Absorbed, Equivalent, and Effective Dose

Radiation dose is a measure of the amount of exposure to radiation. There are three kinds of dose in radiological protection. Absorbed dose is a measureable, physical quantity, while equivalent dose and effective dose are specifically for radiological protection purposes.

Effective dose in particular is a central feature of radiological protection. It sums up any number of different exposures into a single number that reflects, in a general way, the overall risk. The concept may be complex, but it makes radiological protection practical to implement.

Absorbed Dose

Absorbed dose is the amount of energy deposited by radiation in a mass.

The mass can be anything: water, rock, air, people, etc.

Absorbed dose is expressed in milligrays (mGy).

Equivalent Dose

Equivalent dose is calculated for individual organs.

It is based on the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation.

Equivalent dose is expressed in millisieverts (mSv) to an organ.

Effective Dose

Effective dose is calculated for the whole body.

It is the addition of equivalent doses to all organs, each adjusted to account for the sensitivity of the organ to radiation.

Effective dose is expressed in millisieverts (mSv).

More Details

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Absorbed dose is a measurable, physical quantity.

It is expressed in grays (Gy), or, more frequently milligrays (mGy), which are 1/1000th of a gray.

1 gray = 1 joule of energy deposited in 1 kilogram of material i.e. 1 Gy = 1 J/kg.

Equivalent dose = absorbed Dose multiplied the appropriate radiation weighting factor.

The radiation weighting factors are needed because different types of radiation (like alpha, beta, gamma, and neutrons) can have different effects even if the absorbed dose is the same.

Equivalent dose is expressed in sieverts (Sv), or, more frequently, millisieverts (mSv) which are 1/1000th of a sievert, and the organ should always be specified (for example "25 mSv to the skin").

In the simplest cases, for gamma (photon) and beta (electron) radiation, the radiation weighting factor is 1, and therefore, for example, an absorbed dose of 1 mGy in an organ equals an equivalent dose of 1 mSv to that organ.

Effective dose = sum for all organs of (equivalent dose to the organ times the appropriate tissue weighting factor)

The tissue weighting factors are needed because different organs have different levels of sensitivity to radiation, even if the equivalent dose is the same.

Effective dose is expressed in sieverts (Sv), or, more frequently, millisieverts (mSv) which are 1/1000th of a sievert. This is the most frequently used dose in radiological protection. Unless you see mention of a specific organ, a "dose" in Sv or mSv is the effective dose.

In the simplest cases, for uniform whole-body exposure to gamma (photon) or beta (electron) radiation, the radiation weighting factor is 1, and the tissue weighting factors add up to 1, and therefore, for example, an absorbed dose of 1 mGy equals an effective dose of 1 mSv.

Quotes from ICRP Publications

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ICRP Publication 103 paragraphs 107, 108, and 109

In radiation biology, clinical radiology, and radiological protection the absorbed dose, D, is the basic physical dose quantity, and it is used for all types of ionising radiation and any irradiation geometry. It is defined as ... the mean energy imparted to matter ... [divided by the mass]

The SI unit of absorbed dose is J kg-1 and its special name is gray (Gy) ... While it is defined at any point in matter, its value is obtained as an average over a mass ... Absorbed dose is a measurable quantity and primary standards exist to determine its value.

When using the quantity absorbed dose in practical protection applications, doses are averaged over tissue volumes. It is assumed that, for low doses, the mean value of absorbed dose averaged over a specific organ or tissue can be correlated with radiation detriment for stochastic effects in that tissue with an accuracy sufficient for the purposes of radiological protection.

ICRP Publication 103 paragraph 112

The protection quantities are 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. The definition of the protection quantities is based on the average absorbed dose, DT,R in the volume of a specified organ or tissue T (see Table 3), due to radiation of type R (see Table 2). The radiation R is given by the type and energy of radiation either incident on the body or emitted by radionuclides residing within it. The protection quantity equivalent dose in an organ or tissue, HT, is then defined by

where wR is the radiation weighting factor for radiation R. The sum is performed over all types of radiations involved. The unit of equivalent dose is J kg-1 and has the special name sievert (Sv).

ICRP Publication 103 Table 2

ICRP Publication 103 Figure 1

ICRP Publication 103 paragraph 101

... The development of ... effective dose has made a significant contribution to radiological protection as it has enabled doses to be summed from whole and partial body exposure from external radiation of various types and from intakes of radionuclides.

ICRP Publication 103 paragraph 125

The effective dose, E, ... is defined by a weighted sum of tissue equivalent doses as:

where wT is the tissue weighting factor for tissue T and ΣwT = 1. The sum is performed over all organs and tissues of the human body considered to be sensitive to the induction of stochastic effects. These wT values are chosen to represent the contributions of individual organs and tissues to overall radiation detriment from stochastic effects. The unit of effective dose is J kg-1 with the special name sievert (Sv). The unit is the same for equivalent dose and effective dose ... Care must be taken to ensure that the quantity being used is clearly stated.

ICRP Publication 103 Table 3

Read on to learn about Dose limits

See Also

• 

  ICRPædia Guide to the System of Radiological Protection
• 

  ICRP Publication 103 The 2007 Recommendations of ICRP