1. International Atomic Energy Agency ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKE OF RADIONUCLIDES Uncertainties and Performance Criteria

2. International Atomic Energy Agency Interpretation of Measurement Results – Unit Objectives The objective of this unit to identify and define the criteria that are used to characterize the quality of the measurement process for both direct and indirect methods. It will also identify sources of uncertainty in measurement and interpretation and give an estimate of expected magnitudes. At the completion of this unit, the student should understand how to calculate Minimum Detectable Activity and establish adequate accuracy criteria for measurement bias and precision.

3. International Atomic Energy Agency Interpretation of Measurement Results - Unit Outline Measurement Uncertainties Intake and Dose Assessment Uncertainties Performance Criteria: Accuracy Performance Criteria: Sensitivity MDAs - Examples

4. International Atomic Energy Agency Measurement Uncertainties

5. International Atomic Energy Agency Dose determination uncertainties Measurement ?1?1 Direct or indirect measurements Body/organ content, M or Excretion rate, R Interpretation e(g) j m(t) Estimated intake Committed effective dose ?2?2 ?3?3 Body/organ content, M or Excretion rate, R

6. International Atomic Energy Agency Measurement uncertainties Usually most straightforward to estimate Counting statistics dominate at low activities For radionuclides that are, u Easily detected, and u In sufficient quantity, counting statistics are small compared to other uncertainties Systematic uncertainties are important Correction for activity remaining previously measured intakes may be necessary

7. International Atomic Energy Agency Common measurement uncertainties Statistical counting errors Distribution in the body Absorption by overlying tissue (low energy photons) External contamination of the subject or measurement system Calibration errors u Source activity u Simulation accuracy

8. International Atomic Energy Agency Estimated Direct Measurement uncertainties * Source of uncertainty Estimated magnitude 1 σ Chest wall thickness determination 15% to 300% worst case for 17 keV Geometry errors – Subject size and shape departure from single-size calibration model 10% for good geometries (I m arc, linear w/front /back counts) 15-20% for common geometries (linear w/counts from 1 side, 50 cm arc) 40% for poor geometries (detector in contact w/body) Positioning of subject10-15% for whole body * From ANSI 13.30 (1996)

9. International Atomic Energy Agency Typical uncertainties for assessing fission product isotopes * Source of Uncertainty Estimated Uncertainty Depth Length  Width Height-Weight Analysis Technique Calibration Counting Statistics Total Estimated Uncertainty 12% 5% 7% 3% 5% 7% 40% * From Toohey, et al,

10. International Atomic Energy Agency Typical uncertainties for U lung counting Source of Uncertainty Estimated Uncertainty Chest Depth Chest Wall Thickness Activity Location Detector Placement Subject Background Calibration Counting Statistics Total Estimated Uncertainty 12% 15% 5% 10% 5% 40% 90% * From Toohey, et al,

11. International Atomic Energy Agency Typical uncertainties for Pu lung counting Source of errorUncertainty Subject background50% Counting statistics50% Chest wall thickness40% Non-uniform distribution70% Calibration20% Overall uncertainty110% * From Toohey, et al,

12. International Atomic Energy Agency Estimated Indirect Measurement uncertainties * Several parameters contribute to indirect measurement uncertainties The uncertainty associated with most are highly variable Typical uncertainties associated with the radiochemistry are of the order of 3% More details can be found in the USDOE Laboratory Accreditation Program report – ANSI 13.30

13. International Atomic Energy Agency Intake and Dose Assessment Uncertainties

14. International Atomic Energy Agency Some sources of assessment uncertainty Mode of intake Physical and chemical form of material Particle size (AMAD) of the aerosol Time pattern of intake (acute vs. chronic) Errors in biokinetic and dosimetric models Individual variability in biokinetic and dosimetric parameters

15. International Atomic Energy Agency Intake assessment uncertainties Difficult to quantify in routine monitoring - measurements are made at pre-determined times are unrelated to time of intakes Compromise between measurement interpretation quality and the practical limitations linked to measurement frequency Monitoring intervals should be selected so that underestimates due to unknown time of intake are ≤ 3

16. International Atomic Energy Agency Intake assessment uncertainties Practically, this is a maximum since the actual distribution of the exposure in time is unknown Statistically, the error is not systematically the same for all the assessments The random distribution of the exposure makes such an error clearly lower than a factor of 3 If intake occurs just before sampling or measurement, it could be overestimated ≥ 3

17. International Atomic Energy Agency Intake assessment uncertainties Particularly important for excreta monitoring  daily fractions excreted can change rapidly immediately after intake If a high result is found in routine monitoring, it would be appropriate to repeat the sampling or measurement a few days later  adjust the estimate of intake accordingly Samples could also be collected after a period of non-exposure, e.g. after weekend or holiday

18. International Atomic Energy Agency Assessment uncertainties l Models used to describe radionuclide behaviour are used to assess intake and dose l Reliability of dose estimates depends on the accuracy of the models, and limitations on their application l This will depend upon many factors, including: u Knowledge of the time of intake, and u Whether the intake was acute or chronic

19. International Atomic Energy Agency Assessment uncertainties l If the sampling period does not enable the estimation of the biological half-life, assumption of a long body retention may lead to an underestimate of the intake and the committed effective dose l The degree of over- or under-estimation of the dose depends on the body retention pattern

20. International Atomic Energy Agency Assessment uncertainties l Radionuclide behaviour in the body depends upon their physicochemical characteristics l Particle size of inhaled radionuclides is a particularly important for influencing deposition in the respiratory system l Gut absorption factor f 1 substantially influences effective dose following ingestion

21. International Atomic Energy Agency Assessment uncertainties When exposures during routine monitoring are well within limits on intake, default parameters may be sufficient to assess intake If exposures approach or exceed these limits, more specific information on; u Physical form and chemical form of the intake, and u Characteristics of the individual, may be needed to improve the accuracy of the model predictions

22. International Atomic Energy Agency Intake fraction, m(t) depends on several factors Time after intake, d 010100100010000 1 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 m(t) Whole body Lungs Urine Feces Intake pattern (act. vs. chr.) Deposition site Time after intake Particle size Absorption rate (F, M or S) Mode of intake 60 Co, inhalation type M

23. International Atomic Energy Agency Performance Criteria: Accuracy

24. International Atomic Energy Agency Performance criteria Accuracy u Bias (Systematic errors) How well can a given measurement be reproduced. u Repeatability or Precision (Random errors) How close is the mean of a series of measurements to the true value Sensitivity (MDA) What is the lowest value of a quantity that can be measured?

25. International Atomic Energy Agency Performance criteria - Bias where:B ri = relative bias for the ith measurement A i = measured activity A ai = actual activity for the ith measurement Definition:

26. International Atomic Energy Agency Performance criteria - Bias For a test or measurement category, Where:B r = Relative bias for the category n = number of replicate measurements

27. International Atomic Energy Agency Performance criteria – Repeatability * Definition: where:S Br = measurement repeatability for the test or measurement category * Also termed Precision

28. International Atomic Energy Agency Accuracy - How close is close enough? When the activity A ai is at or above the specified Minimum Testing Level (MTL), l Relative bias, B r - 0.25  B r  +0.50 l Relative repeatability, S Br S Br  0.40 These values used by ISO and USDOE Laboratory Accreditation Programme

29. International Atomic Energy Agency MTL Values for Direct Measurements Measurement CategoryTypeRadionuclideMTL I. Transuranium elements via L x- rays Lung 238 Pu9 kBq II. Americium-241 Lung 241 Am0.1 kBq III. Thorium 234 Lung 234 Th in equilibrium w/ parent 238 U 0.5 kBq IV. Uranium-235 Lung 235 U30 kBq V. Fission and activation products Lung Any two: 54 Mn, 58 Co, 60 Co, 144 Ce + 134 Cs & 137 Cs/ 137 Ba 3 kBq 30 kBq 3 kBq VI. Fission and activation products Total body All of: 134 Cs, 137 Cs/ 137m Ba, 60 Co & 54 Mn 3 kBq VII. Radionuclides in the thyroid Thyroid 131 I or 125 I3 kBq

30. International Atomic Energy Agency MTL Values for Indirect Measurements Measurement categoryRadionuclide MTL (per L or per sample) I. BETA activity: average energy < 100keV 3 H, 14 C 35 S 228 Ra 2 kBq 20 kBq 0.9 kBq II. BETA activity: average energy ≤ 100 keV 32 P 89, 90 Sr or 90 Sr 4 Bq III. ALPHA activity: isotopic analysis 228,/230 Th or 232 Th 234/235 U or 238 U 237 Np 238 Pu or 239/240 Pu 241 Am 0.02 Bq 0.01 Bq IV. Elements (mass/volume) Uranium20 μg V. GAMMA (photon) activity 137 Cs/ 137m Ba 60 Co 125 I 2 Bq 0.4 kBq

31. International Atomic Energy Agency Accuracy - How close is close enough? ICRP Publication 75, General Principles for the Radiation Protection of Workers: For external dosimetry a factor of 1.5 at the limits (20 mSv/year) The overall uncertainty in the dose from internal exposure, is likely to be greater than for external exposure

32. International Atomic Energy Agency Accuracy - How close is close enough? ICRP Publication 75, General Principles for the Radiation Protection of Workers: Sampling frequencies should be chosen to avoid errors due to intake uncertainties of more than about a factor 3 For less simple programmes, e.g. for insoluble plutonium, total uncertainties may be about one order of magnitude.

33. International Atomic Energy Agency Performance Criteria: Sensitivity

34. International Atomic Energy Agency Two terms describe sensitivity Minimum Detectable Activity (MDA) (a priori) Minimum activity that can be detected Probability, α, of false positive (Type I error) Probability, β, of false negative (Type II error) Decision level, L C, (a posteriori) The total count value or final measurement of a statistical quantity, L C, at or above which the decision is made that the result is positive Probability, α, of false positive (Type I error)

35. International Atomic Energy Agency Confidence levels and k values α1-βk 0.0010.9993.090 0.0050.9952.576 0.0100.9902.326 0.0250.9751.960 0.0500.9501.645 0.1000.9001.282 0.2000.8000.842 0.2500.7500.675 0.3000.7000.525 0.4000.6000.254 0.500 0

36. International Atomic Energy Agency Standard Deviation where:s = standard deviation of a set of N measurements x i = ith measurement in the set  x = mean of the set of measurements Estimate of the standard deviation for a single measurement: s B = standard deviation of the appropriate blank sample s 0 = standard deviation of the net subject or sample count

37. International Atomic Energy Agency Illustration of L C and MDA relationship 0LcLc MDA (a) (b) α Background Not detected May be Will be Detected kαsBkαsB sBsB (c) s0s0 kβs0kβs0 0 – Value of background distribution L C – The likelihood that the sample distribution characterized by L C was not really positive (false positive) is α MDA – The likelihood that a sample distribution characterized by the MDA will be missed (false negative) is β and is not really positive (false positive) is α

38. International Atomic Energy Agency Minimum detectable activity - MDA Values assigned to MDA depends on the risk of making an error, false positive or false negative. Simplification: Assume β = α, and β = α = 0.05 Then k α = k 1-β = 1.645 = k where: s 0 = standard deviation of net subject counts K = efficiency T = subject counting time

39. International Atomic Energy Agency Minimum detectable activity - MDA where:s B1 = standard deviation in subject counts with no actual activity s B0 = standard deviation in unadjusted blank counts It can be assumed that s B1 = s B0 = s 0, and m = 1 Then, s 0 = s B  2 = 1.415s B, where s B is the standard deviation of a total blank count

40. International Atomic Energy Agency Minimum detectable activity - MDA For direct measurements, MDA becomes: For indirect measurements: where:R = chemical recovery λ = radiological decay constant Δt = elapsed time between reference time and time of count

41. International Atomic Energy Agency Sensitivity - How low is low enough? ICRP Publication 78 and IAEA Safety Guide RS-G-1 * agree; “The recording level for individual monitoring should be derived from the duration of the monitoring period and an annual effective dose of no lower than 1 mSv or an annual equivalent dose of about 10% of the relevant dose limit.”

42. International Atomic Energy Agency Direct measurement MDAs * Measurement categoryOrganMDA I. Transuranium elements via x-raysLungs185 Bq/A II. 241 AmLungs26 Bq III. 234 ThLungs110 Bq IV. 235 ULungs7.4 Bq V. Fission and activation productsLungs740 Bq/A VI. Fission and activation productsWhole body740 Bq/A VII. Radionuclides in the thyroidThyroid740 Bq/A * From ANSI 13.30 A is the number of photons per nuclear transformation – L x-rays for transuranium elements, and gamma rays for fission and activation products

43. International Atomic Energy Agency Indirect measurement MDCs (urine) * Measurement Category NuclideMDC I Beta - Average energy ≤ 100 keV 3 H, 14 C, 35 S 147 Pm 210 P, 228 Ra, 241 Pu 370 Bq/L 0.37 Bq/L 0.19 Bq/L II. Beta – Average energy > 100 KeV 32 P, 89/90 Sr or 90 Sr 131 I 0.74 Bq/L 3.7 Bq/L III. Alpha – Isotopic specific measurements 210 Po, 226 Ra, 228/230/232 Th, 234/235/238 U 237 Np, 238/239/240 Pu, 241 Am, 242/244 Cm 3.7 mBq/L 2.2 mBq/L IV. Mass determinationUranium (natural)5 μg/L V. Gamma or x-raysEmitters with photons ≤ 100 keV2 Bq L -1 /A VI. Gamma or x-raysEmitters with photons > 100 keV2 Bq L -1 /A * From ANSI 13.30 A is the number of photons per nuclear transformation – L x-rays for transuranium elements, and gamma rays for fission and activation products

44. International Atomic Energy Agency Indirect measurement MDAs (faeces) * Measurement Category NuclideMDA VII. Alpha – Isotope specific measurements 234/235/238 U, 228/230/232 Th, 238/239/240 Pu, 241 Am 37 mBq/sample VIII. Beta – Average energy > 100 keV 89/90 Sr or 90 Sr0.74 Bq/sample IX. Gamma or x-raysEmitters with photons ≤ 100 keV2/A Bq/sample X. Gamma or x-raysEmitters with photons > 100 keV2/A Bq/sample * Minimum detectable concentration - From ANSI 13.30 A is the number of photons per nuclear transformation – L x-rays for transuranium elements, and gamma rays for fission and activation products

45. International Atomic Energy Agency MDAs – Examples

46. International Atomic Energy Agency Determination of MDA - Example 90 Sr by Beta Gas Flow Proportional Counting 20 reagent blanks were counted for 1 hour each – 3600 s Total counts 8369537259 7770628853 6673595574 7270656861

47. International Atomic Energy Agency Determination of MDA - Example 90 Sr by Beta Gas Flow Proportional Counting  B = 67.4 counts S B =  [(X i – 67.4) 2 /19] = 9.4 Counting efficiency, K = 0.36 Chemical yield = 0.81

48. International Atomic Energy Agency Determination of MDA - Example Whole body counting for fission and activation products Radionuclide 137 Cs 60 Co OrganBodyLungs Counts in peak region - B98 S B =  B 32.8 Count time, T – s600 Calibration factor, K 1.35  10 -4 2.97  10 -4 MDA - Bq20990

49. International Atomic Energy Agency References HEALTH PHYSICS SOCIETY, Performance Criteria for Radiobioassy, An American National Standard, HPS N13.30-1996 (1996). INTERNATIONAL ATOMIC ENERGY AGENCY, Occupational Radiation Protection, Safety Guide No. RS-G-1.1, ISBN 92-0-102299-9 (1999). INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment of Occupational Exposure Due to Intakes of Radionuclides, Safety Guide No. RS-G-1.2, ISBN 92-0-101999-8 (1999). INTERNATIONAL ATOMIC ENERGY AGENCY, Indirect Methods for Assessing Intakes of Radionuclides Causing Occupational Exposure, Safety Guide, Safety Reports Series No. 18, ISBN 92-0-100600-4 (2002). International Standards Organization, Radiation Protection – Performance Criteria for Radiobioassay – Part 1: General Principles, ISO TC 85/SC2 (1999).