Effective dose (radiation)

Usage for external dose

The calculation of effective dose is required for partial or non-uniform irradiation of the human body because equivalent dose does not consider the tissue irradiated, but only the radiation type. Various body tissues react to ionising radiation in different ways, so the ICRP has assigned sensitivity factors to specified tissues and organs so that the effect of partial irradiation can be calculated if the irradiated regions are known.^[10] 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 cannot be 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.

Usage for internal dose

Effective dose can be calculated for committed dose which is the internal dose resulting from inhaling, ingesting, or injecting radioactive materials.

The dose quantity used is:

Committed effective dose, E(t) is the sum of the products of the committed organ or tissue equivalent doses and the appropriate tissue weighting factors W_T, where t is the integration time in years following the intake. The commitment period is taken to be 50 years for adults, and to age 70 years for children.^[11]

Use of tissue weighting factor W_T

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 disproportionately 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.

Calculating from the equivalent dose:

${\displaystyle E=\sum _{T}W_{T}\cdot H_{T}=\sum _{T}W_{T}\sum _{R}W_{R}\cdot {\bar {D}}_{T,R}}$.

Calculating from the absorbed dose:

${\displaystyle E=\sum _{T}W_{T}\sum _{R}W_{R}\cdot {\frac {\int _{T}D_{R}(x,y,z)\rho (x,y,z)dV}{\int _{T}\rho (x,y,z)dV}}}$

Where

${\displaystyle E}$ is the effective dose to the entire organism

${\displaystyle H_{T}}$ is the equivalent dose absorbed by tissue T

${\displaystyle W_{T}}$ is the tissue weighting factor defined by regulation

${\displaystyle W_{R}}$ is the radiation weighting factor defined by regulation

${\displaystyle {\bar {D}}_{T,R}}$ is the mass-averaged absorbed dose in tissue T by radiation type R

${\displaystyle D_{R}(x,y,z)}$ is the absorbed dose from radiation type R as a function of location

${\displaystyle \rho (x,y,z)}$ is the density as a function of location

${\displaystyle V}$ is volume

${\displaystyle T}$ is the tissue or organ of interest

The ICRP tissue weighting factors 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 United States Nuclear Regulatory Commission still uses the ICRP's 1977 tissue weighting factors in their regulations, despite the ICRP's later revised recommendations.^[15]

UK regulations

The UK Ionising Radiations Regulations 1999 defines its usage of the term effective dose; "Any reference to an effective dose means the sum of the effective dose to the whole body from external radiation and the committed effective dose from internal radiation."^[19]

US effective dose equivalent

The US Nuclear Regulatory Commission has retained in the US regulation system the older term effective dose equivalent to refer to a similar quantity to the ICRP effective dose. The NRC's total effective dose equivalent (TEDE) is the sum of external effective dose with internal committed dose; in other words all sources of dose.

In the US, cumulative equivalent dose due to external whole-body exposure is normally reported to nuclear energy workers in regular dosimetry reports.

• 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