ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF RADIONUCLIDES

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ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO
INTAKES OF RADIONUCLIDES
Interpretation of Measurement Results
2
Introduction
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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.

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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

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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

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Interpretation of monitoring measurements
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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.

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Estimate of intake

• The ICRP has published default values of m(t) in
  Publication 78
• 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 is obtained by the method
  of least squares.
• When more than 10 of the measurements could be
  attributed to previous evaluated intakes a
  correction should be performed.

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Implementing biokinetic models

• 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

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Implementing biokinetic models
ICRP 78 CURVES AND DATA

• Basic assumption for a reference worker in ICRP
  78
• Adult male
• Normal nose breathing at light work
• Breathing rate 1.2 m3/h
• Inhaled aerosol with Activity Median
  Aerodynamic Diameter (AMAD) 5 µm
• Regional Deposition
• ET1 34
• ET2 40
• BB 1.8
• BB 1.1
• AI 5.3
• total 82

The data and curves available in ICRP 78 refers
to these specific conditions of exposure!
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Implementing biokinetic models
ICRP 78 CURVES AND DATA

• General information

Description of the model
Standard assumption for transfer into
systemic phase
Dose coefficients
e(50)
Other informations
ALI0.02/e(50)
In relation to the radionuclide other significant
information are available monitoring techniques
(as for Pu), etc.
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Caesium

• Model Non-recycling model

H, P, Cr, Mn, Co, Zn, Rb, Zr, Ru, Ag, Sb, Ce,
Hg, Cf are as well
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Implementing biokinetic models
ICRP 78 CURVES AND DATA

• Data

m(t)
t
Special monitoring (inhalation)
Special monitoring (ingestion and injection)
Routine monitoring (inhalation)
T
m(T/2)

• Retention (Bq per Bq intake)
• Excretion (Bq/d per Bq intake)

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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

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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 measurement
• Direct
• Whole body
• Lungs

• Indirect
• Urine
• Faeces

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60Co Routine Monitoring Retention
FractionsInhalation
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60Co Retention Fractions - Inhalation
Type M
Type S
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60Co Routine Special Retention FractionsInhalatio
n
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60Co Retention Fractions - Ingestion
Special Monitoring
f1 0.1
f1 0.05
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60Co Retention Fractions - Injection
Special Monitoring
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Intake Estimates - An Example
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Estimate of intake - an example

• Occupational exposure to radioiodine occurs in
  various situations
• I-131 is a common short lived iodine isotope
• Half-life 8 d
• ? particles - average energy 0.19 MeV
• ? - main emission 0.364 MeV
• Rapidly absorbed in blood following intake
• Concentrates in the thyroid
• Excreted predominantly in urine

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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

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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

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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.

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Dose coefficients
1.4 E-08
2.0 E-08
(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.
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Biokinetic model for systemic iodine
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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

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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.

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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

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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

from table A.6.17 (Thyroid) in ICRP 78
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131I intake - Thyroid monitoring
Special monitoring
0.074
Vapor particle
Retention, Bq
or table A.6.17 in ICRP 78
7
Time after intake, d
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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 0.45 mSv
• (4.1104 Bq ? 1.110-8 Sv/Bq ? 103 mSv/Sv)
• This dose may require follow-up investigation

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131I intake - Urine measurement

• One day after the direct thyroid measurement, the
  worker has 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

from table A.6.17 (dairy urinary excretion) in
ICRP 78
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131I intake - Urine measurement

• A committed effective dose of 3 mSv
• (2.7105 Bq ? 1.110-8 Sv/Bq ? 103 mSv/Sv)
• For this example no account is taken of any
  previous intakes

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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

DAC Derived Air Concentration
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Derived air concentrations
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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

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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

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131I - Daily urinary excretion after inhalation
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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

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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 0.27 mSv
• A 2nd urine sample obtained after a few more days
  should be used to verify this conclusion.

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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

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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