The gray is also used in radiation metrology as a unit of the radiation quantitykerma; defined as the sum of the initialkinetic energies of all thecharged particles liberated by unchargedionizing radiation[a] in a sample ofmatter per unit mass. The unit was named after British physicistLouis 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 correspondingcgs unit to the gray is therad (equivalent to 0.01 Gy), which remains common largely in the United States, though "strongly discouraged" in the style guide for U.S.National Institute of Standards and Technology.[3]
The measurement of absorbed dose in tissue is of fundamental importance inradiobiology andradiation 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 solidepithelial tumor ranges from 60 to 80 Gy, whilelymphomas are treated with 20 to 40 Gy. Preventive (adjuvant) doses are typically around 45–60 Gy in 1.8–2 Gyfractions (for breast, head, and neck cancers).
The average radiation dose from a pelvic CT scan is 6 mGy, and that from a selective CT scan of the abdomen and the pelvis is 14 mGy.[7]
Relationship of ICRU/ICRP computed Protection dose quantities and units
The absorbed dose also plays an important role inradiation protection, as it is the starting point for calculating the stochastic health risk of low levels of radiation, which is defined as theprobability 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 theequivalent dose insieverts (Sv), which has the same dimensions as the gray. It is related to the gray by weighting factors described in the articles onequivalent dose andeffective dose.
TheInternational Committee for Weights and Measures states: "In order to avoid any risk of confusion between the absorbed doseD and thedose equivalentH, the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed doseD and the namesievert instead of joules per kilogram for the unit of dose equivalentH."[10]
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 foralpha particles one gray is equivalent to 20 sieverts, and a radiation weighting factor is applied accordingly.
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 arecertain to happen, as opposed to the uncertain effects of low levels of radiation that have aprobability 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.[11] The LD50 dose represents 375 joules for a 75 kg adult.
The gray is used to measure absorbed dose rates in non-tissue materials for processes such asradiation hardening,food irradiation andelectron irradiation. Measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes.
Kerma ("kineticenergyreleased per unitmass") 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 ofbremsstrahlung (X-rays) or fast-moving electrons.
Kerma, when applied to air, is equivalent to the legacyroentgen 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
Using earlyCrookes tube X-Ray apparatus in 1896. One man is viewing his hand with afluoroscope to optimise tube emissions, the other has his head close to the tube. No precautions are being taken.Monument to the X-ray and Radium Martyrs of All Nations erected 1936 at St. Georg hospital in Hamburg, commemorating 359 early radiology workers.
Wilhelm Röntgen discoveredX-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 theInternational Commission on Radiation Units and Measurements, or ICRU,[b] and came into being at the Second ICR in Stockholm in 1928, under the chairmanship ofManne Siegbahn.[12][13][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-filledion chamber. At the first ICRU meeting it was proposed that one unit of X-ray dose should be defined as the quantity of X-rays that would produce oneesu of charge in onecubic centimetre of dry air at 0 °C and 1standard atmosphere of pressure. This unit ofradiation exposure was named the roentgen in honour of Wilhelm Röntgen, who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply togamma radiation.[14] This approach, although a great step forward in standardisation, had the disadvantage of not being a direct measure of the absorption of radiation, and thereby the ionisation effect, in various types of matter including human tissue, and was a measurement only of the effect of the X-rays in a specific circumstance; the ionisation effect in dry air.[15]
In 1940, Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together withWilliam Valentine Mayneord and the radiobiologist John Read, published a paper in which a new unit of measure, dubbed thegram roentgen (symbol: gr) was proposed, and defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation".[16] This unit was found to be equivalent to 88 ergs in air, and made the absorbed dose, as it subsequently became known, dependent on the interaction of the radiation with the irradiated material, not just an expression of radiation exposure or intensity, which the roentgen represented. In 1953 the ICRU recommended therad, equal to 100 erg/g, as the new unit of measure of absorbed radiation. The rad was expressed in coherentcgs units.[14]
In the late 1950s, the CGPM invited the ICRU to join other scientific bodies to work on the development of theInternational System of Units, or SI.[17] The CCU decided to define the SI unit of absorbed radiation as energy deposited by reabsorbed charged particles per unit mass of absorbent material, which is how the rad had been defined, but inMKS units it would be equivalent to the joule per kilogram. This was confirmed in 1975 by the 15th CGPM, and the unit was named the "gray" in honour of Louis Harold Gray, who had died in 1965. The gray was thus equal to 100 rad. Notably, the centigray (numerically equivalent to the rad) is still widely used to describe absolute absorbed doses in radiotherapy.
^Siegbahn, Manne; et al. (October 1929). "Recommendations of the International X-ray Unit Committee".Radiology.13 (4):372–3.doi:10.1148/13.4.372.S2CID74656044.
^"About ICRU - History". International Commission on Radiation Units & Measures. Retrieved2012-05-20.
^abGuill, JH; Moteff, John (June 1960)."Dosimetry in Europe and the USSR".Third Pacific Area Meeting Papers — Materials in Nuclear Applications. Symposium on Radiation Effects and Dosimetry - Third Pacific Area Meeting American Society for Testing Materials, October 1959, San Francisco, 12–16 October 1959. American Society Technical Publication. Vol. 276. ASTM International. p. 64.LCCN60014734. Retrieved2012-05-15.
^Gupta, S. V. (2009-11-19)."Louis Harold Gray".Units of Measurement: Past, Present and Future : International System of Units. Springer. p. 144.ISBN978-3-642-00737-8. Retrieved2012-05-14.
Boyd, M.A. (March 1–5, 2009).The Confusing World of Radiation Dosimetry—9444(PDF). WM2009 Conference (Waste Management Symposium). Phoenix, AZ. Archived fromthe original(PDF) on 2016-12-21. Retrieved2014-07-07. An account of chronological differences between USA and ICRP dosimetry systems.
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