Quantities and units

Very many quantities and units are used in radiation protection. There are far too many to cover in this refresher course. A quantity is represented by a specially chosen symbol, such as l for length, m for mass and t for time. The corresponding unit describes an agreed measure for such a quantity. In the examples above, these are metres, kilograms and seconds, respectively.

We will first cover some general quantities and units. Then we will come to many units used in practice.

Absorbed dose (D)
The name actually says what this quantity is used for, it describes the dose of radiation (energy) absorbed by a material. This quantity is also often simply called dose. The unit of energy is the Joule (J). The unit of absorbed dose is the gray (Gy) and corresponds to 1 Joule per kilogram of material (J/kg).

1 gray = 1 Gy = 1 J/kg

However, one gray is a high dose, so in practice mGy or µGy (one thousandth or one millionth gray) is often used.

Equivalent dose (H)
The equivalent dose is a corrected organ dose.

The organ dose is an average dose for the entire organ. So it does not matter whether 1/3 of the organ received 3 Gy or the whole organ received 1 Gy: the organ dose is 1Gy in both cases.

In addition, a correction is made in this unit for the harmfulness of the type of radiation applied. Not all types of ionising radiation are equally harmful per gray (per amount of energy/kg emitted). The harmfulness of X-rays is taken as the basis for the equation, the radiation weighting factor is (thus) 1.

The unit, to distinguish it from the absorbed dose, is sievert (Sv).

Since all types of radiation other than X-rays are beyond the scope of this course, we can say that when an organ as a whole is exposed to a dose of 1 Gy, the equivalent dose is 1 Sv.

For X-rays: H = 1 x D : 1 Gy on 1 organ = 1Sv equivalent dose

Effective dose (E)
Because the harmful effect of X-rays is not the same for all organs and tissues, a correction is also made for the exposed organ. There are organs that are sensitive to radiation, which have a high weighting factor, and there are organs that are less sensitive, which have a lower weighting factor. So the same organ dose will do more damage to one organ than another. To determine the effect of a dose for a whole individual ('the effective' dose), it is necessary to correct for that difference in sensitivity.

This correction factor is called W_T named. The T stands for Tissue. Here is an overview of the different tissue weighting factors. As with the equivalent dose, the unit of effective dose is sievert (Sv). This is a very large measure, usually mSv or µSv is used.

The sum of all weighting factors is 1: When the whole body is evenly exposed (read: all organs the same equivalent dose), the effective dose is numerically equal to the equivalent dose. When not all organs are exposed (evenly), the effective dose is calculated as follows:

E = (H x W_T)_{organ 1} + (H x W_T)_{organ 2} + (H x W_T)_{organ 3} + ...etc

Example: simplified dose calculation of an intraoral shot:

The entry dose is 2 mGy, the skin is unshielded for a small part in the direct beam.
A small part of the salivary gland lies in the bundle, but is already somewhat shielded by the skin.
The thyroid gland receives a dose of scatter radiation.
All other organs such as bone surface, bone marrow and other organs that receive a small dose of scatter radiation are excluded.

1.From absorbed dose to organ dose:

Offset partial exposure of an organ

D = 2 mGy on cheek ( = 0.1% of skin) H_skin = 0.1% x 2 = 2 µSv

D = 1 mGy on salivary gland (10% of glands) H_{salivary gland} = 10% x 1 = 100 µSv

D = 0.05 mGy on thyroid H_thyroid= 100% x 0.05 = 50 µSv

2.From organ dose to effective dose:

Offset for the sensitivity of an organ: H x WT (table)

By skin exposure = 2 µSv x 0.01 = 0.02 µSv

By salivary gland exposure = 100 µSv x 0.01 = 1 µSv

Due to thyroid exposure = 50 µSv x 0.04 = 2 µSv

Total = 3 µSv

Units in practice
The following is a brief description of the terms used with units in radiology. You will see these units in the quality control reports, or in your screen or on the recordings.

Intreedose

The entry dose is the dose where the X-ray beam first hits the body surface and is often also referred to as skin dose. The unit in which the skin dose is expressed is the gray with the symbol Gy. The entry dose can be used to estimate the effective dose of an examination or to estimate the amount of scatter radiation to bystanders.

Dose-area product (DOP)

The dose-surface product is the measured dose somewhere in the beam before it hits the patient. Because it is a product of dose and area, and both of these show a quadratic relationship with distance to focus, it does not matter where in the beam this measurement takes place, as long as the entire beam falls within the measurement chamber. The abbreviation DAP is also used, which is the abbreviation for the English 'dose area product'. The unit mGy-cm2 is often used.

The above units are used to get an indication of the dose to the client, by testing it against the diagnostic reference level (DRN). The DRN is a level for the exposure of the standard patient below which there is normally 'good medical practice'. With common clinical demands and good diagnostic and technical performance, diagnostic reference levels should not be exceeded during standard procedures.

Sources:

Physics for imaging and radiotherapy; J. Scheurleer, 2017

Introduction to Radiation Hygiene; A.J.J. Bos et al; 2009