Gray (unit)
gray | |
---|---|
SI | |
Unit of | absorbed dose of ionizing radiation |
Symbol | Gy |
Named after | Louis Harold Gray |
Conversions | |
1 Gy in ... | ... is equal to ... |
rad |
The gray (symbol: Gy) is the unit of ionizing radiation dose in the International System of Units (SI), defined as the absorption of one joule of radiation energy per kilogram of matter.[1]
It is used as a unit of the radiation quantity
The gray is also used in radiation metrology as a unit of the radiation quantity kerma; defined as the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation[a] in a sample of matter per unit mass. The unit was named after British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue.[2]
The gray was adopted as part of the International System of Units in 1975. The corresponding
Applications
The gray has a number of fields of application in measuring dose:
Radiobiology
The measurement of absorbed dose in tissue is of fundamental importance in radiobiology and radiation therapy as it is the measure of the amount of energy the incident radiation deposits in the target tissue. The measurement of absorbed dose is a complex problem due to scattering and absorption, and many specialist dosimeters are available for these measurements, and can cover applications in 1-D, 2-D and 3-D.[4][5][6]
In radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy, while lymphomas are treated with 20 to 40 Gy. Preventive (adjuvant) doses are typically around 45–60 Gy in 1.8–2 Gy fractions (for breast, head, and neck cancers).
The average radiation dose from an abdominal X-ray is 0.7 millisieverts (0.0007 Sv), that from an abdominal
Radiation protection
The absorbed dose also plays an important role in radiation protection, as it is the starting point for calculating the stochastic health risk of low levels of radiation, which is defined as the probability of cancer induction and genetic damage.[8] The gray measures the total absorbed energy of radiation, but the probability of stochastic damage also depends on the type and energy of the radiation and the types of tissues involved. This probability is related to the equivalent dose in sieverts (Sv), which has the same dimensions as the gray. It is related to the gray by weighting factors described in the articles on equivalent dose and effective dose.
The
The accompanying diagrams show how absorbed dose (in grays) is first obtained by computational techniques, and from this value the equivalent doses are derived. For X-rays and gamma rays the gray is numerically the same value when expressed in sieverts, but for
Radiation poisoning
The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute exposure to high levels of ionizing radiation. These are effects that are certain to happen, as opposed to the uncertain effects of low levels of radiation that have a probability of causing damage. A whole-body acute exposure to 5 grays or more of high-energy radiation usually leads to death within 14 days. LD1 is 2.5 Gy, LD50 is 5 Gy and LD99 is 8 Gy.[10] The LD50 dose represents 375 joules for a 75 kg adult.
Absorbed dose in matter
The gray is used to measure absorbed dose rates in non-tissue materials for processes such as
Kerma
Kerma ("kinetic energy released per unit mass") is used in radiation metrology as a measure of the liberated energy of ionisation due to irradiation, and is expressed in grays. Importantly, kerma dose is different from absorbed dose, depending on the radiation energies involved, partially because ionization energy is not accounted for. Whilst roughly equal at low energies, kerma is much higher than absorbed dose at higher energies, because some energy escapes from the absorbing volume in the form of bremsstrahlung (X-rays) or fast-moving electrons.
Kerma, when applied to air, is equivalent to the legacy roentgen unit of radiation exposure, but there is a difference in the definition of these two units. The gray is defined independently of any target material, however, the roentgen was defined specifically by the ionisation effect in dry air, which did not necessarily represent the effect on other media.
Development of the absorbed dose concept and the gray
Wilhelm Röntgen discovered X-rays on November 8, 1895, and their use spread very quickly for medical diagnostics, particularly broken bones and embedded foreign objects where they were a revolutionary improvement over previous techniques.
Due to the wide use of X-rays and the growing realisation of the dangers of ionizing radiation, measurement standards became necessary for radiation intensity and various countries developed their own, but using differing definitions and methods. Eventually, in order to promote international standardisation, the first International Congress of Radiology (ICR) meeting in London in 1925, proposed a separate body to consider units of measure. This was called the International Commission on Radiation Units and Measurements, or ICRU,[b] and came into being at the Second ICR in Stockholm in 1928, under the chairmanship of Manne Siegbahn.[11][12][c]
One of the earliest techniques of measuring the intensity of X-rays was to measure their ionising effect in air by means of an air-filled
In 1940, Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with
In the late 1950s, the CGPM invited the ICRU to join other scientific bodies to work on the development of the
The adoption of the gray by the 15th General Conference on Weights and Measures as the unit of measure of the absorption of ionizing radiation, specific energy absorption, and of kerma in 1975[17] was the culmination of over half a century of work, both in the understanding of the nature of ionizing radiation and in the creation of coherent radiation quantities and units.
The following table shows radiation quantities in SI and non-SI units.
Quantity | Unit | Symbol | Derivation | Year | SI equivalent |
---|---|---|---|---|---|
Activity (A) | becquerel | Bq | s−1 | 1974 | SI unit |
curie | Ci | 3.7 × 1010 s−1 | 1953 | 3.7×1010 Bq | |
rutherford | Rd | 106 s−1 | 1946 | 1,000,000 Bq | |
Exposure (X) | coulomb per kilogram | C/kg | C⋅kg−1 of air | 1974 | SI unit |
röntgen | R | esu / 0.001293 g of air | 1928 | 2.58 × 10−4 C/kg | |
Absorbed dose (D) | gray | Gy | J⋅kg−1 | 1974 | SI unit |
erg per gram | erg/g | erg⋅g−1 | 1950 | 1.0 × 10−4 Gy | |
rad
|
rad | 100 erg⋅g−1 | 1953 | 0.010 Gy | |
Equivalent dose (H) | sievert | Sv | J⋅kg−1 × WR | 1977 | SI unit |
röntgen equivalent man | rem | 100 erg⋅g−1 × WR | 1971 | 0.010 Sv | |
Effective dose (E) | sievert | Sv | J⋅kg−1 × WR × WT | 1977 | SI unit |
röntgen equivalent man | rem | 100 erg⋅g−1 × WR × WT | 1971 | 0.010 Sv |
See also
- Dose area product (Gy·cm2)
- International System of Units base units
- Orders of magnitude (radiation)
- Order of magnitude (unit)
- Rad (radiation unit)
- Roentgen equivalent man
- SI derived unit
- Sievert, SI derived unit of dose equivalent radiation
Notes
References
- BIPM). Retrieved 2010-01-31.
- ^ "Rays instead of scalpels". LH Gray Memorial Trust. 2002. Retrieved 2012-05-15.
- ^ "NIST Guide to SI Units – Units temporarily accepted for use with the SI". NIST. National Institute of Standards and Technology. 2 July 2009.
- S2CID 4393848.
- S2CID 18082594.
- PMID 20150687.
- ^ "X-Ray Risk". www.xrayrisk.com.
- S2CID 73326646. ICRP publication 103.
- ^ "CIPM, 2002: Recommendation 2". BIPM.
- ^ "Lethal dose". European Nuclear Society. 5 June 2019.
- S2CID 74656044.
- ^ "About ICRU - History". International Commission on Radiation Units & Measures. Retrieved 2012-05-20.
- ^ LCCN 60014734. Retrieved 2012-05-15.
- ISBN 0-521-22436-5. Retrieved 2012-05-15.
- ISBN 978-3-642-00737-8. Retrieved 2012-05-14.
- ^ "CCU: Consultative Committee for Units". International Bureau of Weights and Measures (BIPM). Retrieved 2012-05-18.
- ISBN 92-822-2213-6, archived(PDF) from the original on 2021-06-04, retrieved 2021-12-16
External links
- Boyd, M.A. (March 1–5, 2009). The Confusing World of Radiation Dosimetry—9444 (PDF). WM2009 Conference (Waste Management Symposium). Phoenix, AZ. Archived from the original (PDF) on 2016-12-21. Retrieved 2014-07-07. An account of chronological differences between USA and ICRP dosimetry systems.