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Fundamental Principles and Recent Developments in Internal Dosimetry

Fundamental Principles and Recent Developments in Internal Dosimetry. 13 th International Congress of the International Radiation Protection Association 13-18 May 2012 Glasgow, Scotland. George Etherington Health Protection Agency, UK. Outline of the lecture.

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Fundamental Principles and Recent Developments in Internal Dosimetry

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  1. Fundamental Principles and Recent Developments in Internal Dosimetry 13th International Congress of the International Radiation Protection Association 13-18 May 2012 Glasgow, Scotland George Etherington Health Protection Agency, UK

  2. Outline of the lecture • Sources of exposure & intake pathways • Internal vs. external dosimetry • Main steps in the calculation of internal dose • Biokinetics: models for the respiratory tract & gastro-intestinal tract • Biokinetics: systemic model for plutonium • Dosimetric methods and models • Use of effective dose • Monitoring of workers • Recent developments will be highlighted in the course of the lecture (The most important are presented in ICRP’s Occupational Intakes of Radionuclides (OIR) report series) N.B. Lecture notes address more topics and give more detail

  3. Sources of exposure: % of average annual dose in UK (2.7 mSv) 84% NATURAL 50% radon gas from the ground 9.5% from food 13% gamma and drink rays from ground and 12% cosmic buildings rays 15% medical < 0.1% discharges 0.1% products < 16% 0.2% fallout 0.2% occupational ARTIFICIAL 42000 42000 Slide title • Text

  4. Routes of entry into the body • Inhalation – mainly workers • Ingestion – mainly members if the public • via wounds – workers • via intact skin – (rather rare)

  5. Differences between internal and external dosimetry, #1 • The source of irradiation is the decay of radionuclides in one or more of the organs of the body Internal External Source organs (S) & Target organs (T)

  6. Differences between internal and external dosimetry, #2 Internal doses can’t be measured directly (and so models have to be used extensively) Internal External 6

  7. Differences between internal and external dosimetry, #3 Internal doses are protracted over time Lung(t), Bq t 7

  8. Differences between internal and external dosimetry, #4 Short range (non-penetrating) radiation (α, β, etc.) make a significant contribution to internal dose External contamination Internal External alpha beta gamma 8

  9. Differences between internal and external dosimetry, #5 The distribution of absorbed dose between organs is often very inhomogeneous Internal External alpha-emitter 9

  10. Summary: main steps in calculation of internal dose from first principles • Model the intake to determine where the radionuclide deposits initially • Model the clearance of the radionuclide • Model the uptaketo organs • Model the retention over time • Calculate the energy deposited in organs over a specified time after intake • Calculate the absorbed dose to each organ over a specified time after intake • Calculate the committed equivalent dose to each organ using the appropriate radiation weighting factors • Calculate the committed effective dose using the appropriate tissue weighting factors BIOKINETICS DOSIMETRY

  11. BIOKINETICS

  12. Biokinetic models • Respiratory tract • Gastro-Intestinal (alimentary) tract • Systemic models for each element / group • simple e.g. tritium, caesium • complex e.g. strontium, plutonium

  13. Biokinetic models • Respiratory tract • Gastro-Intestinal (alimentary) tract • Systemic models for each element / group • simple e.g. tritium, caesium • complex e.g. strontium, plutonium

  14. Compartment models

  15. The respiratory tract

  16. Human Respiratory Tract Model (HRTM) ICRP (1994a). ICRP Publication 66.

  17. Particle deposition: physical processes

  18. Regional deposition (HRTM) AMAD – Activity Median Aerodynamic Diameter AMAD – Activity Median Aerodynamic Diameter

  19. Particle Transport Model (HRTM)

  20. Revision of ET clearance model (OIR) Smith et al., (2012) See poster P02.121

  21. Revision of representation of slow bronchial clearance, #1(HRTM) fs ~ particle geometric diameter 21

  22. Revision of representation of slow bronchial clearance, #2(OIR) New experiments (Sweden and UK): • Have confirmed significant slow airway clearance • Show clearance is not directly related to geometric diameter … • … is related to deposition within BB/bb • … is mainly bronchiolar (bb) • … with a shorter half time (4 vs. 20 days)?

  23. Revision of representation of slow bronchial clearance, #3(OIR) Smith et al., (2012) See poster P02.121

  24. AI3 AI2 AI1 7000 d 700 d 30 d 0.1 0.6 0.3 Revision of representation of alveolar clearance(HRTM)

  25. Revision of representation of alveolar clearance(OIR) Gregoratto et al. Radiat Prot Dosim30 491 (2010) 25

  26. Absorption-to-Blood Model (HRTM)

  27. % t ½ F (fast) 100 10 min M (moderate) 10 10 min 90 140 d S (slow) 0.1 10 min 7000 d 99.9 Default absorption Types(HRTM) Rate, /d 100 100 0.005 100 0.0001 ICRP (1994a). ICRP Publication 66. 27

  28. Default absorption TypesRecent Developments (OIR) 28

  29. Human Alimentary Tract Model (HATM)Key features • Inclusion of all alimentary tract regions in the model • Age and gender-dependent parameter values • Retention in organ walls included • Absorption in all regions • Doses calculated explicitly to target regions ICRP (2007). ICRP Publication 100.

  30. ICRP Publication 30 model Modelling the Human Alimentary TractThe ICRP HAT Model (Publication 100) fA (the alimentary tract transfer factor) is the fraction of activity entering the alimentary tract that is absorbed to blood

  31. Systemic biokinetic models: Recycling model for plutonium ICRP (1993). ICRP Publication 67.

  32. DOSIMETRY

  33. Source and Target Organs Source Organ (S) Target Organ (T) Absorbed fraction, AF (T  S, Ei): the fraction of the energy Ei emitted in S that is absorbed in T Specific Effective Energy, SEE(T  S): the equivalent dose in T per nuclear transformation in S (sieverts) For one radiation type: SEE(T  S) = AF (T  S, Ei) x Ei x Yi x wR / MT Yi – radiation yield; wR – radiation weighting factor; MT – organ/tissue mass

  34. Dosimetric phantoms - MIRD and Voxel Absorbed Fraction (AF) calculations are made using Monte-Carlo radiation transport codes (e.g. MCNP-X) ICRP (2009). ICRP Publication 110. Adult reference computational phantoms.

  35. Voxel phantoms for children

  36. Target cell nuclei Source in airway wall Source on surface Respiratory Tract Dosimetry: Geometric model of airway(HRTM) Air 25481

  37. Alimentary Tract Dosimetry:Single layer epithelium Large intestine

  38. Bone Dosimetry Sources Trabecular bone Cortical bone Targets Red bone marrow Endosteum (inner bone surfaces) An α-emitter on bone surfaces: α-particle tracks 38

  39. Bone Dosimetry:Absorbed Fractions (AF) ICRP (1979). ICRP Publication 30, Part 1. 39

  40. Bone Dosimetry:Absorbed Fractions – Recent developments (OIR) More refined treatment of dependence of AF on: - particle/photon energy - cell structure and bone architecture Macroscopic model of bone derived from CT images (1 mm resolution). Microscopic model derived from micro-CT images (30 um resolution) of trabecular (spongy) bone. Used in Monte Carlo radiation transport calculations of AF 40

  41. Recap BIOKINETIC MODELS provide the total number of nuclear transformations in each SOURCE ORGAN within the dose commitment period (e.g. 50 y for workers) DOSIMETRIC MODELS provide the absorbed dose & equivalent dose in each of the TARGET ORGANS resulting from a single nuclear transformation in each SOURCE ORGAN

  42. Calculation of equivalent and effective dose Bq Gy Sv Sv

  43. 50 50 . . ò ò = = H H H H dt dt 50 50 , , T T T T 0 0 Committed dose

  44. Recent development:Sex averaging in calculation of committed effective dose (E)

  45. Use and misuse of effective dose • (Committed) effective dose is used for regulatory purposes for comparison with dose limits and constraints • Determined for a Reference Person (or Reference Worker), not the individual • Relates to stochastic effects only • Allows summation of doses from different radionuclides and external dose Misuse of effective dose (and equivalent doses to organs) • Not for assessments of doses and risks to individuals • Not for epidemiological studies For these purposes, use: • Absorbed doses in organs / tissues • Data on Relative Biological Effectiveness • Specific risk estimates for the individual

  46. ICRP dose coefficients, Sv per Bq intake • Inhalation and ingestion • Workers and public • Adults, children, fetus Publications: 30 Pts1-4, 48, 54, 56, 61, 66, 67, 68, 69, 70, 71, 72, 78, 88, 89, 95 + CD-ROMs Publication 123 (Occupational Intakes of Radionuclides) Parts 1, 2, 3, 4, 5 (publication from Spring 2013)

  47. Monitoring for internal contamination (1) Body monitoring (also known as in vivo monitoring or direct monitoring)

  48. Monitoring for internal contamination (2) Bioassay sample monitoring (also known as in vitro monitoring or indirect monitoring)

  49. Monitoring for internal contamination (3) • Air sampling • Static air sampling • Personal air sampling

  50. Interpretation of monitoring dataWhole body retention of 60Co as a fraction of intake

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