Gray (unit)

The gray (symbol: Gy) is the SI unit of absorbed radiation dose of ionizing radiation (for example, X-rays), and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter (usually human tissue).^[1]

Named after the British physicist Louis Harold Gray, the SI unit replaces the traditional cgs unit, the rad (equivalent to 0.01 Gy), which remains common in industry in the United States, while "strongly discouraged" in the style guide for U.S. National Institute of Standards and Technology authors.^[2]

Definition

One gray is the absorption of one joule of energy, in the form of ionizing radiation, divided by one kilogram of matter.

$1 \ \mathrm{Gy} = 1\ \frac{\mathrm{J}}{\mathrm{kg}} = 1\ \mathrm{m}^2\cdot\mathrm{s}^{-2}$

For xrays and gamma rays, these are the same units as the sievert (Sv). To avoid any risk of confusion between the absorbed dose (by matter) and the equivalent dose (by biological tissues), one must use the corresponding special units, gray is used instead of the joule per kilogram for absorbed dose and the sievert instead of the joule per kilogram for the dose equivalent. The word "gray" is both the singular and plural spelling.

This SI unit is named after Louis Harold Gray. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Gy). When an SI unit is spelled out in English, it should always begin with a lower case letter (gray), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

The gray was defined in 1975 in honour of Louis Harold Gray (1905–1965), who used a similar concept, "that amount of neutron radiation which produces an increment of energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one röntgen of radiation," in 1940.

Effect on the body

Main article: Radiation poisoning

The gray measures the deposited energy of radiation. The biological effects vary by the type and energy of the radiation and the organism and tissues involved. The sievert attempts to account for these variations.

A whole-body exposure to 5 or more gray of high-energy radiation at one time usually leads to death within 14 days. This dosage represents 375 joules for a 75 kg adult (equivalent to the chemical energy in 20 mg of sugar). Since gray are such large amounts of radiation, medical use of radiation is typically measured in milligray (mGy).

As experienced from follow-up after radiation therapy, epilation may occur on any hair-bearing skin with doses above 1 Gy. It occurs only within the radiation field/s. Hair loss may be permanent with a single dose of 10 Gy, but if the dose is fractionated permanent hair loss may not occur until dose exceeds 45 Gy. The salivary glands and tear glands have a radiation tolerance of about 30 Gy in 2 Gy fractions, a dose which is exceeded by most radical head and neck cancer treatments, potentially causing dryness. Dry mouth (xerostomia) and dry eyes (xerophthalmia) can become irritating long-term problems and severely reduce the patient's quality of life. Similarly, sweat glands in treated skin (such as the armpit) tend to stop working, and the naturally moist vaginal mucosa is often dry following pelvic irradiation.

Dangerously high doses of radiation received by a fetus during the first trimester primarily affect its heart and limbs, while later in pregnancy the brain is the major affected site.^[3]

Dose by source

In radiation therapy, the amount of radiation 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 1.4 mGy, that from an abdominal CT scan is 8.0 mGy, that from a pelvic CT scan is 25 mGy, and that from a selective CT scan of the abdomen and the pelvis is 30 mGy.^[4]

Roentgen

The roentgen is defined as the radiation exposure equal to the quantity of ionizing radiation that will produce one esu of charge in one cubic centimetre of dry air at 0 °C and a standard atmosphere, and is conventionally taken to be worth 0.258 mC/kg (using a conventional air density of about 1.293 kg/m³). Using an air ionization energy of about 35 J/C, we have 1 Gy ≈ 111 R.

See also

• Dose area product (Gy·cm²)
• International System of Units base units
• SI derived unit to which Gray is part
• Orders of magnitude (radiation)
• Roentgen equivalent man
• Rad (unit)
• Sievert, SI derived unit of dose equivalent radiation

References

1. ^ "The International System of Units (SI)". Bureau International des Poids et Mesures (BIPM). http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf. Retrieved 2010-01-31. 
2. ^ "NIST Guide to SI Units - Units temporarily accepted for use with the SI". http://physics.nist.gov/Pubs/SP811/sec05.html#5.2. 
3. ^ medscape.com >What Risk Dose X-Ray Pose to Fetus? From USMLEasy from McGraw-Hill. Posted: 09/16/2010.
4. ^ Baden, Jeffrey M.; Jay B. Brodsky (1985). The Pregnant surgical patient. London: Futura. ISBN 0-87993-238-4. ^{[page needed]}