Dosimetry

Last updated

Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested or inhaled radioactive substances, or externally due to irradiation by sources of radiation.

Health physics

Health physics is the applied physics of radiation protection for health and health care purposes. It is the science concerned with the recognition, evaluation, and control of health hazards to permit the safe use and application of ionizing radiation. Health physics professionals promote excellence in the science and practice of radiation protection and safety. Health physicists principally work at facilities where radionuclides or other sources of ionizing radiation are used or produced; these include hospitals, government laboratories, academic and research institutions, nuclear power plants, regulatory agencies, and manufacturing plants.

Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Exposure can be from a source of radiation external to the human body or due to internal irradiation caused by the ingestion of radioactive contamination.

Ionizing radiation Radiation that carries enough light energy to liberate electrons from atoms or molecules

Ionizing radiation is radiation that carries sufficient energy to detach electrons from atoms or molecules, thereby ionizing them. Ionizing radiation is made up of energetic subatomic particles, ions or atoms moving at high speeds, and electromagnetic waves on the high-energy end of the electromagnetic spectrum.

Contents

Internal dosimetry assessment relies on a variety of monitoring, bio-assay or radiation imaging techniques, whilst external dosimetry is based on measurements with a dosimeter, or inferred from measurements made by other radiological protection instruments.

Internal dosimetry is the science and art of internal ionising radiation dose assessment due to radionuclides incorporated inside the human body.

A radiation dosimeter is a device that measures dose uptake of external ionizing radiation. It is worn by the person being monitored when used as a personal dosimeter, and is a record of the radiation dose received. Modern electronic personal dosimeters can give a continuous readout of cumulative dose and current dose rate, and can warn the wearer with an audible alarm when a specified dose rate or a cumulative dose is exceeded. Other dosimeters, such as thermoluminescent or film types, require processing after use to reveal the cumulative dose received, and cannot give a current indication of dose when being worn.

Dosimetry is used extensively for radiation protection and is routinely applied to monitor occupational radiation workers, where irradiation is expected, or where radiation is unexpected, such as in the aftermath of the Three Mile Island, Chernobyl or Fukushima radiological release incidents. The public dose take-up is measured and calculated from a variety of indicators such as ambient measurements of gamma radiation, radioactive particulate monitoring, and the measurement of levels of radioactive contamination.

Three Mile Island accident nuclear accident

The Three Mile Island accident was a partial meltdown of reactor number 2 of Three Mile Island Nuclear Generating Station (TMI-2) in Dauphin County, Pennsylvania, near Harrisburg and subsequent radiation leak that occurred on March 28, 1979. It was the most significant accident in U.S. commercial nuclear power plant history. On the seven-point International Nuclear Event Scale, the incident was rated a five as an "accident with wider consequences".

Chernobyl City of district significance in Kiev Oblast, Ukraine

Chernobyl or Chornobyl is a ghost city in the Chernobyl Exclusion Zone, situated in the Ivankiv Raion of northern Kiev Oblast, Ukraine, near Ukraine's border with Belarus. Chernobyl is about 90 kilometres (60 mi) north of Kiev, and 140 kilometres (87 mi) southwest of the Belarusian city of Gomel. Before its evacuation, the city had about 14,000 residents.

Fukushima Daiichi nuclear disaster nuclear disaster in Japan

The Fukushima Daiichi nuclear disaster was a nuclear accident at the Fukushima Daiichi Nuclear Power Plant in Ōkuma, Fukushima Prefecture. The disaster was the most significant nuclear incident since the 26 April 1986 Chernobyl disaster and the only other disaster since to be given the Level 7 event classification of the International Nuclear Event Scale.

Other significant areas are medical dosimetry, where the required treatment absorbed dose and any collateral absorbed dose is monitored, and in environmental dosimetry, such as radon monitoring in buildings.

Absorbed dose is a dose quantity which is the measure of the energy deposited in matter by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection, and radiology. It is also used to directly compare the effect of radiation on inanimate matter such as in radiation hardening.

Radon Chemical element with atomic number 86

Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through which thorium and uranium slowly decay into lead and various other short-lived radioactive elements; radon itself is the immediate decay product of radium. Its most stable isotope, 222Rn, has a half-life of only 3.8 days, making radon one of the rarest elements since it decays away so quickly. However, since thorium and uranium are two of the most common radioactive elements on Earth, and they have three isotopes with very long half-lives, on the order of several billions of years, radon will be present on Earth long into the future in spite of its short half-life as it is continually being generated. The decay of radon produces many other short-lived nuclides known as radon daughters, ending at stable isotopes of lead.

Measuring radiation dose

External dose

There are several ways of measuring absorbed doses from ionizing radiation. People in occupational contact with radioactive substances, or who may be exposed to radiation, routinely carry personal dosimeters. These are specifically designed to record and indicate the dose received. Traditionally, these were lockets fastened to the external clothing of the monitored person, which contained photographic film known as film badge dosimeters. These have been largely replaced with other devices such as the TLD badge which uses Thermoluminescent dosimetry or optically stimulated luminescence (OSL) badges.

The film badge dosimeter or film badge is a personal dosimeter used for monitoring cumulative radiation dose due to ionizing radiation.

In physics, optically stimulated luminescence (OSL) is a method for measuring doses from ionizing radiation. It is used in at least two applications:

A number of electronic devices known as Electronic Personal Dosimeters (EPDs) have come into general use using semiconductor detection and programmable processor technology. These are worn as badges but can give an indication of instantaneous dose rate and an audible and visual alarm if a dose rate or a total integrated dose is exceeded. A good deal of information can be made immediately available to the wearer of the recorded dose and current dose rate via a local display. They can be used as the main stand-alone dosimeter, or as a supplement to such as a TLD badge. These devices are particularly useful for real-time monitoring of dose where a high dose rate is expected which will time-limit the wearer's exposure.

The International Committee on Radiation Protection (ICRP) guidance states that if a personal dosimeter is worn on a position on the body representative of its exposure, assuming whole-body exposure, the value of personal dose equivalent Hp(10) is sufficient to estimate an effective dose value suitable for radiological protection. [1] Such devices are known as "legal dosimeters" if they have been approved for use in recording personnel dose for regulatory purposes. In cases of non-uniform irradiation such personal dosimeters may not be representative of certain specific areas of the body, where additional dosimeters are used in the area of concern.

In certain circumstances, a dose can be inferred from readings taken by fixed instrumentation in an area in which the person concerned has been working. This would generally only be used if personal dosimetry had not been issued, or a personal dosimeter has been damaged or lost. Such calculations would take a pessimistic view of the likely received dose.

Internal dose

Internal dosimetry is used to evaluate the committed dose due to the intake of radionuclides into the human body.


Medical dosimetry

Medical dosimetry is the calculation of absorbed dose and optimization of dose delivery in radiation therapy. It is often performed by a professional health physicist with specialized training in that field. In order to plan the delivery of radiation therapy, the radiation produced by the sources is usually characterized with percentage depth dose curves and dose profiles measured by a medical physicist.

In radiation therapy, three-dimensional dose distributions are often evaluated using a technique known as gel dosimetry. [2]

Environmental dosimetry

Environmental Dosimetry is used where it is likely that the environment will generate a significant radiation dose. An example of this is radon monitoring. Radon is a radioactive gas generated by the decay of uranium, which is present in varying amounts in the earth's crust. Certain geographic areas, due to the underlying geology, continually generate radon which permeates its way to the earth's surface. In some cases the dose can be significant in buildings where the gas can accumulate. A number of specialised dosimetry techniques are used to evaluate the dose that a building's occupants may receive.

Measures of dose

External radiation protection dose quantities in SI units Dose quantities and units.png
External radiation protection dose quantities in SI units
Graphic showing relationship of SI radiation dose units SI Radiation dose units.png
Graphic showing relationship of SI radiation dose units

To enable consideration of stochastic health risk, calculations are performed to convert the physical quantity absorbed dose into equivalent and effective doses, the details of which depend on the radiation type and biological context. For applications in radiation protection and dosimetry assessment the (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) have published recommendations and data which are used to calculate these.

Units of measure

There are a number of different measures of radiation dose, including absorbed dose (D) measured in:

Each measure is often simply described as ‘dose’, which can lead to confusion. Non-SI units are still used, particularly in the USA, where dose is often reported in rads and dose equivalent in rems. By definition, 1 Gy = 100 rad and 1 Sv = 100 rem.

The fundamental quantity is the absorbed dose (D), which is defined as the mean energy imparted [by ionising radiation] (dE) per unit mass (dm) of material (D = dE/dm) [3] The SI unit of absorbed dose is the gray (Gy) defined as one joule per kilogram. Absorbed dose, as a point measurement, is suitable for describing localised (i.e. partial organ) exposures such as tumour dose in radiotherapy. It may be used to estimate stochastic risk provided the amount and type of tissue involved is stated. Localised diagnostic dose levels are typically in the 0-50 mGy range. At a dose of 1 milligray (mGy) of photon radiation, each cell nucleus is crossed by an average of 1 liberated electron track. [4]

Equivalent dose

The absorbed dose required to produce a certain biological effect varies between different types of radiation, such as photons, neutrons or alpha particles. This is taken into account by the equivalent dose (H), which is defined as the mean dose to organ T by radiation type R (DT,R), multiplied by a weighting factor WR . This designed to take into account the biological effectiveness (RBE) of the radiation type, [3] For instance, for the same absorbed dose in Gy, alpha particles are 20 times as biologically potent as X or gamma rays. The measure of ‘dose equivalent’ is not organ averaged and now only used for "operational quantities". Equivalent dose is designed for estimation of stochastic risks from radiation exposures. Stochastic effect is defined for radiation dose assessment as the probability of cancer induction and genetic damage. [5]

As dose is averaged over the whole organ; equivalent dose is rarely suitable for evaluation of acute radiation effects or tumour dose in radiotherapy. In the case of estimation of stochastic effects, assuming a linear dose response, this averaging out should make no difference as the total energy imparted remains the same.

Radiation weighting factors WR (formerly termed Q factor)
used to represent relative biological effectiveness
according to ICRP report 103 [6]
RadiationEnergyWR (formerly Q)
x-rays, gamma rays,
beta rays, muons
 1
neutrons < 1 MeV2.5 + 18.2·e−[ln(E)]²/6
1 MeV - 50 MeV5.0 + 17.0·e−[ln(2·E)]²/6
> 50 MeV2.5 + 3.25·e−[ln(0.04·E)]²/6
protons, charged pions  2
alpha rays,
Nuclear fission products,
heavy nuclei
 20

Effective dose

Effective dose is the central dose quantity for radiological protection used to specify exposure limits to ensure that the occurrence of stochastic health effects is kept below unacceptable levels and that tissue reactions are avoided. [7]

It is difficult to compare the stochastic risk from localised exposures of different parts of the body (e.g. a chest x-ray compared to a CT scan of the head), or to compare exposures of the same body part but with different exposure patterns (e.g. a cardiac CT scan with a cardiac nuclear medicine scan). One way to avoid this problem is to simply average out a localised dose over the whole body. The problem of this approach is that the stochastic risk of cancer induction varies from one tissue to another.

The effective dose E is designed to account for this variation by the application of specific weighting factors for each tissue (WT). Effective dose provides the equivalent whole body dose that gives the same risk as the localised exposure. It is defined as the sum of equivalent doses to each organ (HT), each multiplied by its respective tissue weighting factor (WT).

Weighting factors are calculated by the International Commission for Radiological Protection (ICRP), based on the risk of cancer induction for each organ and adjusted for associated lethality, quality of life and years of life lost. Organs that are remote from the site of irradiation will only receive a small equivalent dose (mainly due to scattering) and therefore contribute little to the effective dose, even if the weighting factor for that organ is high.

Effective dose is used to estimate stochastic risks for a ‘reference’ person, which is an average of the population. It is not suitable for estimating stochastic risk for individual medical exposures, and is not used to assess acute radiation effects.

Weighting factors for different organs [8]
OrgansTissue weighting factors
ICRP30(I36)
1979
ICRP60(I3)
1991
ICRP103(I6)
2008
Gonads 0.250.200.08
Red Bone Marrow 0.120.120.12
Colon -0.120.12
Lung 0.120.120.12
Stomach -0.120.12
Breasts 0.150.050.12
Bladder -0.050.04
Liver -0.050.04
Oesophagus -0.050.04
Thyroid 0.030.050.04
Skin -0.010.01
Bone surface0.030.010.01
Salivary glands --0.01
Brain --0.01
Remainder of body0.300.050.12

Dose versus source or field strength

Radiation dose refers to the amount of energy deposited in matter and/or biological effects of radiation, and should not be confused with the unit of radioactive activity (becquerel, Bq) of the source of radiation, or the strength of the radiation field (fluence). The article on the sievert gives an overview of dose types and how they are calculated. Exposure to a source of radiation will give a dose which is dependent on many factors, such as the activity, duration of exposure, energy of the radiation emitted, distance from the source and amount of shielding.

Background radiation

The worldwide average background dose for a human being is about 3.5 mSv per year , mostly from cosmic radiation and natural isotopes in the earth. The largest single source of radiation exposure to the general public is naturally occurring radon gas, which comprises approximately 55% of the annual background dose. It is estimated that radon is responsible for 10% of lung cancers in the United States.

Calibration standards for measuring instruments

Because the human body is approximately 70% water and has an overall density close to 1 g/cm3, dose measurement is usually calculated and calibrated as dose to water.

National standards laboratories such as the National Physical Laboratory, UK (NPL) provide calibration factors for ionization chambers and other measurement devices to convert from the instrument's readout to absorbed dose. The standards laboratories operates as a primary standard, which is normally calibrated by absolute calorimetry (the warming of substances when they absorb energy). A user sends their secondary standard to the laboratory, where it is exposed to a known amount of radiation (derived from the primary standard) and a factor is issued to convert the instrument's reading to that dose. The user may then use their secondary standard to derive calibration factors for other instruments they use, which then become tertiary standards, or field instruments.

The NPL operates a graphite-calorimeter for absolute photon dosimetry. Graphite is used instead of water as its specific heat capacity is one-sixth that of water and therefore the temperature increase in graphite is 6 times higher than the equivalent in water and measurements are more accurate. Significant problems exist in insulating the graphite from the surrounding environment in order to measure the tiny temperature changes. A lethal dose of radiation to a human is approximately 10–20 Gy. This is 10-20 joules per kilogram. A 1 cm3 piece of graphite weighing 2 grams would therefore absorb around 20–40 mJ. With a specific heat capacity of around 700 J·kg−1·K−1, this equates to a temperature rise of just 20 mK.

Dosimeters in radiotherapy (linear particle accelerator in external beam therapy) are routinely calibrated using ionization chambers [9] or diode technology or gel dosimeters. [10]

The following table shows radiation quantities in SI and non-SI units.

Ionising radiation related quantities view    talk    edit
QuantityUnitSymbolDerivationYear SI equivalence
Activity (A) becquerel Bqs−11974SI unit
curie Ci3.7 × 1010 s−119533.7×1010 Bq
rutherford Rd106 s−119461,000,000 Bq
Exposure (X) coulomb per kilogram C/kgC⋅kg−1 of air1974SI unit
röntgen R esu / 0.001293 g of air19282.58 × 10−4 C/kg
Absorbed dose (D) gray Gy J⋅kg−11974SI unit
erg per gramerg/gerg⋅g−119501.0 × 10−4 Gy
rad rad100 erg⋅g−119530.010 Gy
Dose equivalent (H) sievert SvJ⋅kg−1 × WR 1977SI unit
röntgen equivalent man rem100 erg⋅g−119710.010 Sv

Although the United States Nuclear Regulatory Commission permits the use of the units curie, rad, and rem alongside SI units, [11] the European Union European units of measurement directives required that their use for "public health ... purposes" be phased out by 31 December 1985. [12]

Radiation exposure monitoring

Records of legal dosimetry results are usually kept for a set period of time, depending upon the legal requirements of the nation in which they are used.

Medical radiation exposure monitoring is the practice of collecting dose information from radiology equipment and using the data to help identify opportunities to reduce unnecessary dose in medical situations.

See also

Related Research Articles

Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources.

Sievert SI derived unit of equivalent dose of ionizing radiation

The sievert is a derived unit of ionizing radiation dose in the International System of Units (SI) and is a measure of the health effect of low levels of ionizing radiation on the human body. The sievert is of importance in dosimetry and radiation protection, and is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dose measurement and research into the biological effects of radiation.

The gray is a derived unit of ionizing radiation dose in the International System of Units (SI). It is defined as the absorption of one joule of radiation energy per kilogram of matter.

Equivalent dose is a dose quantity H representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the physical quantity absorbed dose, but also takes into account the biological effectiveness of the radiation, which is dependent on the radiation type and energy. In the SI system of units, the unit of measure is the sievert (Sv).

The roentgen equivalent man is an older, CGS unit of equivalent dose, effective dose, and committed dose which are measures of the health effect of low levels of ionizing radiation on the human body.

Kerma is an acronym for "kinetic energy released per unit mass", defined as the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation in a sample of matter, divided by the mass of the sample. It is defined by the quotient .

The rad is a unit of absorbed radiation dose, defined as 1 rad = 0.01 Gy = 0.01 J/kg. It was originally defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by one gram of matter. The material absorbing the radiation can be human tissue or silicon microchips or any other medium.

Radiobiology 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.

In radiobiology, the relative biological effectiveness 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 particles, energies involved, and which biological effects are deemed relevant.

Committed dose equivalent and Committed effective dose equivalent are dose quantities used in the United States system of Radiological Protection for irradiation due to an internal source.

Roentgen (unit) legacy unit of measurement for the kerma of X-rays and gamma rays up to 3 MeV; equals 0.258 mC/kg

The roentgen or röntgen is a legacy unit of measurement for the exposure of X-rays and gamma rays, and is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. In 1928, it was adopted as the first international measurement quantity for ionising radiation to be defined for radiation protection, as it was then the most easily replicated method of measuring air ionization by using ion chambers. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays.

Effective dose is a dose quantity in the International Commission on Radiological Protection (ICRP) system of radiological protection.

The committed dose in radiological protection is a measure of the stochastic health risk due to an intake of radioactive material into the human body. Stochastic in this context is defined as the probability of cancer induction and genetic damage, due to low levels of radiation. The SI unit of measure is the sievert.

References

  1. ICRP pub 103 para 138
  2. C Baldock, Y De Deene, S Doran, G Ibbott, A Jirasek, M Lepage, KB McAuley, M Oldham, LJ Schreiner 2010. Polymer gel dosimetry. Physics in Medicine and Biology 55 (5) R1
  3. 1 2 International Commission on Radiation Units and Measurements (ICRU).Options for Characterizing Energy Deposition. Journal of the ICRU Vol 11 No 2 (2011) Report 86
  4. Feinendegen LE. The cell dose concept; potential application in radiation protection. 1990 Phys. Med. Biol. 35 597
  5. The ICRP says "In the low dose range, below about 100 mSv, it is scientifically plausible to assume that the incidence of cancer or heritable effects will rise in direct proportion to an increase in the equivalent dose in the relevant organs and tissues" ICRP publication 103 paragraph 64
  6. "The 2007 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 103. 37 (2–4). 2007. ISBN   978-0-7020-3048-2. Archived from the original on 16 November 2012. Retrieved 17 May 2012.
  7. ICRP publication 103, paragraph 112
  8. UNSCEAR-2008 Annex A page 40, table A1, retrieved 2011-7-20
  9. Hill R, Mo Z, Haque M, Baldock C, 2009. An evaluation of ionization chambers for the relative dosimetry of kilovoltage x-ray beams. Medical Physics. 36 3971-3981.
  10. Baldock C, De Deene Y, Doran S, Ibbott G, Jirasek A, Lepage M, McAuley KB, Oldham M, Schreiner LJ, 2010. Polymer gel dosimetry. Phys. Med. Biol. 55 R1–R63.
  11. 10 CFR 20.1004. US Nuclear Regulatory Commission. 2009.
  12. The Council of the European Communities (1979-12-21). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC" . Retrieved 19 May 2012.