International Atomic Energy Agency 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 of radioactive material.
International Atomic Energy Agency 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
International Atomic Energy Agency 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
International Atomic Energy Agency 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
International Atomic Energy Agency 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.
International Atomic Energy Agency Estimate of intake l The ICRP has published default values of m(t) in Publication 78 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 is obtained by the method of least squares. l When more than 10% of the measurements could be attributed to previous evaluated intakes a correction should be performed.
International Atomic Energy Agency 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 Implementing biokinetic models
International Atomic Energy Agency Basic assumption for a reference worker in ICRP 78: – Adult male – Normal nose breathing at light work – Breathing rate 1.2 m 3 /h – Inhaled aerosol with Activity Median Aerodynamic Diameter (AMAD) 5 µm –Regional Deposition [%] ET 1 34 ET 2 40 BB 1.8 BB 1.1 AI 5.3 total 82 Implementing biokinetic models ICRP 78 CURVES AND DATA The data and curves available in ICRP 78 refers to these specific conditions of exposure!
International Atomic Energy Agency Implementing biokinetic models ICRP 78 CURVES AND DATA – Description of the model – Standard assumption for transfer into systemic phase – Dose coefficients – Other informations In relation to the radionuclide other significant information are available: monitoring techniques (as for Pu), etc. General information : e(50) ALI=0.02/e(50)
International Atomic Energy AgencyCaesium Model : Non- recycling model H, P, Cr, Mn, Co, Zn, Rb, Zr, Ru, Ag, Sb, Ce, Hg, Cf are as well
International Atomic Energy Agency Retention : (Bq per Bq intake) Excretion : (Bq/d per Bq intake) Implementing biokinetic models ICRP 78 CURVES AND DATA Special monitoring (inhalation) Routine monitoring (inhalation) Special monitoring (ingestion and injection) Data : m(t) m(T/2) t T
International Atomic Energy Agency Retention or excretion 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 type u Direct Whole body Lungs Thyroid u Indirect Urine Faeces
International Atomic Energy Agency 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
International Atomic Energy Agency 60 Co Routine Monitoring Retention Fractions Inhalation
International Atomic Energy Agency 60 Co Retention Fractions - Inhalation Type M Type S
International Atomic Energy Agency 60 Co Routine Special Retention Fractions Inhalation
International Atomic Energy Agency 60 Co Retention Fractions - Ingestion f 1 = 0.1 f 1 = 0.05 Special Monitoring
International Atomic Energy Agency 60 Co Retention Fractions - Injection Special Monitoring
International Atomic Energy Agency Intake Estimates - An Example
International Atomic Energy Agency 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 0.364 MeV u Rapidly absorbed in blood following intake u Concentrates in the thyroid u Excreted predominantly in urine
International Atomic Energy Agency 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 programme is needed l Based on direct thyroid measurement or l Indirect monitoring of urine or workplace samples
International Atomic Energy Agency 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
International Atomic Energy Agency 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.
International Atomic Energy Agency Dose coefficients (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. 2.0 E-08 1.4 E-08
International Atomic Energy Agency Biokinetic model for systemic iodine
International Atomic Energy Agency 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
International Atomic Energy Agency 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.
International Atomic Energy Agency 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
International Atomic Energy Agency 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 from table A.6.17 (Thyroid) in ICRP 78
International Atomic Energy Agency 131 I intake - Thyroid monitoring Time after intake, d Retention, Bq Vapor particle 0.074 7 or table A.6.17 in ICRP 78 Special monitoring
International Atomic Energy Agency 131 I intake - Thyroid monitoring l Thus, m(7) = 0.074, and l Application of the dose coefficients given in the BSS and in the previous table gives, l A committed effective dose of 0.45 mSv (4.110 4 Bq 1.110 -8 Sv/Bq 10 3 mSv/Sv) l This dose may require follow-up investigation
International Atomic Energy Agency 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 from table A.6.17 (dairy urinary excretion) in ICRP 78
International Atomic Energy Agency 131 I intake - Urine measurement l A committed effective dose of 3 mSv (2.710 5 Bq 1.110 -8 Sv/Bq 10 3 mSv/Sv) l For this example no account is taken of any previous intakes
International Atomic Energy Agency 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 DAC; Derived Air Concentration
International Atomic Energy Agency Derived air concentrations
International Atomic Energy Agency 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
International Atomic Energy Agency 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
International Atomic Energy Agency 131 I - Daily urinary excretion after inhalation
International Atomic Energy Agency 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
International Atomic Energy Agency 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 2nd urine sample obtained after a few more days should be used to verify this conclusion.
International Atomic Energy Agency 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
International Atomic Energy Agency 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