IAEA International Atomic Energy Agency Lecture 5 – Internal dose assessment and interpretation of measurement results Postgraduate Educational Course in radiation protection and the Safety of Radiation sources PART V: ASSESSMENT OF EXTERNAL AND INTERNAL EXPOSURES (OTHER THAN MEDICAL) Module V.1 - Assessment of occupational exposure due to intakes of radionuclides

IAEA Dose Assessment - lecture objectives The objective of this lecture is to outline the need for evaluation of information provided by a dosimeter or a measurement of intake, with special emphasis on the indication that a high or unusual exposure occurred, and to identify those steps that may need to be taken. At the completion of this lecture, the student should understand (1) that dosimeter and measurement results alone may not be adequate to assess exposures at or above the investigation level, (2) that more detailed evaluation may be necessary, and (3) what steps should be taken for this assessment. V.2 Lecture 5 - Internal dose assessment2

IAEA Dose Assessment – lecture Outline l Need for special dose assessment l Interpretation of intake measurement results l Estimation of intake and dose V.2 Lecture 5 - Internal dose assessment3

IAEA Need for Special Dose Assessment V.2 Lecture 5 - Internal dose assessment4

IAEA Evaluation of a single exposure : l Below the investigation level, use the internal dose (CED) estimated using standard parameters l Between the investigation level and the annual dose limit, get additional information to improve the assessment of the personal dose equivalent; intake and CED l Above the annual limit, determine the best value of CED V.2 Lecture 5 - Internal dose assessment5

IAEA Estimation of CED l May also be necessary when monitoring data indicate: u Individual doses summed over several monitoring periods exceed the corresponding annual dose limit; u Doses for individuals estimated from workplace monitoring results (air concentration, etc.) exceed the corresponding annual dose limit. V.2 Lecture 5 - Internal dose assessment6

IAEA Interpretation of Intake Measurement Results V.2 Lecture 5 - Internal dose assessment7

IAEA Measurements for internal dose assessment l Direct measurement - the use of detectors placed external to the body to detect ionizing radiation emitted by radioactive material contained in the body. l Indirect measurement - the analysis of excreta, or other biological materials, or physical samples to estimate the body content and/or intake of radioactive material. V.2 Lecture 5 - Internal dose assessment8

IAEA Measurements for internal dose assessment l Direct or indirect measurements provide information about the radionuclides present in: u The body, u Parts of the body, e.g. specific organs or tissues, u A biological sample or u A sample from the working environment. l These data are likely to be used first for an estimation of the intake of the radionuclide V.2 Lecture 5 - Internal dose assessment9

IAEA Measurements for internal dose assessment l Biokinetic models are used for this purpose. l Measurements of body activity can also be used to estimate dose rates directly l Calculation of committed doses from direct measurements still involves the assumption of a biokinetic model, l If sufficient measurements are available to determine retention functions, biokinetic models may not be needed V.2 Lecture 5 - Internal dose assessment10

IAEA Estimated intake Direct Measurements (In vivo) Body/organ content, M Indirect Measurements Excretion rate, M Air concentration Dose rate Committed effective dose m(t) DAC-hr e(g) j m(t) Interpretation of monitoring measurements V.2 Lecture 5 - Internal dose assessment11

IAEA 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 (indirect measurement) – retention fraction - at time t (usually in days) after intake. V.2 Lecture 5 - Internal dose assessment12

IAEA Estimate of intake l The ICRP has published generic values of m(t) l When significant intakes may have occurred, more refined calculations based on individual specific parameters (special dosimetry) should be made l If multiple measurements are available, a single best estimate of intake may be obtained, for example, by the method of least squares V.2 Lecture 5 - Internal dose assessment13

IAEA Retention fraction – m(t) Depends on: Route of intake Absorption type, i.e. chemical form; Type F (fast), Type M (moderate), or Type S (slow) Measurement and sample types u Direct Ø Whole body Ø Lungs Ø Thyroid u Indirect Ø Urine Ø Faeces V.2 Lecture 5 - Internal dose assessment14

IAEA Retention fraction example – 60 Co Intake may be through inhalation, ingestion or injection (wounds) Assigned two absorption types – M and S Assigned two f 1 values for ingestion – 0.01 and 0.05 ICRP 78 considers 4 possibilities for measurement u Direct  Whole body  Lungs u Indirect  Urine  Faeces V.2 Lecture 5 - Internal dose assessment15

IAEA 60 Co Routine Monitoring Retention Fractions - Inhalation V.2 Lecture 5 - Internal dose assessment16

IAEA 60 Co Retention Fractions - Inhalation Type M Type S V.2 Lecture 5 - Internal dose assessment17

IAEA 60 Co Routine Special Retention Fractions Inhalation V.2 Lecture 5 - Internal dose assessment18

IAEA 60 Co Retention Fractions - Ingestion f 1 = 0.1 f 1 = 0.05 Special Monitoring V.2 Lecture 5 -Internal dose assessment19

IAEA 60 Co Retention Fractions - Injection Special Monitoring V.2 Lecture 5 - Internal dose assessment20

IAEA Chronic Intake – Equilibrium Values Equilibrium predicted to occur at the indicated time after exposure begins Values are shown for intakes of: u 1 Bq/year (1/365 Bq per day) u annual intake equivalent to 20 mSv/y 2.8  10 6 Bq/y (7.7  10 3 Bq/d) (Type M) V.2 Lecture 5 - Internal dose assessment21

IAEA Intake Estimates - An Example V.2 Lecture 5 - Internal dose assessment22

IAEA Estimate of intake - an example l Occupational exposure to radioiodine occurs in various situations l I-131 is a common short lived iodine isotope: u Half-life = 8 d u  particles - average energy 0.19 MeV u  - main emission energy = 0.36 MeV u Rapidly absorbed in blood following intake u Concentrates in the thyroid u Excretion predominantly in urine V.2 Lecture 5 - Internal dose assessment23

IAEA Estimate of intake - an example l After intake, I-131 may be detected directly in the thyroid, or indirectly in urine samples l If occupational exposure to I-131 can occur, a routine monitoring program is needed l Based on direct thyroid measurement or l Indirect monitoring of urine or workplace samples V.2 Lecture 5 - Internal dose assessment24

IAEA Estimate of intake - an example l Choice of monitoring method depends on various factors: u Availability of instrumentation u Relative costs of the analyses u Sensitivity that is needed l Direct measurement of activity in the thyroid offers the most accurate dose assessment l Other methods may be adequate and may be better suited to the circumstances V.2 Lecture 5 - Internal dose assessment25

IAEA Estimate of intake - an example l Chemical form of the radionuclide is a key parameter in establishing biokinetics l All common forms of iodine are readily taken up by the body l For inhalation of particulate iodine, lung absorption type F is assumed l Elemental iodine vapour is assigned to class SR-1 with absorption type F l Absorption of iodine from the gastrointestinal tract is assumed to be complete, i.e. f 1 = 1. V.2 Lecture 5 - Internal dose assessment26

IAEA Dose coefficients (a) For lung absorption types see para of RS-G-1.2 (b) For inhalation of gases and vapours, the AMAD does not apply for this form. V.2 Lecture 5 - Internal dose assessment27

IAEA Biokinetic model for systemic iodine V.2 Lecture 5 - Internal dose assessment28

IAEA Radioiodine biokinetics l 30% of iodine reaching the blood is assumed transported to the thyroid l The other 70% is excreted directly in urine l Biological half-time in blood is taken to be 6 h l Iodine incorporated into thyroid hormones leaves the gland with a biological half-life of 80 d and enters other tissues V.2 Lecture 5 - Internal dose assessment29

IAEA Radioiodine biokinetics l Iodine is retained in these tissues with a biological half-life of 12 d. l Most iodine (80%) is subsequently released and available in the circulation for uptake by the thyroid or direct urinary excretion l Remainder is excreted via the large intestine in the faeces l The physical half-life of I-131 is short, so this recycling is not important for committed effective dose. V.2 Lecture 5 - Internal dose assessment30

IAEA 131 I intake - Thyroid monitoring l A routine monitoring programme l 14 day monitoring period l Thyroid content of 3000 Bq 131 I is detected in a male worker l Based on workplace situation, exposures are assumed due to inhalation of particulates l Intakes by ingestion would lead to the same pattern of retention and excretion V.2 Lecture 5 - Internal dose assessment31

IAEA 131 I intake - Thyroid monitoring l Intake pattern is not known l Assume an acute intake occurred in the middle of the monitoring period l 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 V.2 Lecture 5 - Internal dose assessment32

IAEA 131 I intake - Thyroid monitoring l Thus, m(7) = 0.074, and l Application of the dose coefficients given in the GSR and in the previous table gives, a committed effective dose of 0.45 mSv (4.1  10 4 Bq  1.1  Sv/Bq  10 3 mSv/Sv) l This dose may require follow-up investigation V.2 Lecture 5 - Internal dose assessment33

IAEA 131 I intake - Urine measurement l One day after the direct thyroid measurement, the worker has a 24-h urine sample l Sample assay shows 30 Bq of 131 I l From the biokinetic model for a type F particulate, m(8) for daily urinary excretion is 1.1 E-04 V.2 Lecture 5 - Internal dose assessment34

IAEA 131 I intake - Urine measurement l A committed effective dose of 3 mSv (2.7  10 5 Bq  1.1  Sv/Bq  10 3 mSv/Sv) l For this example no account is taken of any previous intakes V.2 Lecture 5 - Internal dose assessment35

IAEA 131 I intake - Workplace air measurements l Workplace air measurements showed 131 I concentrations that were low but variable l Maximum concentrations between 10 and 20 kBq/m 3 (12 to 25 times the DAC) for short periods several times in several locations l At the default breathing rate of 1.2 m 3 /h, worker could receive an intake of 24 kBq in one hour without respiratory protection V.2 Lecture 5 - Internal dose assessment36

IAEA Derived air concentrations V.2 Lecture 5 - Internal dose assessment37

IAEA 131 I intake - Workplace air measurements l If worker had worked for one hour without respiratory protection, or l Somewhat longer with limited respiratory protection l The intake estimated from air monitoring would be consistent with that determined by bioassay (direct and indirect) measurements V.2 Lecture 5 - Internal dose assessment38

IAEA 131 I intake - Dose assessment l Intake discrepancy suggests at least one of the default assumptions is not correct l Significant individual differences in uptake and metabolism cannot generally account for discrepancies of nearly a factor of 10 l The rate of 131 I excretion in urine decreases markedly with time after intake - a factor of more than 1000 over the monitoring period V.2 Lecture 5 - Internal dose assessment39

IAEA 131 I - Daily urinary excretion after inhalation V.2 Lecture 5 - Internal dose assessment40

IAEA 131 I intake - Dose assessment l Assumption of the time of intake is a probable source of error l If the intake occurred 3 days before the urine sample was submitted l Intake estimated from the urine measurement would be 21 kBq l Intake from the thyroid measurement would be 25 kBq l The agreement would be satisfactory V.2 Lecture 5 - Internal dose assessment41

IAEA 131 I intake - Dose assessment l From the biokinetic model, the fraction of inhaled 131 I retained in the thyroid only changes by about a factor of 3 over the monitoring period l Without more information, the new assumption is more reliable for dose assessment l The committed effective dose for this example would then be 0.27 mSv l A 2 nd urine sample obtained after a few more days should be used to verify this conclusion. V.2 Lecture 5 - Internal dose assessment42

IAEA 131 I intake - Dose assessment l Committed effective dose from thyroid monitoring is relatively insensitive to assumptions about the time of intake l However, there is rapid change in urinary excretion with time after exposure l Result - direct measurement provides a more reliable basis for interpreting routine radioiodine monitoring measurements l Urine screening may still be adequate to detect significant intakes V.2 Lecture 5 - Internal dose assessment43

IAEA 131 I intake - Dose assessment l Air concentrations that substantially exceed a DAC should trigger individual monitoring l However, because of direct dependence on: u Period of exposure u Breathing rates u Levels of protection and u Other factors known only approximately l Intake based on air monitoring for 131 I are less reliable than from individual measurements V.2 Lecture 5 - Internal dose assessment44

IAEA References GSR INTERNATIONAL ATOMIC ENERGY AGENCY, Occupational Radiation Protection, Safety Guide No. RS-G-1.1, ISBN (1999). INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment of Occupational Exposure Due to Intakes of Radionuclides, Safety Guide No. RS-G-1.2, ISBN (1999). INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment of Occupational Exposure Due to External Sources of Radiation, Safety Guide RS-G-1.3 (1999). V.2 Lecture 5 - Internal dose assessment45

IAEA References INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION/ INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Conversion Coefficients for Use in Radiological Protection Against External Radiation, ICRP Publication 74, Pergamon Press, London and New York (1997) or ICRU Publication 57, ICRU, 7910 Woodmont Ave., Bethesda, MD USA (1998). INTERNATIONAL ATOMIC ENERGY AGENCY, Indirect Methods for Assessing Intakes of Radionuclides Causing Occupational Exposure, Safety Guide, Safety Reports Series No. 18, ISBN (2002). V.2 Lecture 5 - Internal dose assessment46

IAEA References INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Limits for Intakes of Radionuclides by Workers, ICRP Publication 30, Part 1, Annals of the ICRP 2(3/4), Pergamon Press, Oxford (1979). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Limits for Intakes of Radionuclides by Workers, ICRP Publication 30, Part 2, Annals of the ICRP 4(3/4), Pergamon Press, Oxford (1980). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Limits for Intakes of Radionuclides by Workers, ICRP Publication 30, Part 3 (including addendum to Parts 1 and 2), Annals of the ICRP 6(2/3), Pergamon Press, Oxford (1981). V.2 Lecture 5 - Internal dose assessment47

IAEA References INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Dose Coefficients for Intakes of Radionuclides by Workers, ICRP Publication 68. Annals of the ICRP 24(4), Elsevier Science Ltd., Oxford (1994). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Individual Monitoring for Internal Exposure of Workers: Replacement of ICRP Publication 54, ICRP Publication 78, Annals of the ICRP 27(3-4), Pergamon Press, Oxford (1997). NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Evaluating the Reliability of Biokinetic and Dosimetric Models and Parameters Used to Assess Individual Doses for Risk Assessment Purposes, NCRP Commentary No.15, NCRP, Bethesda (1998). NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS - Uncertainties in Internal Radiation Dose Assessment, Report No. 164 (2009) V.2 Lecture 5 - Internal dose assessment48