by the injected boron-10, forming excited (boron-11) which breaks down into lithium-7 and a helium-4 alpha particle both of these produce closely spaced ionizing radiation.This concept is described as a binary system using two separate components for the therapy of cancer. Each component in itself is relatively harmless to the cells, but when combined together for treatment they produce a highly cytocidal (cytotoxic) effect which is lethal (within a limited range of 5-9 micrometers or approximately one cell diameter). Clinical trials, with promising results, are currently carried out in Finland and Japan.

When radioactive compounds enter the human body, the effects are different from those resulting from exposure to an external radiation source. Especially in the case of alpha radiation, which normally does not penetrate the skin, the exposure can be much more damaging after ingestion or inhalation. The radiation exposure is normally expressed as a committed dose.

History

Although radiation was discovered in late 19th century, the dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when Wilhelm Röntgen intentionally subjected his fingers to X-rays in 1895. He published his observations concerning the burns that developed, though he misattributed them to ozone, a free radical produced in air by X-rays. Other free radicals produced within the body are now understood to be more important. His injuries healed later.

As a field of medical sciences, radiobiology originated from Leopold Freund's 1896 demonstration of the therapeutic treatment of a hairy mole using a new type of electromagnetic radiation called x-rays, which was discovered 1 year previously by the German physicist, Wilhelm Röntgen. After irradiating frogs and insects with X-rays in early 1896, Ivan Romanovich Tarkhanov concluded that these newly discovered rays not only photograph, but also "affect the living function". At the same time, Pierre and Marie Curie discovered the radioactive polonium and radium later used to treat cancer. 

The genetic effects of radiation, including the effects on cancer risk, were recognized much later. In 1927 Hermann Joseph Muller published research showing genetic effects, and in 1946 was awarded the Nobel prize for his findings.

More generally, the 1930s saw attempts to develop a general model for radiobiology. Notable here was Douglas Lea, whose presentation also included an exhaustive review of some 400 supporting publications.

Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as patent medicine and radioactive quackery. Examples were radium enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood. Curie later died of aplastic anemia caused by radiation poisoning. Eben Byers, a famous American socialite, died of multiple cancers (but not acute radiation syndrome) in 1932 after consuming large quantities of radium over several years; his death drew public attention to dangers of radiation. By the 1930s, after a number of cases of bone necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market.

In the United States, the experience of the so-called Radium Girls, where thousands of radium-dial painters contracted oral cancers (but no cases of acute radiation syndrome), popularized the warnings of occupational health associated with radiation hazards. Robley D. Evans, at MIT, developed the first standard for permissible body burden of radium, a key step in the establishment of nuclear medicine as a field of study. With the development of nuclear reactors and nuclear weapons in the 1940s, heightened scientific attention was given to the study of all manner of radiation effects.

The atomic bombings of Hiroshima and Nagasaki resulted in a large number of incidents of radiation poisoning, allowing for greater insight into its symptoms and dangers. Red Cross Hospital surgeon Dr. Terufumi Sasaki led intensive research into the Syndrome in the weeks and months following the Hiroshima bombings. Dr Sasaki and his team were able to monitor the effects of radiation in patients of varying proximities to the blast itself, leading to the establishment of three recorded stages of the syndrome. Within 25–30 days of the explosion, the Red Cross surgeon noticed a sharp drop in white blood cell count and established this drop, along with symptoms of fever, as prognostic standards for Acute Radiation Syndrome. Actress Midori Naka, who was present during the atomic bombing of Hiroshima, was the first incident of radiation poisoning to be extensively studied. Her death on August 24, 1945 was the first death ever to be officially certified as a result of radiation poisoning (or "Atomic bomb disease").

Areas of interest

The interactions between organisms and electromagnetic fields (EMF) and ionizing radiation can be studied in a number of ways:

• Radiation physics
• Radiation chemistry
• molecular and cell biology
• Molecular genetics
• Cell death and apoptosis
• High and low-level electromagnetic radiation and health
• Specific absorption rates of organisms
• Radiation poisoning
• Radiation oncology (radiation therapy in cancer)
• Bioelectromagnetics
• Electric field and Magnetic field - their general nature.
• Electrophysiology - the scientific study of the electrical properties of biological cells and tissues.
• Biomagnetism - the magnetic properties of living systems (see, for example, the research of David Cohen using SQUID imaging) and Magnetobiology - the study of effect of magnets upon living systems. See also Electromagnetic radiation and health
• Bioelectromagnetism - the electromagnetic properties of living systems and Bioelectromagnetics - the study of the effect of electromagnetic fields on living systems.
• Electrotherapy
• Radiation therapy
• Radiogenomics
• Electroconvulsive therapy
• Transcranial magnetic stimulation - a powerful electric current produces a transient, spatially focussed magnetic field that can penetrate the scalp and skull of a subject and induce electrical activity in the neurons on the surface of the brain.
• Magnetic resonance imaging - a very powerful magnetic field is used to obtain a 3D image of the density of water molecules of the brain, revealing different anatomical structures. A related technique, functional magnetic resonance imaging, reveals the pattern of blood flow in the brain and can show which parts of the brain are involved in a particular task.
• Embryogenesis, Ontogeny and Developmental biology - a discipline that has given rise to many scientific field theories.
• Bioenergetics - the study of energy exchange on the molecular level of living systems.
• Biological psychiatry, Neurology, Psychoneuroimmunology
• Bioluminescence - a marked phosphorescence found in fungi, deep-sea creatures etc., as against Biophoton - a much weaker electromagnetic radiation, thought by Alexander Gurwitsch, its discoverer, to be a form of signalling.

The activity of biological and astronomical systems inevitably generates magnetic and electrical fields, which can be measured with sensitive instruments and which have at times been suggested as a basis for "esoteric" ideas of energy.

Radiation sources for experimental radiobiology

Radiobiology experiments typically make use of a radiation source which could be:

• An isotopic source, typically ¹³⁷Cs or ⁶⁰Co.
• A particle accelerator generating high energy protons, electrons or charged ions. Biological samples can be irradiated using either a broad, uniform beam or using a microbeam, focused down to cellular or subcellular sizes.
• A UV lamp.