14.1 - Principles Of Radiation Protection

Data on Radiation Protection

Dose Limits

The latest guidelines from the ICRP are summarised in the table below (Report 103)

Type of Limit Occupational Public
Equivalent Dose 20 mSv/year* 1 mSv/year
Effective Dose
Lens of eye 150 mSv/year 15 mSv/year
Skin 500 mSv/year 50 mSv/year
Hands and Feet 500 mSv/year N/A

* Note that 20 mSv/year is average yearly dose over 5 years and it is possible to exceed this dose in a single year.

Basis for Dose Limits

Radiation is present in everyday life from cosmic and terrestial sources. The average yearly dose for a normal person is 3 mSv, most of which is due to inhaled radon gas. This must be accounted for when considering radiation doses to the public and to workers in the area.
The use of radiation is on the proviso that the negative factors are accepted by society. It is known that radiation may cause both:

  • Deterministic effects, which occur once a threshold dose is reached and increase in severity as dose increases above this threshold, and
  • Stochastic effects, which are due to permanent DNA damage and may lead to secondary malignancies or heritable effects in a person's offspring. Stochastic effects become more likely with increasing dose, but have no threshold dose and their severity is not related to the dose. There is no direct evidence in humans that heritable effects occur, but evidence is present in other mammalian species.

Stochastic effects are the main reason for dose limits, as deterministic effects typically have threshold doses of at least several gray. Evidence suggests that secondary malignancies become more common with increasing dose, and therefore dose limits are provided to limit the likelihood of this occuring.

Radiation Protection Definitions

Units in Radiation Protection

Absorbed Dose (Gy)

The amount of radiation energy (in joules (J)) absorbed per unit mass (kg). Special Unit: Gray (Gy). Absorbed dose does not take into account the radiation quality or the tissue in question.

Equivalent Dose (Sv)

The absorbed dose in Gy, corrected by application of the radiation weighting factor WR. This accounts for the increased toxicity caused by densely ionising radiations such as alpha particles.

Effective Dose (Sv)

The equivalent dose in Sv, corrected by application of tissue weighting factors for individual tissues WT. The weighting factors for every tissue add up to 1.

Radiation Weighting Factor (WR)

The correction factor applied to the absorbed dose to account for the altered biological effect of different radiations. The radiation weighting factors recommended by the ICRP are:

Radiation Quality Radiation Weighting Factor
Photons 1
Electrons, Muons 1
Protons, Charged Pions 2
Alpha particles, fission products, fission fragments 20
Neutrons Continuous curve, peaks at energies about 10 MeV

Tissue Weighting Factor (WT)

The correction factor applied to the equivalent dose to account for the varied risks of stochastic effects in different tissues. The tissue weighting factors recommended by the ICRP are:

Tissue Type Tissue Weighting Factor (per tissue)
Lung, stomach, colon, bone marrow, breast, remainder of body 0.12
Gonads 0.08
Thyroid, oesophagus, liver, bladder 0.04
Bone surface, skin, salivary glands, brain 0.01
Total 1

Definitions in Radiation Protection

Deterministic Effects

Effects that only occur once a threshold dose is reached, and increase in severity as dose increases over the threshold. If the dose falls below the threshold dose then no effect is seen. Threshold doses vary between tissues and between individuals. Deterministic effects include most commonly seen radiation side effects, both early and late.

Stochastic Effects

Effects that have no threshold dose and whose severity is not related to dose. Stochastic effects become more likely as dose increases. Stochastic effects include secondary malignancies and inheritable effects.

Radiation Incident

A radiation incident is an abnormal event, where a source of ionising radiation is uncontrolled temporarily, and where a person is exposed to no more than twice the effective dose than would have otherwise been received. For instance, if a patient was scheduled to receive a dose of 2 Gy but due to machine error received 3 Gy, it would be classed as a radiation incident.

Radiation Accident

A radiation accident is a more severe abnormal event, due to a source of ionising radiation being out of control, and:

  • The source remains out of control,
  • Dispersal of radioactive material takes place, or
  • A person is exposed to over twice the effective dose than would have been expected in the situation

If a HDR source fails to retract, and a staff member receives a significant dose of radiation during retrieval of the source, this would be a radiation accident.

ALARA Principle

ALARA is an acronym for "As Low As Reasonably Achievable". It is a concept that takes into account the dangers of radiation but also the economics and practicality of radiation protection. ALARA can be acheived by three methods:

  • Time: As radiation dose is dependent on the time exposed, it makes sense to limit the time a person is exposed to radiation. This can be accomplished by practising the handling of radioactive materials with dummy runs, or by placing infrequently used facilities (eg. storerooms) in areas with a potentially higher radiation exposure.
  • Distance: Radiation loses its intensity over distance as per the inverse square law. Doubling the distance from the source leads to a four-fold reduction in dose. The ALARA principle in this scenario is that radiation sources should be kept distant from the public and staff, but not so distant that it becomes too difficult to reach them.
  • Shielding: Radiation is attenuated by shielding. By placing shielding in the path of radiation the dose on the distant side of the shield will be significantly reduced (depending on the radiation quality and the shield thickness). ALARA principles state that shielding should be cost effective (concrete is better than metal) and should reduce the dose rate beyond the shield to insignificant levels (not to zero).