Radiation dose

Question 1

what is radiation?

Answer 1

Radiation is a means by which energy is transported through space or through matter

Question 2

what are some examples of radiation?

Answer 2

-Light, X-rays and radio waves are all forms of electromagnetic radiation
-They exhibit both wave and particle like behaviour
-There is no mass associated with the transfer of energy
-Particles (with mass) can be viewed as radiations as they carry energy
-Sound can be also viewed as a form of radiation as it has the properties of waves and carries energy

Question 3

Give examples of ionising and non ionising radiation

Answer 3

Ionising
X-rays, g-rays, a-particles, b-particles, neutrons, cosmic rays

Non-ionising
Light, ultraviolet, infra-red, radio waves, microwaves, ultrasound

Question 4

what are the different measures of radiation dose

Answer 4

Absorbed dose
Equivalent dose
Effective dose

Question 5

absorbed dose

Answer 5

-can cause biological damage
XR photons transfer energy into a material as kinetic energy of charged particles (electrons)
These electrons deposit their energy along ionization tracks
The energy per unit mass deposited by these electrons is the absorbed dose

Question 6

equivalent dose

Answer 6

-Measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effect of different types of ionizing radiation
-Equivalent dose is therefore a less fundamental quantity than radiation absorbed dose but is more biologically significant.

Question 7

how is equivalent dose calculated

Answer 7

Calculated by multiplying the average absorbed dose to the organ or tissue (DT) by a weighting factor (wR)

The weighting factor is selected for the type and energy of the radiation incident on the body

Question 8

why do different tissues of the body respond to radiation differently

Answer 8

The probability for stochastic effects that result from a given equivalent dose will generally depend upon the particular tissue or organ irradiated

A tissue weighting factor (wT) is introduced to account for this, giving rise to the concept of effective dose

Question 9

what is effective dose

Answer 9

proportional to the stochastic health effects of radiation in humans
It takes into account the radio-sensitivity of different organs and also the equivalent dose to each organ

Question 10

what are the practical measures of radiation dose?

Answer 10

Entrance surface dose (ESD)
Dose area product (DAP)
Diagnostic reference levels

Question 11

how do we estimate effective dose?

Answer 11

Measure the Entrance Surface Dose (ESD) or DAP for a specific examination (e.g. chest exam)

Lookup the effective dose for that examination in a published table

These tables are compiled using mathematical models to estimate internal organ doses

Question 12

what are diagnostic reference levels used for

Answer 12

More commonly, diagnostic reference levels are used to gauge radiation received from a particular examination
National Diagnostic Reference Levels
Dose measurement audits for specific examinations can be undertaken within your department
ESD or DAP levels can be compared to the published diagnostic reference levels
They can also be used to estimate effective dose (risk) for specific examinations

Question 13

what are some examples of practical dose measurements in radiography:

Answer 13

Entrance Surface Dose measured with:
Dosimeter
Directly
Phantom
TLD
Dose Area Product (DAP)

Question 14

entrance surface dose

Answer 14

The entrance surface dose (ESD) is equivalent to the absorbed dose in air at the surface of skin on the central beam axis at the position of the patient or phantom surface

Backscattered radiation from the patient is included in the measurement

It is also referred to as the entrance air kerma or the entrance surface air kerma (kerma - kinetic energy released in the medium)

Question 15

how do we measure entrance surface dose

Answer 15

Can be measured with an ionisation chamber (dosimeter) close to the patient’s skin – this is generally not practical

Can be measured using a suitable test phantom – but may not represent the patient or spread of patients accurately

Question 16

Dose area product (DAP)

Answer 16

DAP is a quantity that is becoming more common in assessing diagnostic reference levels in radiography
It is defined as theabsorbed dose(in air) multiplied by the area irradiated, expressed inGy·cm2

DAP reflects not only the dose within the radiation field but also the area oftissueirradiated
Therefore, it may be a better indicator of the overall risk of inducing cancer than the dose within the field

Question 17

deterministic effects

Answer 17

Deterministic Effects
There are definable threshold doses for these effects below which no damage, in terms of measurable clinical response, can be detected

Above the threshold, the severity of effect increases with dose in a way that can be predicted

Question 18

stochastic effects

Answer 18

A stochastic effect is one that is governed by the laws of chance
Since a single ionising event may cause radiation damage to the DNA it is usual to assume that there is no threshold dose for stochastic effects of ionising radiation
No radiation dose, no matter how small, is safe!

Question 19

what are some key points from data regarding stochastic effects

Answer 19

-The risk of cancer is not the same for all parts of the body. Many organs are affected, but not all to the same degree
-There is a long latent period before cancer develops- excess leukaemia occurred between 5-14 years but the risk of solid tumours was still increasing 40 years after the bombs
-There is no evidence of a threshold dose for most tumour types
-In terms of relative risk, cancer was highest in those under 10 years of age at the time of exposure

Question 20

heritable effects

Answer 20

Less clear evidence
Strong circumstantial evidence
Plants, bacteria, fruit flies and mice
For the purposes of risk estimation the dose of radiation that would double the natural incidence of germ line mutation is estimated from animal work to be approximately 1 Sv

Question 21

models of radiation cancer risk

Answer 21

Risks of very low doses are uncertain, as they are based on relatively few studies
The smallest dose that has provided clear evidence of an increased cancer risk is approximately 200 mSv
Breast cancer after multiple lung radiographs of patients with tuberculosis
Japanese atom bomb survivors

Question 22

radiation risk to individuals

Answer 22

Medical exposures deliver radiation doses to individuals or members of specific groups (men vs women, young vs old)

In some cases, average risk factors based on the total population will be misleading

Question 23

what is the risk of long-term stochastic effects

Answer 23

The health effects of radiation to be considered when deciding whether a diagnostic examination should be carried out are the stochastic effects:
Cancer and hereditary disease

Question 24

exposure of the foetus

Answer 24

When a patient is found to be pregnant only after an X-ray examination expert advice should be sought
Careful estimate of dose to the fetus
< 2 mGy will carry a risk well below both the natural incidence of malignant disease and the other risks of pregnancy