Compartmental modelling of dendrites

File:SI Radiation dose units.png

Graphic showing relationship of SI radiation dose units

The effective dose in radiation protection and radiology is a measure of the cancer risk to a whole organism due to ionizing radiation delivered non-uniformly to part(s) of its body. It takes into account both the type of radiation and the nature of each organ being irradiated. 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 measurement of effective dose is required because equivalent dose does not consider the type and amount of tissue exposed. An equivalent dose applied to only a portion of the body will carry lower risk than if the same equivalent dose was applied to 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. An effective dose will carry the same overall effective risk to the whole organism 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.

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.

Computation

Weighting factors for different organs

Organs	Tissue weighting factors
	ICRP26 1977	ICRP60 1990	ICRP103 2007
Gonads	0.25	0.20	0.08
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
Bladder	-	0.05	0.04
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

A uniform field of penetrating radiation deposits the same amount of energy per unit mass in any organic material it hits. This is the absorbed dose, a quantity that is independent of body weight, organ size or tissue mass. To obtain an effective dose, the absorbed dose is first corrected for the radiation type to give the equivalent dose, and then corrected for the tissue receiving the radiation. 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.

The sum of effective doses to each region of the body add up to the whole-body effective dose for the body. If only part of the body is radiated, then only the regions radiated are used to add up an effective dose which can be compared to organisms that receive an equivalent dose uniformly over its whole body. Effective doses can be calculated and given out for specific procedures, such as a head CT scan.

More precisely, the effective dose of radiation (E) is found by calculating a weighted average of the equivalent dose (H_T) in different body tissues, with the weighting factors (W_T) designed to reflect the different importance of tissue types to the danger to the whole organism. Where the dose is only applied to a portion of a tissue or organ, it needs to be averaged across the entire mass of the tissue or organ in order to be representative of that tissue type.

$E=\sum _T{W}_T\cdot H_T=\sum _T{W}_T\sum _R{W}_R\cdot {\overline{D}}_{T,R}$.

$E=\sum _T{W}_T\sum _R{W}_R\cdot \frac{\underset{T}{\int }{D}_R(x,y,z)\rho (x,y,z)dV}{\underset{T}{\int }\rho (x,y,z)dV}$

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

${\overline{D}}_{T,R}$ 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 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 tissue weighting factors always add up 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 organism.

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.

The effective dose is a reasonable way to assess the health effects of beta and alpha radiation on the skin and eyes, or the effects of radiotherapy applied selectively to a part of the body. Effective doses are important in the calculation of committed dose due to internal exposures. Because background radiation has inhalation and ingestion components committed dose is required to calculate the dose from background radiation. The effective dose is not intended as a measure of acute or threshold effects of radiation exposure such as erythema, radiation sickness or death.

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.

In some situations the effective dose gives the best estimate of the excess relative risk of cancer. The Biologic Effects of Ionizing Radiation (BEIR) reports contain the relationship between dose and cancer calculated largely from dose reconstructions for A-bomb survivors.

History

The concept of effective dose was introduced in 1975. 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

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See also

• Radioactivity
• Collective dose
• Total effective dose equivalent
• Deep-dose equivalent
• Cumulative dose
• Committed dose equivalent
• Committed effective dose equivalent