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

2. Interpretation of Measurement Results – Unit Objectives
The objective of this unit to outline the steps and issues involved in using the results of internal exposure measurements to establish intake and committed effective dose. The unit includes an example that illustrates the principles involved.
At the completion of this unit, the student should understand how to approach the problem of measurement result interpretation, as well as how to find the necessary information resources.

3. Interpretation of Measurement Results - Unit Outline
Introduction
Intake Estimate – An Example

4. Introduction

5. 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.
Direct measurement addresses the use of detectors placed external to the body to detect ionizing radiation emitted by radioactive material contained in the body. It has usually been referred to as whole body counting, but includes other terms such as lung counting, organ counting or wound counting.
Indirect measurement, in contrast, refers to the analysis of excreta (urine, feces, sweat), or other materials such as blood, breath or saliva to estimate the body content of radioactive material. Indirect measurement also makes use of measurements made in the workplace not necessarily associated with a specific individual. This could, for example, include either personal air sampling or workplace air sampling.

6. 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
Direct or indirect measurements provide information about the amount(s) of radionuclides present in the body, in parts of the body such as specific organs or tissues, in a biological sample or in a sample from the working environment. The first use of these data is likely to be an estimation of the intake of the radionuclide by the worker.

7. 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
Biokinetic models which describe body and organ contents, and activity in excreta, as a function of time following intake, and exposure models which relate intake to workplace conditions, are used for this purpose. Alternatively, measurements of activity in the body can be used to estimate dose rates directly. The calculation of committed doses from direct measurements still involves the assumption of a biokinetic model if sufficient measurements are not available to determine retention functions.

8. Interpretation of monitoring measurements
Estimated
intake
Direct
Measurements
(In vivo)
Body/organ
content, M
Indirect
Excretion rate, M
Air concentration
Dose
rate
Committed
effective dose
m(t)
DAC-hr
e(g)j
Direct and indirect measurements have the same objective - to arrive at an accurate estimate of the committed effective dose received by an individual as a result of exposure to internally deposited radionuclides. Both methods may arrive at this estimate through determination of the estimated intake of radioactive material. Direct measurement methods can also be used to determine the dose rate to the organ or tissue directly and thereby arrive at the effective dose.
Either or both methods may be used to determine committed effective dose for an individual. The selection of methods depends on a number of factors including: the nuclide or nuclides to be assayed, and their radiological properties, the levels of exposure, and facilities available to make the determination.

9. 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
In order to calculate an estimate of intake, the measured body content or excretion rate, M, is divided by the fraction m(t) of the intake retained in the whole body (direct measurement) or having been excreted from the body (indirect measurement) at time t (usually in days) after intake:

10. Estimate of intake The ICRP has published generic values of m(t)
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 may be obtained, for example, by the method of least squares
The ICRP has published generic values of m(t) for selected radionuclides in tissues or excreta, together with retention functions for systemic activity. Further information is provided in ICRP Publication 78 using more recent biokinetic models.
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 may be obtained, for example, by the method of least squares

11. 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 type
Direct
Whole body
Lungs
Thyroid
Indirect
Urine
Faeces
Retention fractions, m(t), vary significantly with the element and its chemical form. This dictates the absorption types assigned to the radionuclide. Tritium, for example, is given only one type, while others such as ruthenium may have various chemical forms that fall into all three types.
Retention fraction also depends on whether the amount retained is being measured directly through organ or whole body measurement, or the excretion rates are measured through urine or faecal samples.

12. 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 bioassay
Direct
Whole body
Lungs
Indirect
Urine
Faeces

13. 60Co Routine Monitoring Retention Fractions Inhalation
Except as noted, the table and figures are for acute intakes. For routine monitoring, the intake is assumed to have occurred at the mid-point of the monitoring period. The length of the monitoring period is shown. Note that, in general, the excretion values for retention of S type material intakes are lower than those for M type material. This reflects the slower elimination of S type material. This is also reflected in the higher values for lung and whole body indicating a higher retention of the S type material since it is eliminated more slowly. These are shown graphically in the accompanying figures.

14. 60Co Retention Fractions - Inhalation
Type M
Type S

15. 60Co Routine Special Retention Fractions Inhalation
Special monitoring is undertaken following a known or suspected intake of radioactive material. The time after intake that the measurement is made, or sample collected is shown. Note the values at 4 days post exposure are the same as those shown for a routine monitoring program with a 7 day period. The mid-point of that period is about 4 days, so the values should be equivalent.

16. 60Co Retention Fractions - Ingestion
Special Monitoring
f1 = 0.1
f1 = 0.05
Only the special monitoring data table is presented for intake by ingestion. However, the figures show retention fractions for whole body counting, urine and faecel analysis. Data for lung retention is not included because intake was by ingestion, not inhalation. The retention fractions for f1 = 0.05 and f1 = 0.1 are very similar.

17. 60Co Retention Fractions - Injection
Special Monitoring
It is presumed that intake by injection – puncture – would automatically result in special monitoring so that a routine monitoring data table is not included.

18. Chronic Intake – Equilibrium Values
Equilibrium predicted to occur at the indicated time after exposure begins
Values are shown for intakes of:
1 Bq/year (1/365 Bq per day)
Annual intake equivalent to 20 mSv/y – 2.8 106 Bq/y (7.7  103 Bq/d)
Chronic intake conditions are more complicated than for acute intakes. The time course of intake can be highly uneven. The only data that can reasonably be presented as generic or default guidance must assume chronic intakes at a constant level.

19. Intake Estimates - An Example

20. Estimate of intake - an example
Occupational exposure to radioiodine occurs in in various situations
I-131 is a common short lived iodine isotope:
Half-life = 8 d
 particles - average energy 0.19 MeV
 - main emission 0.36 MeV
Rapidly absorbed in blood following intake
Concentrates in the thyroid
Excreted predominantly in urine
Occupational exposure due to radioiodine occurs in the nuclear industry, in nuclear medicine and in research. One common exposure is due to I-131, a short lived radioisotope (half-life 8 d) which decays with the emission of both beta particles (average energy for main emission 0.19 MeV) and gamma radiation (main emission 0.36 MeV). Iodine is rapidly absorbed into the circulation following inhalation or ingestion, is concentrated in the thyroid, and is excreted predominantly in urine.

21. 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
After an intake, I-131 may be detected directly by measurement of activity in the thyroid, or indirectly in urine samples. Where occupational exposures due to I-131 can occur, a routine monitoring programme may be based on direct thyroid measurement or on indirect monitoring of urine or workplace samples. The choice of monitoring method will depend on factors such as the availability of instrumentation locally (since the isotope is short lived) and the relative costs of the analyses, as well as on the sensitivity that is needed. Although direct measurement of activity in the thyroid provides the basis for the most accurate dose assessment, other methods may provide adequate monitoring and may be better suited to particular circumstances.

22. 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

23. 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.

24. Dose coefficients (a) For lung absorption types see para. 6.13.
(b) For inhalation of gases and vapours, the AMAD does not apply for this form.

25. Biokinetic model for systemic iodine

26. 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
For adults, it is assumed that, of the iodine reaching the blood, 30% is transported to the thyroid gland and the other 70% is excreted directly in urine via the urinary bladder. The 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, where it is retained with a biological half-life of 12 d.

27. 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.
Most iodine (80%) is subsequently released and is available in the circulation for uptake by the thyroid or direct urinary excretion; the remainder is excreted via the large intestine in the faeces. Because of the short physical half-life of I-131, this recycling is not important in terms of the committed effective dose.

28. 131I intake - Thyroid monitoring
A routine monitoring programme
14 day monitoring period
Thyroid content of 3000 Bq 131I 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
As an example, suppose that in a routine monitoring programme, with a monitoring period of 14 days, a thyroid content of 3000 Bq I-131 is detected in a male worker. Because of the operations under way in this workplace, it is assumed that any exposures will be due to inhalation of a particulate rather than vapour form (although for I-131 this assumption is not critical). Similarly, intakes by ingestion would also lead to the same pattern of retention and excretion [8, 9], and the same committed effective dose calculated from the monitoring data.

29. 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
If the intake pattern is not known, it should be assumed that an acute intake occurred in the middle of the monitoring period, provided that intakes are uncommon. With this assumption, it can be shown from the biokinetic model that 7.4% of the radioactive substance inhaled in a particulate (type F) form with a default AMAD of 5 is retained in the thyroid after 7 d. Thus, m(7) = 0.074, and the example monitoring result from the previous paragraph would indicate an intake of 41 kBq. Application of the dose coefficients given in the BSS and in the previous table gives a committed effective dose of 450 from such an intake. Such a dose may require follow- up investigation.

30. 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 450 µSv
(4.1•104 Bq  1.1•10-8 Sv/Bq  106 µSv/Sv)
This dose may require follow-up investigation
Thus, m(7) = 0.074, and the example monitoring result from the previous paragraph would indicate an intake of 41 kBq. Application of the dose coefficients given in the BSS and in the previous table gives a committed effective dose of 450 µSv from such an intake. Such a dose may require follow-up investigation.

31. 131I intake - Urine measurement
One day after the direct thyroid measurement, the worker 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
One day after the direct thyroid measurement, the worker in the example submits a 24-h urine sample, which is found to contain 30 Bq of I From the biokinetic model for a type F particulate, m(8) for daily urinary excretion is 1.1 E-04. On this basis, an intake of 270 kBq, and a committed effective dose of 3 mSv (for an aerosol with an AMAD of 5 μm), would be calculated. For this example no account is taken of any previous intakes.

32. 131I intake - Urine measurement
A committed effective dose of 3,000 µSv
(2.7•105 Bq  1.1•10-8 Sv/Bq  106 µSv/Sv)
For this example no account is taken of any previous intakes
One day after the direct thyroid measurement, the worker in the example submits a 24-h urine sample, which is found to contain 30 Bq of I From the biokinetic model for a type F particulate, m(8) for daily urinary excretion is 1.1 E-04 [9]. On this basis, an intake of 270 kBq, and a committed effective dose of 3 mSv (for an aerosol with an AMAD of 5 ), would be calculated. For this example no account is taken of any previous intakes.

33. 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
In the example, a review of workplace air measurements over the monitoring period, in the facilities where the exposure may have occurred, demonstrated that concentrations of I-131 were generally low but variable. Maximum concentrations between 10 and 20 (12 to 25 times the DAC value, see Table A–II) were recorded for short periods several times during the period, and in several locations. At the default breathing rate of 1.2 m3/h, an intake of 24 kBq can be received while working for one hour without respiratory protection in a concentration of 20 kBq/m3.

34. Derived air concentrations

35. 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
Were the worker to have done so, or to have worked for a somewhat longer period with limited respiratory protection, the intake calculated from air monitoring would be consistent, within the accuracy normally achievable by such methods, with that calculated from bioassay measurements.

36. 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
The large discrepancy between the estimates of intake calculated on the basis of the direct thyroid measurement and of the measurement of radioactive material excreted in urine suggests that at least one of the default assumptions used to derive these estimates is not correct. Although there are significant individual differences in iodine uptake and metabolism, these differences cannot generally account for a discrepancy of a factor of nearly ten.

37. 131I - Daily urinary excretion after inhalation
This slide illustrates the rapid decrease of urinary excretion following an 131I inhalation. It is clear that accurate assessment of time of intake is essential.

38. 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
On the other hand, the rate of excretion of I-131 in urine decreases markedly with time after intake, by a factor of more than 1000 over the monitoring period, so the default assumption concerning the time of intake is a probable source of error. If the intake were assumed to have occurred three days before the urine sample was submitted (i.e. two days before the end of the monitoring period), rather than at the midpoint of the monitoring period (eight days before the sample), the intake estimated from the urine measurement would be 21 kBq, and that from the thyroid measurement would be 25 kBq, a satisfactory agreement.

39. 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 270 µSv
A 2nd urine sample obtained after a few more days should be used to verify this conclusion.
According to the biokinetic model, the fraction of inhaled I-131 retained in the thyroid only changes by about a factor of three over the whole monitoring period. In the absence of better evidence from a review of the sources of possible workplace exposure, this refined assumption provides a more reliable basis for dose assessment. The committed effective dose for this example would then be 270 µSv. A second urine sample obtained after a few more days should be used to verify this conclusion.

40. 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
The committed effective dose calculated from direct thyroid monitoring results is relatively insensitive to assumptions about the time of intake. It is because of the rapid change in urinary excretion with time after exposure that direct measurement provides a much more reliable basis for interpreting routine monitoring measurements for radioiodine, although urine screening may still be adequate to detect significant intakes.

41. 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
The measurement of air concentrations substantially exceeding a DAC would have triggered individual monitoring of workers who had been present in the workplace. However, because of their direct dependence on the period of exposure, breathing rates, levels of protection and other factors that will be known only approximately, estimates of intake based on air monitoring for I-131 are much less reliable than those based on individual measurements.

42. References
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR ORGANISATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION, WORLD HEALTH ORGANIZATION, International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, Safety Series No. 115, IAEA, Vienna (1996).
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, Indirect Methods for Assessing Intakes of Radionuclides Causing Occupational Exposure, Safety Guide, Safety Reports Series No. 18, ISBN (2002).
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).

43. References
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).
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).