The ICRP System of Radiological Protection

1
The ICRP System of Radiological Protection
2
Overview

• Historical Background
• ICRP Publication 60
• More Recent Publications
• Controllable Dose

3
Aims and Scope

• To be of help to regulatory and advisory agencies
  at national, regional and international levels,
  mainly by providing advice on fundamental
  principles
• To provide guidance to management, their advisors
  and individuals, such as radiologists, who have
  to make decisions about the use of ionising
  radiations
• To provide an appropriate standard of protection
  for man without unduly limiting the beneficial
  practices giving rise to radiation exposure

4
Limitations

• Confined to protection against ionising
  radiations.
• Consider that the standard of environmental
  control needed to protect man to the degree
  currently thought desirable will ensure that
  other species are not put at risk. Occasionally,
  individual members of non-human species might be
  harmed, but not to the extent of endangering
  whole species or creating imbalance between
  species.

5
Dosimetric Considerations

• HT ? wRDT,R
• E ??wTHT

6
Radiation Weighting Factors

• Photons 1
• Electrons and muons 1
• Neutrons lt10 keV 5
• 10 - 100 keV 10
• 100 keV - 2 MeV 20
• 2 MeV - 20 MeV 10
• gt20 MeV 5
• Protons, other than recoil protons, gt2 MeV 5
• Alpha particles, fission fragments, heavy
  nuclei 20

7
Tissue Weighting Factors
8
The Inclusion of Non-Fatal Cancer

• Fk.F(1-k)/kF(2-k)
• Fk.P
• P(2-k)k

9
Significant Statements on Biological Effects - 1

• A distinction is made between change, damage,
  harm and detriment
• Data on hormesis are not sufficient to take them
  into account
• Detriment is restricted to health detriment
• The biological information needed in radiation
  protection is drawn, to the maximum extent
  possible, from data on radiation effects in
  humans
• Non-stochastic effects are renamed
  deterministic

10
Significant Statements on Biological Effects - 2

• Cancers induced by radiation do not have
  distinguishable characteristics
• Individuals may differ somewhat in their
  sensitivities to the induction of cancer by
  radiation
• No stochastic effects other than cancer and
  benign tumours are induced by radiation
• There continues to be a justifiable basis for
  using a linear dose-response relationship without
  threshold for cancer induction at low doses and
  dose rates

11
Significant Statements on Biological Effects - 3

• There is sufficient evidence to justify making an
  allowance for non-linearity when interpreting
  data for low-LET radiation at high doses and dose
  rates
• A Dose and Dose Rate Effectiveness Factor of 2 is
  used for all equivalent doses resulting from
  absorbed doses below 0.2 Gy and from higher
  absorbed doses when the dose rate is less than
  0.1 Gy/h

12
Significant Statements on Biological Effects - 4

• The probability of fatal cancer at low doses and
  dose rates is estimated as 0.04 per Sv for
  workers and 0.05 per Sv for whole populations
  including children
• In the case of lung cancer from inhaled radon
  progeny, it is reasonable to express the
  attributable risk coefficient per unit of radon
  exposure and not per unit dose to the lung or
  bronchial epithelium

13
Significant Statements on Biological Effects - 5

• Radiation has not been identified as a cause of
  hereditary disorders in man
• Nominal hereditary effect coefficients of 0.01
  per Sv for the whole population and 0.006 per Sv
  for workers adequately represent the severity
  weighted number of hereditary defects to be
  expected in all generations
• With additional weighting for years of life lost,
  the corresponding numbers are 0.013 per Sv and
  0.008 per Sv

14
Significant Statements on Biological Effects - 6

• The induction of malformations due to antenatal
  exposure is taken to be a deterministic effect
  occurring from the third week after conception
  (threshold 0.1 Gy)
• With antenatal exposures between 8 and 25 weeks
  post-conception, there is a general shift in the
  IQ distribution with increasing dose and an
  increase in the number of children classified as
  severely mentally retarded
• At doses of around 0.1 Sv, no effect would be
  detectable on the general distribution of IQ

15
The Conceptual Framework of Radiological
Protection - Objective

• To prevent the occurrence of deterministic
  effects by keeping doses below the relevant
  thresholds, and to ensure that all reasonable
  steps are taken to reduce the induction of
  stochastic effects

16
General Principles for Proposed and Continuing
Practices

• Justification
• Optimisation of protection
• Individual dose and risk limits

17
Justification

• No practice involving exposures to radiation
  should be adopted unless it produces a sufficient
  benefit to the exposed individuals or to society
  to offset the radiation detriment it causes

18
Optimisation

• The magnitude of individual doses, the number of
  people exposed, and the likelihood of incurring
  exposures should all be kept as low as reasonably
  achievable, economic and social factors being
  taken into account.
• This procedure should be constrained by
  restrictions on the doses to individuals, or the
  risks to individuals in the case of potential
  exposures, so as to limit the inequity likely to
  result from the inherent economic and social
  judgements.

19
Dose and Risk Limits

• The exposure of individuals from all relevant
  practices should be subject to dose limits, or to
  some control of risks in the case of potential
  exposures
• Dose limits are aimed at ensuring that no
  individual is exposed to radiation risks that are
  judged to be unacceptable in any normal
  circumstances
• Not all sources are susceptible to control by
  action at the source and it is necessary to
  specify the sources to be included as relevant
  before selecting a dose limit

20
General Principles for Intervention

• The proposed intervention should do more good
  than harm
• The form, scale and duration of the intervention
  should be optimised, i.e. the net benefit should
  be maximised
• Dose limits do not apply in intervention, but
  there will be some level of projected dose above
  which, because of serious deterministic effects,
  intervention will almost always be justified

21
Comments on the Conceptual Framework

• Justification requires only that the net benefit
  of a practice be positive - to search for the
  best of all the available options is usually a
  task beyond the responsibility of radiological
  protection agencies
• Justification applies to the introduction of new
  practices and the review of existing practices
• Optimisation should be first applied at the
  design stage of any project

22
Comments on the Conceptual Framework

• Potential exposures need to be considered as part
  of the assessment of practices, but may also lead
  to calls for intervention - their implications
  need to be considered in both contexts
• In principle, risk limits should be associated
  with potential exposures
• There is an on-going requirement to assess the
  effectiveness of any established system of
  protection

23
Proposed and Continuing Practices Occupational
Exposure

• Of the components of exposure to natural sources,
  only radon in workplaces and work with minerals
  containing natural radionuclides can reasonably
  be regarded as the responsibility of the
  operating management.
• However, even these two components should be
  regarded as excluded from occupational exposure
  and treated separately, unless the relevant
  regulatory agency has ruled otherwise, either for
  a defined geographical area or for defined
  practices.
• Unless specific controls are in force relating to
  natural sources, exposures to such sources need
  not be included in radiation monitoring results.

24
Proposed and Continuing Practices Occupational
Exposure

• If dose constraints are used, they should be
  applicable to broad classes of operations, e.g.
  routine operations of nuclear power plants
• Occupational dose limits apply to all
  occupational exposures, including those resulting
  from minor mishaps and misjudgements

25
Proposed and Continuing Practices Occupational
Exposure

• The recommended limit is 20 mSv/y, averaged over
  5 years, with the further provision that
  effective dose shall not exceed 50 mSv in any one
  year
• No special restrictions are required on the
  subsequent exposure of an individual who has
  exceeded a dose limit

26
Proposed and Continuing Practices Occupational
Exposure

• The effective dose limit is the boundary of
  tolerability
• Dose limits for skin and lens of the eye are 500
  mSv and 150 mSv, respectively
• Annual Limits on Intake (ALIs) are based on a
  committed effective dose of 20 mSv
• Once pregnancy is declared, the conceptus should
  be protected by applying a supplementary
  equivalent dose limit to the surface of the
  womans abdomen of 2 mSv for the remainder of
  pregnancy and by limiting intakes of
  radionuclides to about 1/20 of the ALI

27
Proposed and Existing Practices Medical Exposure

• Each procedure (diagnostic or therapeutic) is, in
  principle, subject to a separate justification
• Diagnostic and therapeutic procedures causing
  exposures of the abdomen of women likely to be
  pregnant should be avoided unless there are
  strong clinical indications

28
Proposed and Existing Practices Public Exposure

• Dose limits and dose constraints are applicable
  in relation to the mean dose to the critical
  group
• The dose limits relate only to practices -
  radionuclides already present in the environment,
  whether from natural or human-related sources,
  are excluded
• The limit for public exposure is 1 mSv in a year
• Dose limits for skin and lens of the eye are 50
  mSv and 15 mSv per year, respectively

29
Proposed and Existing Practices Potential
Exposure

• Where doses, should they occur, will not be in
  excess of dose limits, it is adequate to use the
  product of the expected dose and its probability
  of occurrence as if this were a dose that is
  certain to occur
• If the dose is in excess of dose limits, this
  simple approach is inadequate
• Risk constraints should be defined applicable to
  the attributable probability of death

30
Proposed and Existing Practices Interactive
Situations

• Detriment due to public exposure should not be
  treated differently from that due to occupational
  exposure
• The sum of effective doses from each type of
  exposure from a given source should be used in
  optimisation procedures

31
Protection in Intervention

• The comparison of benefits and detriments should,
  in the first place, be made for those at risk,
  but impacts on the rest of society should be
  considered
• Social costs, including an allowance for anxiety,
  should be included

32
Protection in Intervention

• Exposures of emergency teams following accidents
  should be limited by operational controls and the
  doses should be treated separately from normal
  doses
• Exposures in the control of an accident and in
  immediate and urgent remedial work should not
  result in effective doses of more than about 0.5
  Sv, except for life-saving actions
• Once the emergency is under control, remedial
  work should be treated as part of occupational
  exposure

33
Implementation of the Recommendations

• No specific dose level is recommended for
  distinguishing between controlled and supervised
  areas
• Operating management should provide guides to
  designers and operators on the maximum levels of
  exposure that the management expects to occur in
  defined operations and on the reliability needed
  to limit potential exposures
• These guides are not targets, but an envelope
  within which designers and operators should work
  to achieve optimisation

34
Implementation of the Recommendations

• Exemption of sources is an important regulatory
  function
• Appropriate bases for exemption are that a source
  gives rise to small individual and collective
  doses and/or that no reasonable control
  procedures can achieve significant reductions in
  those doses
• The extent to which small individual doses should
  be included in the estimation of collective doses
  depends on the degree to which the contribution
  from these doses influences the choice between
  options

35
Recent Publications

• 77 Radiological Protection Policy for the
  Disposal of Radioactive Waste
• 78 Individual Monitoring for Internal Exposure
  of Workers
• 79 Genetic Susceptibility to Cancer
• 80 Radiation Dose to Patients from
  Radiopharmaceuticals
• 81 Radiation Protection Recommendations as
  Applied to the Disposal of Long-lived Solid
  Radioactive Waste

36
Forthcoming Publications

• Dose Estimation to the Embryo and Fetus
• Risk Estimation for Multi-factorial Diseases
• Principles for Protection of the Public in
  Situations of Prolonged Exposure
• Reference Man Anatomy, Physiology and
  Elemental Composition
• Dosimetric Model for the Gastrointestinal Tract

37
Genetic Susceptibility - 1

• Current estimates of cancer risk already include
  an unknown contribution from genetically
  radiosensitive sub-populations
• The likely contribution to radiation risk from
  familial cancer disorders is too low to generate
  an unacceptable distortion of estimates of cancer
  risk in most populations
• There is insufficient knowledge to judge the
  contribution to risk from mutations of low
  penetrance

38
Genetic Susceptibility - 2

• Because of the high risk of spontaneous cancer in
  familial disorders, low doses of radiation are
  unlikely to impact significantly on lifetime
  cancer risk
• At high doses, e.g. in radiotherapy, the relative
  risk may become important
• The utility of genetic testing is currently
  limited by technical factors and concerns over
  predictive power its future value in an
  occupational context is open to doubt and would
  be subject to ethical scrutiny

39
Controllable Dose

• Is the linear, no-threshold concept more than a
  convenient regulatory fiction?
• Will contaminated land cleanup and
  decommissioning require too great an expenditure
  of resources?
• Is collective dose evaluated over long-timescales
  to large populations meaningful?
• Why should we distinguish occupational, public
  and medical exposures?
• Why should we distinguish practices and
  interventions?

40
The Principle

• If the risk of harm to the health of the most
  exposed individual is trivial, then the total
  risk is trivial - irrespective of how many people
  are exposed.

41
The Rules

• Doses should not exceed about 30 mSv and this
  level should be approached only if there is
  benefit to the individual or the dose is
  difficult to reduce or prevent
• At around 3 mSv, there may be a need to reduce or
  prevent doses, particularly if there is no
  benefit to the individual
• A dose of 0.3 mSv should be the maximum to an
  individual who receives no direct benefit from
  one source of radiation
• A dose of 0.03 mSv presents a trivial risk to an
  individual