Equivalent dose

Physical size
Surname	Equivalent dose
Formula symbol	${\displaystyle H}$
Derived from	Absorbed dose ${\displaystyle D}$
Size and unit system unit dimension SI Sv L 2 · T −2

The dose equivalent is a dose quantity for radiation exposure through ionizing radiation . It is used in radiation protection and is a measure of the biological effect of exposure with regard to stochastic risks (cancer and hereditary defects), taking into account the type of radiation. The basis is the energy transmitted by the radiation. The different effectiveness of the radiation types involved with regard to stochastic risks is taken into account by weighting factors. The unit of measurement for the dose equivalent is the Sievert (Sv) .

overview

Equivalent dose as a generic term for body doses as well as for dose measurement parameters for external radiation exposure. The derivation from the absorbed dose for a type of radiation is illustrated. If there are different types of radiation, the resulting equivalent doses add up. In practical applications with external radiation exposure, dose measurements are used to estimate the body doses that cannot be directly measured.

The dose equivalent is used to quantify stochastic radiation risks in humans. This takes place in the form of the body dose ( German Radiation Protection Act (StrlSchG) , Section 5 (19)). The body dose is an umbrella term for that

• Organ equivalent dose ( equivalent dose averaged over an organ) (Section 5 (27) StrlSchG) and the
• Effective dose (mean value formed over the entire body from the organ equivalent doses, taking into account the organs' radiation sensitivity) (Section 5 (11) StrlSchG).

The body doses are the subject of legal regulations and limit values ​​are set for them.

However, body doses are not measurable. They have to be estimated from quantities that are accessible to a measurement, in connection with measurement rules and models.

In practical radiation protection, dose measurements are mainly used to estimate body doses in the event of external radiation exposure . These relate to measuring points in accordance with standardized measuring methods and stipulations of the ICRU (see section # Equivalent dose as dose measurement variable ).

For internal radiation exposure , the estimation of body doses is based on model calculations by the ICRP (see section # Equivalent dose as body dose for internal radiation exposure ).

All methods for determining equivalent doses have in common that they are derived from the absorbed dose by multiplying it by weighting factors . Depending on the procedure, these are defined differently (see section #Weighting factors ).

The adjacent figure illustrates these relationships, which are described in detail in the following sections.

Weighting factors

The weighting factors are dimensionless. Physically, the units of measurement for absorbed dose and the equivalent dose based on it are therefore the same. In order to make the difference clear, the unit of measurement of the dose equivalent is called “Sievert (Sv)” in contrast to the unit of measurement “Gray (Gy)” of the absorbed dose. Refer to Gray and Sievert (unit) for details on units of measure .

With the same absorbed dose, the various types of radiation (symbolized below by the letter ) differ in their effects, sometimes considerably. Alpha particles are many times more effective than photons of gamma radiation or X-rays with the same absorbed dose and their weighting factor is correspondingly higher. There are two concepts for deriving the weighting factor of a type of radiation: ${\displaystyle R}$

• In one concept, observations of stochastic health effects are used to determine the effectiveness of a type of radiation compared to photons as reference radiation. This weighting factor is called the radiation weighting factor . It is used to determine the body doses . ${\displaystyle w_{R}}$
• The other concept is based on a theoretical model that derives the biological effectiveness of radiation from a biophysical point of view from its linear energy transfer capacity (LET). One with a low LET acts as reference radiation. The weighting factor derived in this way is called the quality factor . It is used in the context of external radiation exposure for the dose parameters . ${\displaystyle Q_{R}}$

Dose equivalent as a dose measure

See also : quality factor

The equivalent dose of a type of radiation, symbolized by , is a dose measure for local and personal dose monitoring in the event of external radiation exposure (see German Radiation Protection Ordinance (StrlSchV) , Annex 18, Part A ). The quality factor of the type of radiation is used as a weighting factor to determine it . It is defined by the ICRU for standardized soft tissue. The values ​​can be found in Annex 18, Part D of the StrlSchV . ${\displaystyle H_{R}}$ ${\displaystyle Q_{R}}$ ${\displaystyle R}$

Mathematically, the equivalent dose (in Sv) for one type of radiation is obtained by multiplying the absorbed dose (in Gy) by the quality factor . ${\displaystyle H_{R}}$ ${\displaystyle R}$ ${\displaystyle D_{R}}$ ${\displaystyle Q_{R}}$

${\displaystyle H_{R}=Q_{R}\cdot D_{R}}$

If more than one type of radiation interacts with different energy doses and quality factors, the respective equivalent doses add up. ${\displaystyle R}$ ${\displaystyle D_{R}}$ ${\displaystyle Q_{R}}$

${\displaystyle H_{gesamt}=\sum _{R}H_{R}}$

The important characteristics of the dose equivalent as a dose measurement variable are ${\displaystyle H_{R}}$

• Local dose . The local dose is determined in particular to delimit radiation protection areas and to determine protective measures. For such purposes, the local dose rate is usually measured, which expresses the increase in the local dose per unit of time, usually given in microsieverts per hour (µSv / h).
• Personal dose . The personal dose is a dose measure that is measured at the point where the dosimeter is worn. It is not a body dose, but in accordance with Section 65 StrlSchV it is basically used to monitor whether the specified limit values ​​for body doses are adhered to for people exposed to radiation.

The further differentiation of these two measured dose quantities is characterized by the methods on which the calibration of corresponding measuring devices or dosimeters is based. The standardized depths of the respective measuring points in phantoms (including the " ICRU sphere "), where the energy dose generated by the radiation is measured, are decisive . One receives the variety of applications in the radiation protection practice appropriate ${\displaystyle d}$

• for the local dose as a more extensive dose measurement variable, the very frequently used ambient dose equivalent . The measuring point is in the ICRU sphere at a depth of 10 mm. Further dose parameters of the local dose are directional equivalent doses , which are fixed to certain spatial directions. ${\displaystyle H^{*}(10)}$
• for the personal dose, the frequently used deep personal dose, whose measuring point in the ICRU sphere is also at a depth of 10 mm, as well as the surface personal dose and the eye lens personal dose with measuring points 0.07 and 3 mm deep. ${\displaystyle H_{p}(10)}$ ${\displaystyle H_{p}(0,07)}$ ${\displaystyle H_{p}(3)}$

Equivalent dose as the body dose for external radiation exposure

See also : Organ Dose  and Effective Dose

Body doses for external radiation are the organ equivalent dose and the effective dose . ${\displaystyle H_{T}}$ ${\displaystyle E}$

The organ equivalent dose relates to the absorbed dose of the type of radiation averaged over an organ or tissue , weighted with the radiation weighting factor . Its values ​​can be found in Annex 18, Part C, No. 1 of the StrlSchV . ${\displaystyle T}$ ${\displaystyle D_{T}}$ ${\displaystyle R}$ ${\displaystyle w_{R}}$

${\displaystyle H_{T}=w_{R}\cdot D_{T,R}}$

If types of radiation with different values ​​for and energy doses act on the organ , the corresponding equivalent doses add up. ${\displaystyle R}$ ${\displaystyle w_{R}}$ ${\displaystyle D_{T,R}}$ ${\displaystyle T}$

${\displaystyle H_{T}=\sum _{R}w_{R}\cdot D_{T,R}}$

The effective dose is the weighted summation of the organ equivalent doses of the organs concerned . Here are organ-dependent weighting factors used, which express the relative radiation sensitivity of organs to each other with respect. Stochastic damage. They must not be confused with the aforementioned factors and . Their values ​​can be found in Annex 18, Part C No. 2 of the StrlSchV . ${\displaystyle E}$ ${\displaystyle H_{T}}$ ${\displaystyle T}$ ${\displaystyle w_{T}}$ ${\displaystyle Q_{R}}$ ${\displaystyle w_{R}}$

${\displaystyle E=\sum _{R}w_{T}\cdot H_{T}}$

See the " Effective Dose " article for more details .

Derivation of the body dose from the dose measured variable for external radiation exposure

The derivation of body doses from the measured dose quantities is one of the key tasks in radiation protection. However, it is limited to external radiation exposure, in particular to

• Deep personal dose . In the case of penetrating radiation, it can in many cases be equated to the body dose with sufficient accuracy, especially in the case of photon radiation (synonymous with ). With a low dose and a largely homogeneous radiation field, it corresponds to the effective dose with sufficient accuracy. The prerequisite is a whole-body exposure that is as homogeneous as possible. ${\displaystyle H_{p}(10)}$ ${\displaystyle Q_{R}=w_{R}=1}$
• Surface personal dose . In the context of skin, hand and foot dosimetry, it can be equated directly to the relevant body dose. ${\displaystyle H_{p}(0,07)}$

Under less favorable conditions, in the case of external radiation exposure, adapted conversion coefficients must be developed from the data of the radiation fields in conjunction with suitable computer-aided models and anthropomorphic phantoms, with which body doses can be estimated from measured variables.

Equivalent dose as the body dose for internal radiation exposure

See also : Follow-up organ dose  and Follow-up effective dose

In the case of internal radiation exposure, i. H. upon irradiation by radionuclides that are supplied to the body and incorporated by him, the organ equivalent dose and the effective dose occurs as body dose to the site, the sequence organ equivalent dose or the effective dose . The future exposure to the radionuclides remaining in the body is also included in these doses, which are determined for the time of intake.

No dose parameters are defined for internal radiation exposure. Other measured variables must be used, including indirect ones, such as determination of the activity of urine and stool samples.

The easiest way to estimate subsequent doses is to use dose coefficients directly from the intake data. In addition to the radionuclide and the supplied activity, this also includes data on the chemical and physical form of the supplied radioactive substance. ${\displaystyle I}$

The dose coefficients for subsequent organ equivalent doses and for the effective subsequent dose are a function of the intake data and they relate to an integration time . The integration time for adults is 50 years. ${\displaystyle h_{T}(t)}$ ${\displaystyle e(t)}$ ${\displaystyle t}$

The corresponding equivalent doses are simply the product of the added activity (in Bq ) with the relevant dose coefficient (in Sv / Bq). ${\displaystyle I}$

${\displaystyle H_{T}=h_{T}(50)\cdot I}$

${\displaystyle E=e(50)\cdot I}$

The radiation weighting factor for the radionuclide under consideration as well as the biokinetic and metabolic processes are taken into account in the dose coefficients. There are compilations of the dose coefficients for the relevant radionuclides in connection with the Radiation Protection Ordinance and as publications by the ICRP, whereby a distinction is also made between coefficients for the general public and for the professional sector.

scope of application

See also : Radiation Risk  and Radiation Damage

In radiation protection, equivalent doses are used in a dose range of up to a few 100 mSv, where stochastic effects are known to occur or (at low doses) are suspected and where deterministic effects are not yet decisive. In the case of significantly higher doses with the decisive deterministic effects, radiation doses are given solely in the form of the absorbed dose in gray (Gy). Radiation therapy is a typical area of ​​application for this.

Order of magnitude of equivalent doses (examples)

• As a result of natural radiation exposure, a person living in Germany receives an average effective dose of 2.1 mSv per year. The natural local dose rate (ambient dose equivalent rate) outdoors in Germany is between 0.05 and 0.18 µSv / h, depending on local conditions. For details, see the article Radiation exposure .
• Through the "civilization radiation exposure", especially through medical applications, a person living in Germany receives an average effective dose of 1.7 mSv per year.
• With a chest x-ray (pa image) the patient receives an effective dose of about 0.018 mSv, with a CT scan of the chest about 5.1 mSv.

• In Germany, the limit value for an individual in the population due to planned exposure situations is 1 mSv effective dose per year in addition to natural and medical radiation exposure in accordance with Section 80 Radiation Protection Act (StrlSchG) .
• The limit value for occupationally exposed persons according to § 78 StrlSchG is 20 mSv effective dose per year in Germany. No more than 400 mSv may accumulate over a career.
• In the event of a radiological emergency in Germany, according to Section 93 StrlSchG, the effective dose should fall below a reference value of 100 mSv in the first year after occurrence. The emergency dose value according to § 4 Emergency Dose Value Ordinance, which serves as a criterion for the appropriateness of an evacuation, is 100 mSv effective dose in seven days for an intended reference person who is constantly unprotected in the open. For details, see the Radiological Emergency article .
• For emergency responders, a limit of 250 mSv per use and life applies when it comes to saving human lives. To protect property, it is 15 mSv per use.
• Clinical symptoms of radiation sickness occur with short-term whole-body or large-volume partial body irradiation in the dose range above 1 Gy.

Historical

The term equivalent dose was used for dose measurements and for body doses until 1991 using the quality factor as a weighting factor. With the ICRP publication 60, the radiation weighting factor was introduced with regard to the body dose . This does not affect the use and definition of the term equivalent dose for the measured variable. ${\displaystyle Q}$ ${\displaystyle w_{R}}$

The dose equivalent was previously given in Rem ( roentgen equivalent man ). 1 Sv is equal to 100  Rem .

Individual evidence

1. ↑ Dose coefficients for calculating radiation exposure (Part II and III on internal exposure of individuals in the general public and for occupational exposure), published online as Appendix 160 a and b to the Federal Gazette of August 28, 2001
2. ↑ For incorporation in the professional area, the ICRP provides newer dose coefficients with the publications Occupational Intakes of Radionuclides Part 2, 3 and 4 (ICRP publications 134, 137 and 141), including an electronic appendix for download in the form of a database view for PC (executable file) zip file (installed 85.4 MB)
3. ↑ ^{a b} page of the Federal Office for Radiation Protection (BfS) on natural radiation exposure, online
4. ↑ Page of the BfS on the monitoring of the local gamma dose rate, online
5. ↑ Orientation aid for imaging procedures , recommendation of the Radiation Protection Commission (SSK), 3rd, revised edition, adopted at the 300th meeting of the SSK on June 27, 2019, PDF download 1.58 MB
6. ^ Committee on Fire Brigade Matters, Disaster Control and Civil Defense: Fire Brigade Service Regulation 500: Units in NBC operation , January 2012
7. ^ International Commission on Radiological Protection (ICRP): The 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, Ann. ICRP 21 (1-3), 1991