Lecture 29. Radiation protection

1. Lecture 29. Radiation protection

2. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

3. General philosophy Second International Congress of Members prepare X-ray Radiology in Stockholm, 1928 protection recommendations • The congress set up the International X-Ray and Radium Protection • Committee; after World War II it morphed into two commissions: • The International Commission on Radiological • Protection (ICRP) • 2. The International Commission on Radiation Units and • Measurements (ICRU)

4. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

5. Stochastic Effects and Deterministic Effects • Stochastic Effects. • No occupational exposure should be permitted until • the age of 18 years; • The effective dose in any year should not exceed 50 mSv • (5 rem); • The individual worker’s lifetime effective dose should not • exceed age in years x 10 mSv (1 rem) • These limits apply to the sum of the effective dose from external • radiation and the committed effective dose from internal • exposures.

6. Deterministic Effects • 150 mSv (15 rem) per year for the lens of the eye. • 500 mSv (50 rem) per year for localized areas of the skin and the hands and feet The additional limits for deterministic effects are required because the weighting factors

7. Quantities and Units Dose The quantity used to measure the “amount” of ionizing radiation is the absorbed dose, or simply dose. This is defined as the energy absorbed per unit mass, and its unit is joules per kilogram, the gray (Gy), named after the British physicist who contributed to the development of ionization chamber theory. In the past the unit was the rad (radiation absorbed dose), defined as an energy absorption of 100 erg/g. Consequently, 1 Gy equals 100 rad.

8. Radiation Weighting Factor (WR) Some radiations are biologically more effective, for a given dose, than others. This is taken into account by weighting the absorbed dose by a factor related to a quality of radiation. A radiation weighting factor (WR) is a dimensionless multiplier used to place biologic effects (risks) from exposure to different types of radiation on a common scale. Radiation weighting factors are chosen by the ICRP as representative of RBE, applicable to low doses and low dose rates, and for biologic end points relevant to stochastic late effects.

9. Radiation Weighting Factor (WR) Weighting factors recommended by the ICRP for different types of radiation

10. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

11. Equivalent dose – relative weighting factors In radiation protection, the equivalent dose is the product of the absorbed dose averaged over the tissue or organ and the radiation weighting factor selected for the type and energy of radiation involved. Equivalent dose = absorbed dose x radiation weighting factor If the absorbed dose is measured in gray, the equivalent dose is in sievert (Sv), named after the Swedish physicist who designed early ionization chambers. 1 Sv of either neutrons or X-rays does result in equal biologic effect (1 Gy of these radiations does not produce the same effect). The ICRP has recommended a new name for this quantity: Radiation weighted dose

12. Equivalent dose – relative weighting factors If a radiation field is made up of a mixture of radiations, the equivalent dose is the sum of the individual doses of the various types of radiations, each multiplied by the appropriate radiation weighting factor. Thus, if a tissue or organ were exposed to 0.15 Gy (15 rad) of cobalt-60 gamma-rays plus 0.02 of 1-MeV neutrons, the equivalent dose would be: (0.15 x 1) + (0.02 x 20) = 0.55 Sv, or 55 rem

13. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

14. Effective dose – tissue weighting factors The sum of all of the weighted equivalent doses in all the tissues or organs irradiated is called the effective dose, which is expressed by the formula: Effective dose = Σ absorbed dose x WR xWT for all tissues or organs exposed. The table on the next slide lists the tissue weighting factors recommended by the ICRP.

15. Effective dose – tissue weighting factors

16. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

17. Committed Equivalent Dose In the case of external radiation, the absorbed dose is delivered at the time of exposure, but for radionuclides, the total absorbed dose is distributed over time, as well as to different tissues in the body. To take into account the varying time distributions of dose delivery, the ICRP defined the committed equivalent dose as the integral over 50 years of the equivalent dose in a given tissue after intake of a radionuclide. This time was chosen to correspond to the working life of a person.

18. Committed Effective Dose If the committed equivalent doses to individual organs or tissues resulting from the intake of a radionuclide are multiplied by the appropriate tissue weighting factors and then summed, the result is the committed effective dose.

19. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

20. Collective Equivalent Dose The quantities referred to previously all relate to the exposure of the individual. They become appropriate for application to the exposure of a group or population by the addition of the term collective. Thus, the collective equivalent dose is the product of the average equvalent dose to a population and the number of persons exposed. The unit is person-sievert

21. Collective Effective Dose The collective effective dose is also the product of the average effective dose to a population and the number of persons exposed. The unit is again the person-sievert

22. Collective Committed Effective Dose In the case of a population ingesting or inhaling radionuclides that deposit their dose over a prolonged period of time, the integral of the effective dose over the entire population out to a period of 50 years is called collective committed effective dose. These collective quantities can be thought of as representing the total consequences of exposure of a population or a group

23. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

24. Dose limits for occupational and public exposure Aims and Objectives of Radiation Protection • ICRP: The objectives of radiation protection are: • to prevent clinically significant radiation-induced deterministic • effects by adhering to dose limits that are below the apparent or • practical threshold, and • to limit the risk of stochastic effects (cancer and hereditary • effects) to a reasonable level in relation to societal needs, values, • and benefits gained.

25. Aims and Objectives of Radiation Protection The difference in shape of the dose-response relationships for deterministic and stochastic effects.

26. Aims and Objectives of Radiation Protection A L A R A The objectives of radiation protection can be achieved by reducing all exposure to as low as reasonably achievable (ALARA) and by applying dose limits for controlling occupational and general public exposures. For radiation protection purposes, it is assumed that the risk of stochastic effects is strictly proportional to dose without threshold throughout the range of dose and dose rates of importance in radiation protection.

27. Dose limits for occupational and public exposure Tolerance dose – a dose to which workers could be exposed continuously without any evident deleterious acute effects. The maximum permissible dose was designed to ensure that the probability of the occurrence of injuries was so low that the risk would be readily acceptable to the average person.

28. Dose limits for occupational exposure

29. Risks associated with current recommended limits Risk estimates for radiation-induced cancer and hereditary effects

30. Risks associated with current recommended limits

31. General philosophy • Stochastic and deterministic effects • Effective dose – relative weighting factors • Equivalent dose – tissue weighting factor • Committed dose • Collective exposure dose • Dose limits for occupational and public • exposure • ICRP and NCRP

32. 1946 the NCRP (the National Council on Radiation Protection) • received a charter from Congress as an independent body to • provide advice and recommendations on matters pertaining to • radiation protection in the United States. It is still the basis of • radiation protection policy in the United States today, though • legal responsibility for radiation safety is variously in the • hands of the: • Nuclear Regulatory Commission • Department of Energy • State or city bureaus of radiation control

33. Organizations • Committees that summarize and analyze data and suggest risk • estimates for radiation-induced cancer and hereditary effects: • International level: United Nations Scientific Committee • on the Effects of Atomic Radiation (UNSEAR); • United States committee appointed by the National Academy • of Sciences, is known as BEAR (Biological Effects of Ionizing • Radiations) Committee. • Committees that formulate the concepts for use in radiation • protection and recommend maximum permissible levels: • International level: International Commission on Radiological • Protection (ICRP); • United States: the NCRP. It normally follows the ICRP; • United States: the Environmental Protection Agency (EPA), • the Nuclear Regulatory Commission, the U.S. Occupational • Safety and Health Administration, the Department of Energy.

34. Limits for occupational exposure. The NCRP recommends the limits for occupational exposure summarized in the following slides. These limits do not include natural background radiation or radiation for medical purposes In the United States the Environmental Protection Agency (EPA) provide the guidance to federal agencies. In agreement states, the Nuclear Regulatory Commission formulates rules for by-product materials from reactors. Table on the next slide lists “agreement states” as of 2003.

36. ICRP and NCRP At the present time, there is a difference in the recommendations of the national and international bodies regarding the maximum permissible effective dose for occupational exposure (stochastic effects). The differences are highlighted in the Table on the next slide.

37. ICRP and NCRP compared

38. Doses to which individuals are exposed vary enormously by several orders of magnitude. Figure 15.2 compares the ranges of doses used in medicine with doses received occupationally and from natural sources. This chart uses the new SI units of gray and sievert

39. Comparison of the ranges of doses used in medicine with doses received occupationally and from natural sources. This chart uses the older units based on the rad and the rem