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ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF RADIONUCLIDES

ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF RADIONUCLIDES. Interpretation of Measurement Results. Introduction. Measurements for internal dose assessment.

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ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF RADIONUCLIDES

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  1. ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF RADIONUCLIDES Interpretation of Measurement Results

  2. Introduction

  3. Measurements for internal dose assessment • Direct measurement - the use of detectors placed external to the body to detect ionizing radiation emitted by radioactive material contained in the body. • Indirect measurement - the analysis of excreta, or other biological materials, or physical samples to estimate the body content of radioactive material.

  4. Measurements for internal dose assessment • Direct or indirect measurements provide information about the radionuclides present in: • The body, • Parts of the body, e.g specific organs or tissues, • A biological sample or • A sample from the working environment. • These data are likely to be used first for an estimation of the intake of the radionuclide

  5. Measurements for internal dose assessment • Biokinetic models are used for this purpose. • Measurements of body activity can also be used to estimate dose rates directly • Calculation of committed doses from direct measurements still involves the assumption of a biokinetic model, • If sufficient measurements are available to determine retention functions, biokinetic models may not be needed

  6. Interpretation of monitoring measurements Direct Measurements (In vivo) Indirect Measurements m(t) Excretion rate, M Air concentration m(t) Body/organ content, M Estimated intake DAC-hr e(g)j Dose rate Committed effective dose

  7. Estimate of intake Where M is the measured body content or excretion rate, m(t) is the fraction of the intake retained in the whole body (direct measurement) or having been excreted from the body in a single day (indirect measurement) – retention or excretion fraction - at time t (usually in days) after intake.

  8. Estimate of intake • The ICRP has published default values of m(t) in Publication 78 • When significant intakes may have occurred, more refined calculations based on individual specific parameters (special dosimetry) should be made • If multiple measurements are available, a single best estimate of intake is obtained by the method of least squares. • When more than 10% of the measurements could be attributed to previous evaluated intakes a correction should be performed.

  9. Implementing biokinetic models • The ICRP Publication 78 “Individual Monitoring for internal exposure of workers - replacement of ICRP Publication 54” provides a general guidance on the design of individual monitoring programmes and the interpretation of results of estimates of intakes of radionuclides by workers. • A reference worker is assumed in relation to the biokinetic models and the parameter values describing the scenario of contamination. Radionuclides are selected for their potential importance in occupational exposure. • This publication replaces the previous one ICRP Publication 54 “Individual Monitoring for intakes of radionuclides by workers: design and interpretation” taking into account: - new protection quantities and new set of exposure conditions (ICRP 60) - new general principles for radiation protection of workers (ICRP 75) - respiratory tract model of ICRP 66 - revised biokinetic models when available for selected radionuclides

  10. Implementing biokinetic models ICRP 78 CURVES AND DATA • Basic assumption for a reference worker in ICRP 78: – Adult male – Normal nose breathing at light work – Breathing rate 1.2 m3/h – Inhaled aerosol with Activity Median Aerodynamic Diameter (AMAD) 5 µm – Regional Deposition [%] ET1 34 ET2 40 BB 1.8 BB 1.1 AI 5.3 total 82 The data and curves available in ICRP 78 refers to these specific conditions of exposure!

  11. Implementing biokinetic models ICRP 78 CURVES AND DATA • General information : – Description of the model – Standard assumption for transfer into systemic phase – Dose coefficients e(50) – Other informations ALI=0.02/e(50) In relation to the radionuclide other significant information are available: monitoring techniques (as for Pu), etc.

  12. Caesium • Model : Non-recycling model H, P, Cr, Mn, Co, Zn, Rb, Zr, Ru, Ag, Sb, Ce, Hg, Cf are as well

  13. Implementing biokinetic models ICRP 78 CURVES AND DATA • Data : • Retention : (Bq per Bq intake) • Excretion : (Bq/d per Bq intake) t m(t) Special monitoring (inhalation) Special monitoring (ingestion and injection) Routine monitoring (inhalation) T m(T/2)

  14. Retention or excretion fraction – m(t) • Direct • Whole body • Lungs • Thyroid • Indirect • Urine • Faeces Depends on: • Route of intake • Absorption type, i.e. chemical form; Type F (fast), Type M (moderate), or Type S (slow) • Measurement and sample type

  15. Retention fraction example – 60Co • Intake may be through inhalation, ingestion or injection (wounds) • Assigned two absorption types – M and S • Assigned two f1 values for ingestion – 0.01 and 0.05 • ICRP 78 considers 4 possibilities for measurement • Direct • Whole body • Lungs • Indirect • Urine • Faeces

  16. 60Co Routine Monitoring Retention FractionsInhalation

  17. 60Co Retention Fractions - Inhalation Type M Type S

  18. 60Co Routine Special Retention FractionsInhalation

  19. 60Co Retention Fractions - Ingestion Special Monitoring f1 = 0.1 f1 = 0.05

  20. 60Co Retention Fractions - Injection Special Monitoring

  21. Intake Estimates - An Example

  22. Estimate of intake - an example • Occupational exposure to radioiodine occurs in various situations • I-131 is a common short lived iodine isotope: • Half-life = 8 d •  particles - average energy 0.19 MeV •  - main emission 0.364 MeV • Rapidly absorbed in blood following intake • Concentrates in the thyroid • Excreted predominantly in urine

  23. Estimate of intake - an example • After intake, I-131 may be detected directly in the thyroid, or indirectly in urine samples • If occupational exposure to I-131 can occur, a routine monitoring programme is needed • Based on direct thyroid measurement or • Indirect monitoring of urine or workplace samples

  24. Estimate of intake - an example • Choice of monitoring method depends on various factors: • Availability of instrumentation • Relative costs of the analyses • Sensitivity that is needed • Direct measurement of activity in the thyroid offers the most accurate dose assessment • Other methods may be adequate and may be better suited to the circumstances

  25. Estimate of intake - an example • Chemical form of the radionuclide is a key parameter in establishing biokinetics • All common forms of iodine are readily taken up by the body • For inhalation of particulate iodine, lung absorption type F is assumed • Elemental iodine vapour is assigned to class SR-1 with absorption type F • Absorption of iodine from the gastrointestinal tract is assumed to be complete, i.e. f1 = 1.

  26. Dose coefficients 1.4 E-08 2.0 E-08 (a) For lung absorption types see para. 6.16 of RS-G-1.2 (b) For inhalation of gases and vapours, the AMAD does not apply for this form.

  27. Biokinetic model for systemic iodine

  28. Radioiodine biokinetics • 30% of iodine reaching the blood is assumed transported to the thyroid • The other 70% is excreted directly in urine • Biological half-time in blood is taken to be 6 h • Iodine incorporated into thyroid hormones leaves the gland with a biological half-life of 80 d and enters other tissues

  29. Radioiodine biokinetics • Iodine is retained in these tissues with a biological half-life of 12 d. • Most iodine (80%) is subsequently released and available in the circulation for uptake by the thyroid or direct urinary excretion • Remainder is excreted via the large intestine in the faeces • The physical half-life of I-131 is short, so this recycling is not important for committed effective dose.

  30. 131I intake - Thyroid monitoring • A routine monitoring programme • 14 day monitoring period • Thyroid content of 3000 Bq131I is detected in a male worker • Based on workplace situation, exposures are assumed due to inhalation of particulates • Intakes by ingestion would lead to the same pattern of retention and excretion

  31. 131I intake - Thyroid monitoring • Intake pattern is not known • Assume an acute intake occurred in the middle of the monitoring period • From the biokinetic model, 7.4% of the radioactivity inhaled in a particulate (type F) form with a default AMAD of 5 is retained in the thyroid after 7 d from table A.6.17 (Thyroid) in ICRP 78

  32. 131I intake - Thyroid monitoring Special monitoring 0.074 Vapor particle Retention, Bq or table A.6.17 in ICRP 78 7 Time after intake, d

  33. 131I intake - Thyroid monitoring • Thus, m(7) = 0.074, and • Application of the dose coefficients given in the BSS and in the previous table gives, • A committed effective dose of 0.45 mSv (4.1•104 Bq  1.1•10-8 Sv/Bq  103 mSv/Sv) • This dose may require follow-up investigation

  34. 131I intake - Urine measurement • One day after the direct thyroid measurement, the worker has a 24-h urine sample • Sample assay shows 30 Bq of 131I • From the biokinetic model for a type F particulate, m(8) for daily urinary excretion is 1.1 E-04 from table A.6.17 (dairy urinary excretion) in ICRP 78

  35. 131I intake - Urine measurement • A committed effective dose of 3 mSv (2.7•105 Bq  1.1•10-8 Sv/Bq  103 mSv/Sv) • For this example no account is taken of any previous intakes

  36. 131I intake - Workplace air measurements • Workplace air measurements showed 131I concentrations that were low but variable • Maximum concentrations between 10 and 20 kBq/m3 (12 to 25 times the DAC) for short periods several times in several locations • At the default breathing rate of 1.2 m3/h, worker could receive an intake of 24 kBq in one hour without respiratory protection DAC; Derived Air Concentration

  37. Derived air concentrations

  38. 131I intake - Workplace air measurements • If worker had worked for one hour without respiratory protection, or • Somewhat longer with limited respiratory protection • The intake estimated from air monitoring would be consistent with that determined by bioassay (direct and indirect) measurements

  39. 131I intake - Dose assessment • Intake discrepancy suggests at least one of the default assumptions is not correct • Significant individual differences in uptake and metabolism cannot generally account for discrepancies of nearly a factor of 10 • The rate of 131I excretion in urine decreases markedly with time after intake - a factor of more than 1000 over the monitoring period

  40. 131I - Daily urinary excretion after inhalation

  41. 131I intake - Dose assessment • Assumption of the time of intake is a probable source of error • If the intake occurred 3 days before the urine sample was submitted • Intake estimated from the urine measurement would be 21 kBq • Intake from the thyroid measurement would be 25 kBq • The agreement would be satisfactory

  42. 131I intake - Dose assessment • From the biokinetic model, the fraction of inhaled 131I retained in the thyroid only changes by about a factor of 3 over the monitoring period • Without more information, the new assumption is more reliable for dose assessment • The committed effective dose for this example would then be 0.27 mSv • A 2nd urine sample obtained after a few more days should be used to verify this conclusion.

  43. 131I intake - Dose assessment • Committed effective dose from thyroid monitoring is relatively insensitive to assumptions about the time of intake • However, there is rapid change in urinary excretion with time after exposure • Result - direct measurement provides a more reliable basis for interpreting routine radioiodine monitoring measurements • Urine screening may still be adequate to detect significant intakes

  44. 131I intake - Dose assessment • Air concentrations that substantially exceed a DAC should trigger individual monitoring • However, because of direct dependence on: • Period of exposure • Breathing rates • Levels of protection and • Other factors known only approximately • Intake based on air monitoring for 131I are less reliable than from individual measurements

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