Gray (unit)

# Gray (unit)

Overview
The gray is the SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...

Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

(for example, X-rays), and is defined as the absorption of one joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

by one kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

(usually human tissue).
Discussion

Encyclopedia
The gray is the SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...

Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

(for example, X-rays), and is defined as the absorption of one joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

by one kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

(usually human tissue).

Named after the British physicist Louis Harold Gray
Louis Harold Gray
Louis Harold Gray was a British physicist who worked mainly on the effects of radiation on biological systems, inventing the field of radiobiology as he went. A summary of his work is given below...

The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by...

(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
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

authors.

## Definition

One gray is the absorption of one joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

of energy, in the form of ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

, divided by one kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

.

For xrays and gamma rays, these are the same units as the sievert
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

(Sv). To avoid any risk of confusion between the absorbed dose
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...

(by matter) and the equivalent dose
Equivalent dose
The equivalent absorbed radiation dose, usually shortened to equivalent dose, is a computed average measure of the radiation absorbed by a fixed mass of biological tissue, that attempts to account for the different biological damage potential of different types of ionizing radiation...

(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.
The gray was defined in 1975 in honour of Louis Harold Gray
Louis Harold Gray
Louis Harold Gray was a British physicist who worked mainly on the effects of radiation on biological systems, inventing the field of radiobiology as he went. A summary of his work is given below...

(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
Röntgen
The roentgen is a unit of measurement for exposure to ionizing radiation , and is named after the German physicist Wilhelm Röntgen...

## Effect on the body

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
Sievert
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

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
Radiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...

, epilation
EPilation
EPilation is the first full-length release from Brisbane musician Tara Simmons. Although a full-length, EPilation contains no new material and is a compilation of Tara's first two EPs Pendulum and All The Amendments...

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 gland
Salivary gland
The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose...

s 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
Xerostomia
Xerostomia is the medical term for the subjective complaint of dry mouth due to a lack of saliva. Xerostomia is sometimes colloquially called pasties, cottonmouth, drooth, or doughmouth. Several diseases, treatments, and medications can cause xerostomia. It can also be exacerbated by smoking or...

) and dry eyes (xerophthalmia
Xerophthalmia
Xerophthalmia is a medical condition in which the eye fails to produce tears. It may be caused by a deficiency in vitamin A and is sometimes used to describe that lack, although there may be other causes....

) 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.

Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

during the first trimester primarily affect its heart and limbs, while later in pregnancy the brain is the major affected site.

## Dose by source

Radiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...

, 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.

## Roentgen

The roentgen is defined as the radiation exposure equal to the quantity of ionizing radiation that will produce one esu
Statcoulomb
The statcoulomb or franklin or electrostatic unit of charge is the physical unit for electrical charge used in the centimetre-gram-second system of units and Gaussian units. It is a derived unit given by...

of charge in one cubic centimetre
Cubic centimetre
A cubic centimetre is a commonly used unit of volume extending the derived SI-unit cubic metre, and corresponds to the volume of a cube measuring 1 cm × 1 cm × 1 cm...

of dry air at 0 °C and a standard atmosphere
Standard atmosphere
Standard atmosphere may refer to:* A standard reference value for air pressure:** Atmosphere , an approximation of the value at sea level** Atmospheric pressure, other reference values* A model of how atmospheric pressure varies with altitude:...

, 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.

• Dose area product
Dose area product
Dose area product is a quantity used in assessing the radiation risk from diagnostic X-rays examination and interventional procedures. It reflects not only the dose within the radiation field but also the area of tissue irradiated. Therefore, it may be a better indicator of the overall risk of...

(Gy·cm2)
• International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

base units
• SI derived unit
SI derived unit
The International System of Units specifies a set of seven base units from which all other units of measurement are formed, by products of the powers of base units. These other units are called SI derived units, for example, the SI derived unit of area is square metre , and of density is...

to which Gray is part