External dose quantities used in radiation protection and dosimetry

The radiation dose quantity effective dose is the tissue-weighted sum of the equivalent doses in all specified tissues and organs of the body and represents the stochastic risk (probability of cancer induction and genetic effects) of ionizing radiation delivered non-uniformly to body parts. [1] [2] It takes into account the type of radiation and the nature of each organ or tissue being irradiated. It is the central quantity for dose assessments in radiological protection. [3]

The calculation of effective dose is required because equivalent dose does not consider the type of tissue irradiated, but only the radiation type. Various body tissues react differently to ionizing radiation, so the International Commission on Radiological Protection (ICRP) has assigned sensitivity factors to specified tissues and organs of the human body so that the effect of partial or non-uniform irradiation can be calculated if the irradiated regions are known.[4] A radiation field irradiating only a portion of the body will carry lower risk than if the same field irradiated the whole body. To take this into account, the effective doses to the component parts of the body which have been irradiated are calculated and summed. This becomes the effective dose for the whole body, dose quantity E. It is a "protection" dose quantity which can be calculated, but not measured in practice.

An effective dose will carry the same effective risk to the whole body regardless of where it was applied, and it will carry the same effective risk as the same amount of equivalent dose applied uniformly to the whole body.

The effective dose is not intended as a measure of deterministic or other effects of radiation although it is used to estimate inherited effects.[5]

The SI unit for effective dose is the sievert (Sv) which is one joule/kilogram (J/kg). The older unit, the rem is still used, primarily in the USA. The effective dose replaces the former effective dose equivalent, and is sometimes incorrectly called the dose equivalent, but should not be confused with the equivalent dose.[6]

Graphic showing relationship of SI radiation dose units

## Use

According to the ICRP, the main uses of effective dose are the prospective dose assessment for planning and optimisation in radiological protection, and demonstration of compliance with dose limits for regulatory purposes. [7] The ICRP also states that 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.[8]

## Calculation of effective dose

Ionising radiation deposits energy in the matter being irradiated. The quantity used to express this is the absorbed dose, a physical quantity that is independent of radiation type and tissue type. To allow consideration of the radiological risk, the dose quantities equivalent dose and effective dose have been devised by the International Commission on Radiation Units and Measurements (ICRU) and the ICRP to calculate the stochastic biological effect of the absorbed dose.

To obtain an effective dose, the absorbed organ dose DT is first corrected for the radiation type using factor WR to give a weighted average of the equivalent dose quantity HT, received in different body tissues, and the result is further corrected for the tissues or organs being irradiated using factor WT, to produce the effective dose quantity E.

The sum of effective doses to all organs and tissues of the body represents the effective dose for the whole body. If only part of the body is irradiated, then only those regions are used to calculate the effective dose. The tissue weighting factors summate to 1.0, so that if an entire body is radiated with uniformly penetrating external radiation, the effective dose for the entire body is equal to the equivalent dose for the entire body.

### Use of tissue weighting factor WT

The ICRP tissue weighting factors are given in the accompanying table, and the equations used to calculate from either absorbed dose or equivalent dose are also given.

Some tissues like bone marrow are particularly sensitive to radiation, so they are given a weighting factor that is disproportionally large relative to the fraction of body mass they represent. Other tissues like the hard bone surface are particularly insensitive to radiation and are assigned a disproportionally low weighting factor.

Weighting factors for different tissues[9]
Organs Tissue weighting factors
ICRP26
1977
ICRP60
1990[10]
ICRP103
2007[11]
Red Bone Marrow 0.12 0.12 0.12
Colon - 0.12 0.12
Lung 0.12 0.12 0.12
Stomach - 0.12 0.12
Breasts 0.15 0.05 0.12
Liver - 0.05 0.04
Oesophagus - 0.05 0.04
Thyroid 0.03 0.05 0.04
Skin - 0.01 0.01
Bone surface 0.03 0.01 0.01
Salivary glands - - 0.01
Brain - - 0.01
Remainder of body 0.30 0.05 0.12
Total 1.00 1.00 1.00

Calculating from the equivalent dose:

$E = \sum_T W_T \cdot H_T = \sum_T W_T \sum_R W_R \cdot \bar{D}_{T,R}$.

Calculating from the absorbed dose:

$E = \sum_T W_T \sum_R W_R \cdot \frac{\int_{T}D_R (x,y,z)\rho(x,y,z)dV}{\int_{T}\rho(x,y,z)dV}$

Where

$E$ is the effective dose to the entire organism
$H_T$ is the equivalent dose absorbed by tissue T
$W_T$ is the tissue weighting factor defined by regulation
$W_R$ is the radiation weighting factor defined by regulation
$\bar{D}_{T,R}$ is the mass-averaged absorbed dose in tissue T by radiation type R
$D_R (x,y,z)$ is the absorbed dose from radiation type R as a function of location
$\rho(x,y,z)$ is the density as a function of location
$V$ is volume
$T$ is the tissue or organ of interest

The ICRP tissue weighting factors (not to be confused with relative biological effectiveness factors used to calculate equivalent dose) are chosen to represent the fraction of health risk, or biological effect, which is attributable to the specific tissue named. These weighting factors have been revised twice, as shown in the chart above.

The US Nuclear Regulatory Commission still endorses the ICRP's 1977 tissue weighting factors in their regulations, in spite of the ICRP's later revised recommendations.[12]

## Related Quantities

### Effective Dose Equivalent

The US Nuclear Regulatory Commission has encoded into regulation the older term effective dose equivalent to refer to effective dose. The NRC's total effective dose equivalent (TEDE) is a sum of effective or equivalent doses with committed dose.

### Cumulative equivalent

Cumulative equivalent dose due to external whole-body exposure is normally reported to nuclear energy workers in regular dosimetry reports. In the US, three different effective doses are typically reported:

• deep-dose equivalent, (DDE) which is properly a whole-body equivalent dose
• shallow dose equivalent, (SDE) which is actually the effective dose to the skin
• eye dose equivalent (not defined in the NRC glossary)

### Increased Cancer Risk

One sievert (100 rem) of effective dose carries with it a 4% chance of developing a fatal cancer in an average adult, and a 0.8% chance of hereditary defect in future offspring. This average is used in radiation protection although the risk to a given exposed individual depends on many factors, particularly age as cancers may develop years or decades after exposure. There is also an increased risk of survivable cancers, including some that will never be diagnosed.

The Biologic Effects of Ionizing Radiation (BEIR) reports contain the relationship between dose and cancer calculated largely from dose reconstructions for A-bomb survivors.[13][14]

## History

The concept of effective dose was introduced in 1975.[15] It was then included in 1977 as “effective dose equivalent” into Publication 26 by the International Commission on Radiological Protection, an international body that provides guidance on the risk caused by radiation. In 1990, ICRP publication 60 shortened the name to "effective dose." This quantity is sometimes incorrectly referred to as the "dose equivalent" because of the earlier name, and that misnomer in turn causes confusion with equivalent dose. The tissue weighting factors were revised in 1990 and 2007 because of new data.

## References

1. ^ ICRP publication 103, glossary
2. ^ ICRP publication 103, para 104 and 105
3. ^ ICRP publication, 103 para 103
4. ^ ICRP publication 103, para 22 & glossary
5. ^ Wrixon, A D (2007). "New ICRP recommendations". Journal of Radiation Protection 28: 161–168. Bibcode:2008JRP....28..161W. doi:10.1088/0952-4746/28/2/R02.
6. ^ "Glossary". Tutorial on the Regulatory Control of Nuclear Power Plants. International Atomic Energy Agency. Retrieved 8 May 2012.
7. ^ ICRP publication 103 executive summary para j
8. ^ ICRP publication 103 para 101
9. ^ UNSCEAR-2008 Annex A page 40, table A1, retrieved 2011-7-20
10. ^ "1990 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 60 21 (1-3). 1991. ISBN 978-0-08-041144-6. Retrieved 17 May 2012.
11. ^ "The 2007 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 103 37 (2-4). 2007. ISBN 978-0-7020-3048-2. Retrieved 17 May 2012.
12. ^ 10 CFR 20.1003. US Nuclear Regulatory Commission. 2009. Retrieved 25 November 2012.
13. ^ "Report in Brief Beir VII: Health Risks from Exposure to Low Levels of Ionizing Radiation". Earth and Life Studies at the National Academies. The National Academies. Retrieved 25 August 2012.
14. ^ Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII – Phase 2. National Academies Press. 2006. p. 248. ISBN 0-309-53040-7.
15. ^ Jacobi W (1975). "The concept of effective dose - A proposal for the combination of organ doses". Radiat. Environ. Biophys (12): 101–109.