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

1

International Atomic Energy Agency

ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES 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.


Interpretation of Measurement Results - Unit Outline

Measurement Uncertainties

Intake and Dose Assessment Uncertainties

Performance Criteria: Accuracy

Performance Criteria: Sensitivity

MDAs - Examples


Measurement Uncertainties


Dose determination uncertainties

Measurement

?1

Direct or indirectmeasurements

Body/organ content, Mor

Excretion rate, R

Interpretation

e(g)j

m(t)

Estimatedintake

Committedeffective dose

?2

?3

Body/organ content, Mor

Excretion rate, R


Measurement uncertainties

Usually most straightforward to estimate Counting statistics dominate at low activities For radionuclides that are,

Easily detected, and 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


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

Source activity Simulation accuracy


Estimated Direct Measurement uncertainties*

Source of uncertainty

Estimated magnitude

1 σChest 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 subject 10-15% for whole body

* From ANSI 13.30 (1996)


Typical uncertainties for assessing fission product isotopes*

Source of UncertaintyEstimated

Uncertainty

DepthLength WidthHeight-WeightAnalysis TechniqueCalibrationCounting StatisticsTotal Estimated Uncertainty

12%5%7%3%5%7%

40%

* From Toohey, et al,


Typical uncertainties for U lung counting

Source of UncertaintyEstimated

UncertaintyChest DepthChest Wall ThicknessActivity LocationDetector PlacementSubject BackgroundCalibrationCounting StatisticsTotal Estimated Uncertainty

12%15%5%5%

10%5%

40%90%



Typical uncertainties for Pu lung counting

Source of error Uncertainty

Subject background 50%

Counting statistics 50%

Chest wall thickness 40%

Non-uniform distribution 70%

Calibration 20%

Overall uncertainty 110%



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 N 13.30 and ISO 12790-1


INTRODUCTION OF SF

• The recently developed IDEAS Guidelines for the assessment of internal doses from monitoring data suggest default measurement uncertainties (i.e. scattering factors, SF) to be used for different types of monitoring data.

• The SF values represent the geometric standard deviation of the distribution of all results, supposed to be approximated by log-normal distribution.


INTRODUCTION OF SF

• The IDEAS guidelines consider two types of uncertainty : • Type A : connected to counting statistic

and decreasing with the increasing of activity and counting time (Poisson distribution)

• Type B : all other components of uncertainty also connected with inter and intra-subject variability (e.g. in excretion)


INTRODUCTION OF SF

SF values are important. For these issues.

• They are needed to assess the uncertainty in the estimated intake and dose.

• They determine the relative weighting of data in fitting process and can effect the estimated intake when different types of monitoring data are used simultaneously.

• They enable rogue data to be identified objectively• They enable objective (statistical) criteria (goodness-of-fit)

to be calculated, which are used to determine whether the predictions of the biokinetic model (with a given set of parameter values) used to assess the intake and dose are inconsistent with the measurement data.


INTRODUCTION OF SF

• The IDEAS Guidelines assume the overall uncertainty on an individual monitoring value can be described in terms of a lognormal distribution and the SF is defined as the geometric standard deviation (GSD).

• This approximation is valid if Type A errors are relatively small (<30%). Thus, it is assumed that if the measurements could be repeated, hypothetically at the same time, then the distribution of the measurement results could be described by a lognormal distribution.


INTRODUCTION OF SF

• SF values depend on type of monitoring measurement. Default values are reported in the following slides.

• When the type A component of the uncertainty is small (< 30%) the type B component alone could be used for uncertainty.

BA SFSFSF 22 lnlnexp


SF default values for in-vivo measurements

SF values depend on type of monitoring measurement. For in-vivo measurement types:

In vivo measurements

SF values

(Type B uncertainty)

Low photon energy

E < 20 keV2.1

Intermediate photon energy

20 keV < E < 100 keV1.3

High photon energy

E > 100 keV1.2


SF default values for in-vivo measurements

SF values depend on type of monitoring measurement. For in-vitro measurement types:

In vitro

measurements

SF values

(Type B uncertainty)

URINE

For HTO after inhalation1.1

URINE

Normalized 24 h excretion1.7

URINE

Spot urine data2.0

FECES

Inhalation (Pu-Am)2.5

FECES

Wound (Pu)3.1


Intake and Dose Assessment Uncertainties


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


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



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



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


Assessment uncertainties

Models used to describe radionuclide behavior are used to assess intake and dose

Reliability of dose estimates depends on the accuracy of the models, and limitations on their application

This will depend upon many factors, including: Knowledge of the time of intake, and Whether the intake was acute or chronic



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

The degree of over- or under-estimation of the dose depends on the body retention pattern



Radionuclide behavior in the body depends upon their physicochemical characteristics

Particle size of inhaled radionuclides is a particularly important for influencing deposition in the respiratory system

Gut absorption factor f1 substantially influences effective dose following ingestion



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;

Physical form and chemical form of the intake, and

Characteristics of the individual,

may be needed to improve the accuracy of the model predictions


Intake fraction, m(t) depends on several factors

Time after intake, d0 10 100 1000 10000

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 (acut. vs. chr.) Deposition siteTime after intake Particle size Absorption rate (F, M or S) Mode of intake

60Co, inhalation type M


Performance Criteria:Accuracy


Performance criteria

Accuracy Bias (Systematic errors)

How well can a given measurement be reproduced.

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?


Performance criteria - Bias

ai

aiiri A

AAB

where: Bri = relative bias for the ith measurement

Ai = measured activityAai = actual activity for the ith

measurement

Definition:


Performance criteria - Bias

For a test or measurement category,

Where: Br = Relative bias for the category

n = number of replicate measurements

n

BBB

n

iri

rir

1


Performance criteria – Repeatability*

Definition:

where: SBr = measurement repeatability for the test or measurement category

* Also termed Precision

11

2

n

BBS

n

irri

Br


Accuracy - How close is close enough?

When the activity Aai is at or above the specified Minimum Testing Level (MTL),

Relative bias, Br

- 0.25 Br +0.50

Relative repeatability, SBr

SBr 0.40

These values used by ISO and USDOE Laboratory Accreditation Program


MTL Values for Direct Measurements

Measurement Category Type Radionuclide MTL

I. Transuranium elements via L x-rays

Lung 238Pu 9 kBq

II. Americium-241 Lung 241Am 0.1 kBq

III. Thorium 234Lung

234Th in equilibrium w/ parent 238U

0.5 kBq

IV. Uranium-235 Lung 235U 30 kBq

V. Fission and activation products

Lung

Any two: 54Mn, 58Co, 60Co, 144Ce + 134Cs & 137Cs/137Ba

3 kBq

30 kBq

3 kBq

VI. Fission and activation productsTotal body

All of: 134Cs, 137Cs/137mBa, 60Co & 54Mn

3 kBq

VII. Radionuclides in the thyroid Thyroid 131I or 125I 3 kBq


MTL Values for Indirect Measurements

Measurement category Radionuclide MTL(per L or per sample)

I. BETA activity: average energy < 100keV

3H, 14C35S228Ra

2 kBq

20 kBq

0.9 kBq

II. BETA activity: average energy ≤ 100 keV

32P89, 90Sr or 90Sr

4 Bq

III. ALPHA activity: isotopic analysis

228,/230Th or 232Th234/235U or 238U237Np238Pu or 239/240Pu

241Am

0.02 Bq

0.02 Bq

0.01 Bq

0.01 Bq

0.01 Bq

IV. Elements (mass/volume) Uranium 20 μg

V. GAMMA (photon) activity

137Cs/137mBa60Co125I

2 Bq

2 Bq

0.4 kBq



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



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 programs, e.g. for insoluble plutonium, total uncertainties may be about one order of magnitude.


Performance Criteria: Sensitivity


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, LC, (a posteriori)

The total count value or final measurement of a statistical quantity, LC, at or above which the decision is made that the result is positive

Probability, α, of false positive (Type I error)


Confidence levels and k values

α 1-β k0.001 0.999 3.090

0.005 0.995 2.576

0.010 0.990 2.326

0.025 0.975 1.960

0.050 0.950 1.645

0.100 0.900 1.282

0.200 0.800 0.842

0.250 0.750 0.675

0.300 0.700 0.525

0.400 0.600 0.254

0.500 0.500 0


Standard Deviation

11

2

N

xxs

N

ii

where: s = standard deviation of a set of N measurementsxi = ith measurement in the set

x = mean of the set of measurements

Estimate of the relative standard deviation for a single measurement:

sB = standard deviation of the appropriate blank samples0 = standard deviation of the net subject or sample count

i

i

Xx

s


Illustration of LC and MDA relationship

0 Lc MDA

(a) (b)α

Background

Not detected May be

Will be

Detected

kαsB

sB

(c)s0kβs0

0 – Value of background distribution

LC – The likelihood that the sample distribution characterized by LC 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


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 = 0.05Then kα = k1-β = 1.645 = k

where: s0 = standard deviation of net subject countsK = efficiencyT = subject counting time

TK

s.

TK

skMDA

329332 00



202

210

1BB s

mss

where: sB1 = standard deviation in subject counts with no actual activity

sB0 = standard deviation in unadjusted blank counts

It can be assumed that sB1 = sB0 = s0, and m = 1Then, s0 = sB2 = 1.415sB, where sB is the standard deviation of a total blank count



TK

s.MDA B

3654

For direct measurements, MDA becomes:

For indirect measurements:

where: R = chemical recoveryλ = radiological decay constant Δt = elapsed time between reference time and time of count

tB

eTRK

s.MDA

3654


Direct measurement MDAs*

Measurement category Organ MDAI. Transuranium elements via x-rays Lungs 185 Bq/A

II. 241Am Lungs 26 Bq

III. 234Th Lungs 110 Bq

IV. 235U Lungs 7.4 Bq

V. Fission and activation products Lungs 740 Bq/A

VI. Fission and activation products Whole body 740 Bq/A

VII. Radionuclides in the thyroid Thyroid 740 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


Indirect measurement MDCs (urine)*

Measurement Category

Nuclide MDC

I Beta - Average energy ≤ 100 keV

3H, 14C, 35S147Pm210Pb, 228Ra, 241Pu

370 Bq/L

0.37 Bq/L

0.19 Bq/L

II. Beta – Average energy > 100 KeV

32P, 89/90Sr or 90Sr131I

0.74 Bq/L

3.7 Bq/L

III. Alpha – Isotopic specific measurements

210Po, 226Ra, 228/230/232Th, 234/235/238U237Np, 238/239/240Pu, 241Am, 242/244Cm

3.7 mBq/L2.2 mBq/L

IV. Mass determination Uranium (natural) 5 μg/L

V. Gamma or x-rays Emitters with photons ≤ 100 keV 2 Bq L-1/A

VI. Gamma or x-rays Emitters with photons > 100 keV 2 Bq L-1/A

* From ANSI 13.30A is the number of photons per nuclear transformation – L x-rays for transuranium elements, and gamma rays for fission and activation products


Indirect measurement MDAs (faeces)*

Measurement Category

Nuclide MDA

VII. Alpha – Isotope specific measurements

234/235/238U, 228/230/232Th, 238/239/240Pu, 241Am

37 mBq/sample

VIII. Beta – Average energy > 100 keV

89/90Sr or 90Sr 0.74 Bq/sample

IX. Gamma or x-rays Emitters with photons ≤ 100 keV 2/A Bq/sample

X. Gamma or x-rays Emitters with photons > 100 keV 2/A Bq/sample

* Minimum detectable concentration - From ANSI 13.30A is the number of photons per nuclear transformation – L x-rays for transuranium elements, and gamma rays for fission and activation products


MDAs – Examples


Determination of MDA - Example

90Sr by Beta Gas Flow Proportional Counting

20 reagent blanks were counted for 1 hour each – 3600 s

Total counts

83 69 53 72 59

77 70 62 88 53

66 73 59 55 74

72 70 65 68 61



90Sr by Beta Gas Flow Proportional Counting

B = 67.4 counts

SB = [(Xi – 67.4)2/19] = 9.4

Counting efficiency, K = 0.36

Chemical yield = 0.81

Bq.

..

..MDA 0440

8103603600

349654



Whole body counting for fission and activation products

Radionuclide 137Cs 60Co

Organ Body Lungs

Counts in peak region - B 9 8

SB =B 3 2.8

Count time, T – s 600 600

Calibration factor, K 1.3510-4 2.9710-4

MDA - Bq 209 90

TK

s.MDA B

3654


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).

Documents

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