1
International Atomic Energy Agency
ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKE OF
RADIONUCLIDES
Biokinetic Models
3 International Atomic Energy Agency
Trough Wound
Trough SkinInhalation
Ingestion
From ICRP 30 to ICRP 100
ICRP 66
NCRP Report No. 156
No model but some ICRP’s…..
Biokinetic Models First of all, the entrance of intake
Excretion
4 International Atomic Energy Agency
Metabolic vs. Dosimetric models
Modeling - Mathematical descriptions used to describe the processes involved in physical movement of radionuclides in the body following intake, and the deposition of energy that constitutes exposure
Biokinetic modeling includes two types of models
Metabolic models
Dosimetric models
5 International Atomic Energy Agency
Biokinetic models
Describe deposition and movement of radioactive material through the body
Depend on the intake mode, element, chemical form and physical form, and particle size (inhalation)
Tissues (including fluids) and organs, termed “Compartments”
Transfer routes
Transfer rates, Excretion routes
A BIntake
C
Urine
Faeces
a
b
6 International Atomic Energy Agency
Dosimetric models
Address the micro and macro distribution of the radionuclide within the tissues or organs where significant deposition may occur
Take into account the radiosensitivity of the deposition site tissues or organs - wT
Include consideration of wR, especially for alpha emitting radionuclides
Depend on the decay properties of the radionuclide - particle type and energy
Address contribution to other target organs
7 International Atomic Energy Agency
ICRP recommendations on biokinetics
ICRP Recommendations on: Assessing radionuclide intake, and Resulting doses, From monitoring data.
For occupationally workers, a suite of models to represent radionuclide behaviour after entry by: Inhalation or Ingestion
8 International Atomic Energy Agency
Routes of intake, transfers and excretion
Transfercompartment
Respiratory Tract Model
Liver
Kidney
Urinary bladderOtherorgans
Subcutaneoustissue
LymphnodesS
kin
Ski
n
GastroIntest.Tract model
UrineUrine FaecesFaeces
IngestionIngestionExhalationExhalationInhalationInhalation
Extrinsic removalExtrinsic removal
WoundWound
SweatSweat
Direct absorptionDirect absorption
9 International Atomic Energy Agency
Other routes of intake
For other routes of exposure, intakes are only likely to occur as a result of accidents
Almost no internationally accepted models for: Entry through intact skin or Wounds
Exception - HTO Readily absorbed through intact skin. Assumed to give additional tritium intake Equal to 50% of the inhaled tritium
10 International Atomic Energy Agency
Tissue weighting factors, wT
wT introduced to calculate committed effective dose equivalent from individual tissue dose equivalents
Provided a common way of expressing external and internal doses
ICRP used wT in biokinetic models for dose equivalents to organs and tissues from: Inhalation and Ingestion
Earlier models didn’t fully describe biokinetics
11 International Atomic Energy Agency
Tissue or organ Tissue weighting factor (wT)
Gonads 0.20 Bone marrow (red) 0.12 Colon (c) 0.12 Lung (d) 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Liver 0.05 Oesophagus 0.05 Thyroid 0.05 Skin 0.01 Bone surface 0.01 Remainder (e) 0.05
ICRP defined tissue weighting factors
’90 recommendation
’07 recommendation
0.08
0.040.120.040.040.04
0.12
Salivary gland
Brain0.01
0.01
12 International Atomic Energy Agency
InhalationRespiratory
tractmodel
Gastrointestinaltract
model
Ingestion
Faecal excretion
Transfer compartment
Tissuecompartment
1
Tissuecompartment
2
Tissuecompartment
3
Tissuecompartment
i
a1 a2 a3 ai
Excretion
Urinarybladder
Urinary excretion Systemic faecal excretion
Gastrointestinaltract model
fu ff
General model for radionuclides kinetics
13 International Atomic Energy Agency
Description of biokinetic models
Uptake factors and biological half time:
• If the biological half time within compartment i , Ti,
and a fraction aij of the activity in compartment i to
be transferred to compartment j are given, the transfer rate ij from i to j is calculate by
i
ijij T
a
693.0
ln 2=0.693
14 International Atomic Energy Agency
Description of biokinetic models/2
• On the other hand, if activity is transferred from compartment i to compartments 1, … ,n with transfer rates li1, li2,… ……., lin then
the overall biological half-time Ti
in compartment i is calculated by
• and the uptake factor aij to
compartment j by
n
kik
iT
1
2ln
n
kik
ijija
1
Transfer rates
15 International Atomic Energy Agency
ICRP Biokinetic models
ICRP biokinetic models are to be used in normal situations, e.g. doses from routine monitoring measurements.
Evaluation of accident doses needs specific information: Time and pattern of intake, Physicochemical form of the radionuclides, Individual characteristics (e.g. body mass).
16 International Atomic Energy Agency
Individual specific data
Individual specific data may be obtained through special monitoring, i.e. repeated direct measurements of:
Whole body,
Specific sites and/or
Excretion measurements
18 International Atomic Energy Agency
Definitions
Aerodynamic diameter
The diameter of the unit density sphere that has the same terminal settling velocity in air as the particle of interest
AMAD - Activity median aerodynamic diameter
50% of the activity (aerodynamically classified) in the aerosol is associated with particles of aerodynamic diameter (dae) greater than the AMAD. A log-normal distribution is usually assumed
22 International Atomic Energy Agency
Definitions/3
Thermodynamic diameter
The diameter of a spherical particle that has the same diffusion coefficient in air as the particle of interest (practically equal to the geometric diameter)
AMTD - Activity median thermodynamic diameter
50% of the activity (thermodynamically classified) in the aerosol is associated with particles of thermodynamic diameter (dth) greater than the AMTD
23 International Atomic Energy Agency
Respiratory tract model
Extrathoracic (ET) ET1, anterior nasal passage, ET2, posterior nasal and oral
passages, the pharynx and larynx
Thoracic Bronchial (BB: trachea, and
main bronchi), Bronchiolar (bb: bronchioles) Alveolar-interstitial (AI: the
gas exchange region).
Lymphatic tissue (for ET and TH)
Extrathoracic
ThoracicBB
Bronchial
ET2
ET1
Larynx
Trachea
Oral partNasal part
Main bronchi
Bronchi
Bronchioles bb
Al
Anterior nasal passage
Posterior nasal passage
Pharynx
bb
Al
Alveolar duct + alveoli
Respiratory bronchioles
Terminal bronchiolesBronchioles
Bronchiolar
Alveolar -interstitial
Average lung dose
24 International Atomic Energy Agency
Respiratory tract model/2
• Geometrical model
If cut in this section……
Epithelium tissue structure to show source & target
25 International Atomic Energy Agency
Respiratory tract model/3
• Target and source tissue in bronchial epithelium
26 International Atomic Energy Agency
Physiological parameters/2Lung volume to estimate respiration rate
27 International Atomic Energy Agency
Physiological parameters/3
Volumes related to light work Activity Air breathed during 1 day
(m3) Sleep (8 h) 3.6
Occupational (5.5h light exercise + 2.5h rest sitting)
9.6
Non-occupational (4h rest + 3h light exercise+1h heavy
exercise)
9.7
Total 23
Ventilation rates
Activity Tidal
Volume (L)
Ventilation rate (m3/h)
Respiration frequency
(min-1) Sleep 0.625 0.45 12
Rest (sitting awake) 0.75 0.54 12 Light exercise 1.25 1.5 20 Heavy exercise 1.923 3 26
28 International Atomic Energy Agency
Respiratory tract model features
Deposition of inhaled particulates: Calculated for each RT region Both inhalation and exhalation are
considered, as a function of: Particle size, Breathing parameters and/or Work load, Assumed independent of chemical form
29 International Atomic Energy Agency
Respiratory tract model features/2
Default deposition parameters: Age dependent Range of particle sizes:
0.6 nm activity median thermodynamic diameter (AMTD) to
100 m activity median aerodynamic diameter (AMAD).
For occupationally exposed individuals, based on average daily patterns of activity
30 International Atomic Energy Agency
Respiratory tract model features/3
Inhalation dose coefficients:
AMAD of 5 m - Now considered most likely for the workplace
AMAD of 1 m - Previous workplace default value (ICRP 30)
AMAD of 1 m - Default for the public
31 International Atomic Energy Agency
Inhalation - Deposition model
Evaluates fractional deposition in each region
Aerosol sizes of practical interest - 0.6 nm to 100 μm
ET regions Measured deposition efficiencies related to:
Particle size Airflow
Scaled by anatomical dimensions
32 International Atomic Energy Agency
Inhalation - Deposition model/2
Thoracic airways - theoretical model for gas transport and particle deposition is used
Calculates particle deposition in BB, bb, and AI regions
Quantifies effects of lung size & breathing rate
Regions treated as a series of filters
Efficiency is evaluated considering both:
Aerodynamic processes (gravitational settling, inertial impaction)
Thermodynamic processes (diffusion)
33 International Atomic Energy Agency
Inhalation - Deposition model/3
Regional deposition fractions calculated for lognormal particle size distributions
Geometric standard deviations (g) - a function of the median particle diameter
From 1.0 at 0.6 nm to 2.5 above ~ 1 μm Deposition parameters are given for three
reference levels of exertion for workers Sitting Light exercise Heavy exercise
34 International Atomic Energy Agency
Region Deposition (%) of 5 m AMAD
ET1 34
ET2 40
BB 1.8
bb 1.1
AI 5.3
Total 82
Respiratory tract - Deposition
36 International Atomic Energy Agency
Clearance from the respiratory tract
Clearance from the respiratory tract is treated as two competing processes:
Particle transport(by mucociliary clearance or translocation to lymph nodes), and
Absorption to blood
37 International Atomic Energy Agency
Particle transport
Treated as a function of deposition site
Independent of particle size and material
Modeled using several regional compartments with different clearance half-times, e.g.
AI region given 3 compartments, Clearing to bb with biological half-lives of
about 35, 700 and 7000 days.
39 International Atomic Energy Agency
Simultaneous differential equation for Clearance model
based on whole compartment model
41 International Atomic Energy Agency
Particle transport/2
Similarly, bb and BB have fast and slow clearance compartments
Clearance from the AI region also involves transfer to lymphatic tissue
For bb, BB and ET;
Compartments to represent material sequestered in tissue and transported to lymphatic tissue
42 International Atomic Energy Agency
Absorption into blood
Depends on the physicochemical form of the radionuclide
Independent of deposition site - Except ET1 (no absorption is assumed).
Changes in dissolution and absorption with time are allowed
43 International Atomic Energy Agency
Absorption into blood/2
Particles in initial
state
Particles in transformed
state
Body fluids
Deposition
spt
spst
44 International Atomic Energy Agency
Alternative mode of indication of absorption
st
srrpt
srrsp
ss
ssfs
ssfss
1
45 International Atomic Energy Agency
Absorption into blood/3
Material specific dissolution rates preferred Use default absorption parameters if no specific
information is available:
F (fast) -100% absorbed with a half-time of 10min
M (moderate) -90% absorbed with a half-time of 140days
S (slow) -99.9% absorbed with a half-time of 7000days
Broadly correspond to lung classes D (days), W (weeks) and Y (years), but lung classes referred to overall lung clearance rates
46 International Atomic Energy Agency
Absorption rates
Expressed as:
Approximate biological half-lives, and
Corresponding amounts of material deposited in each region that reach body fluids
All the material deposited in ET1 is removed by extrinsic means, such as nose blows
47 International Atomic Energy Agency
Absorption rates/2
In other regions, most material not absorbed is cleared to the gastrointestinal tract by particle transport.
Small amounts transferred to lymph nodes are absorbed into body fluids at the same rate as in the respiratory tract.
48 International Atomic Energy Agency
Absorption rates - Default values
Model parameters
(d-1) F (Fast) M (moderate) S (slow)
sp 100 10 0.1
spt 0 90 100
st - 0.005 0.0001
49 International Atomic Energy Agency
Absorption rates – Alternative presentation
Model parameters F (Fast) M (moderate) S (slow)
fr 1 0.1 0.001
Sr (d-1) 100 100 100
Ss (d-1) - 0.005 0.0001
50 International Atomic Energy Agency
Deposition of gases and vapours
Respiratory tract deposition is material specific
Inhaled gas molecules contact airway surfaces
Return to the air unless they dissolve in, or react with, the surface lining
Fraction of an inhaled gas or vapor deposited depends on its solubility and reactivity
Regional deposition of a gas or vapor obtained from in-vivo experimental studies
51 International Atomic Energy Agency
Solubility/Reactivity (SR) classes Description Examples Class SR-0 Insoluble and non-reactive:
negligible deposition in the respiratory tract.
41Ar, 85Kr, 133Xe
Class SR-1 Soluble or reactive: deposition may occur throughout the respiratory tract.
Tritium gas, 14CO, 131I vapour, 195Hg vapour
Class SR-2 Highly soluble or reactive: total deposition in the extrathoracic airways (ET2). For the purpose of calculation they are treated as they were injected directly into the blood.
3H in organic compounds and tritiated water
SR0
Insoluble and nonreactive
Soluble and reactive
Deposition ET : 100% Instantaneous transfer to blood : type "V"
SR1
SR2
Rn Xe N2 H2 He SF6
Soluble or reactive gas
Reactivity ->
Solubility
->
Deposition : ET1 : 10% ET2 : 20 % BB : 10 % bb : 20 % AI : 40 %
Transfer to blood : Type "V' or "F"
CO2 O3 HF HTO SO2
Elemental I CO Hg vapour Ni carbonyl
52 International Atomic Energy Agency
Deposition of gases and vapours
Guidance on the deposition and clearance of gases and vapours similar to particulates
Default SR classes and absorption types Type F Type V, very rapid absorption
recommended for elements for which inhalation of gas or vapor form is important
Only low mass concentrations of gases and vapours are considered.
53 International Atomic Energy Agency
Dosimetric modelCorrespondence between source regions and
compartments in the clearance model.
Target regions Source regions Source Compartments in clearance
model ET1-sur ET1 ET2-sur ET2+TET2 ET2-seq ETseq+TETseq LN-ET LNET+TLNET BB-gel BB1+TBB1 BB-sol BB2+TBB2 BB-seq BBseq+TBBseq bb-gel bb1+Tbb1 bb-sol bb2+Tbb2 bb-seq bbseq+Tbbseq AI AI1+AI2+AI3+TAI1+TAI2+TAI3 LN-TH LNTH+TLNTH
TargettissuesET1-basET2-basLN-ETBB-basBB-secbb-secAILN-TH
54 International Atomic Energy Agency
BB
bb
Al
ET2
0.1 1 10 100AMAD (m)
100
10
1
0.1
0.01
Reg
ion
al d
epo
siti
on
(%
)
ET1
Influence of particle size on deposition in various regions of the respiratory tract
55 International Atomic Energy Agency
Effect of particle size on aerosol deposition
Committed effective dose for Type M and S 239Pu compounds decreases with increasing AMAD
Reflects decreasing deposition in the AI region and BB and bb with increasing AMAD
In this case, the assumption of Type M characteristics is more restrictive than Type S for the calculation of effective dose
Other aerosol characteristics have slight influence on the committed effective dose
56 International Atomic Energy Agency
Adult male
Light work (5.5 h) + sitting (2.5 h)
Type M
Type S
0.1 1 10 100AMAD (m)
10-3
10-4
10-5
10-6
Co
mm
itte
d e
ffe
cti
ve
do
se
pe
r u
nit
in
tak
e (
Sv
/Bq
)
Influence of AMAD on the committed effective dose
239Pu