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Office of Research and DevelopmentSafety, Health, and Environmental Management Office
Bioassay Screening Method for an Athyrotic I-125 Lab Worker
Ritchie Buschow, MEM, CLSORSO, U.S. EPA RTP, NCSpring 2020 NCHPS MeetingWilmington, NC
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Disclaimer
• This presentation is provided as a courtesy, informational briefing to the NC HPS audience and is not intended for dissemination outside of this immediate audience. The content of this presentation, while prepared by an EPA federal employee, are strictly his express opinion and are not to be construed to be agency policy, interpretation or regulation.
• Any mention of a specific commercial product, Trade name or service does not constitute an endorsement or recommendation for use.
Support Health Effects Research of
Pollutants Inhibiting Iodine Uptake
in the Thyroid
3
What is the
Definition of
Athyrotic?
An enlarged Thyroid is not fun! Surgical
removal is usually necessary
The Problem with an Athyrotic
Individual working with
radioiodine
• Traditional bioassay method is to use a
NaI crystal to detect thyroid uptake of
radioiodine compounds
• ICRP 56 three compartmental model no
longer valid
• So, how do you perform an in-vivo
measurement when the person has their
thyroid removed or the gland becomes
totally dysfunctional?
Capintec CAPTUS 3000
Portable Thyroid Uptake
System
Internal Dose Assessment
Thyroid Scanfor I-125 & I-131 Urine Analysis
for H-3, C-14, P-32, & S-35
Picture Credit: (Thyroid gland)
http://www.uos.harvard.edu/ehs/radsafety/pur_i125.shtml6
10 CFR 20 Limits on Intake
•Allowable Limit on Intake (ALI):–Inhalation: 40 µCi
–Ingestion: 60 µCi
•Derived Air Concentration (DAC): 3 x 10-8 µCi/mL
But what about thyroid burden?Predetermined Action Levels (PALs)
> 1.0 µ Ci (specific actions required) and > 5.0 µ Ci (additional actions required) – from Reg. Guide 8.20 (September 2014)
7
TABLE 2
Activity Levels Above Which Bioassay
for Radioiodine Is Necessary
Activity Limit applies to these quantities used over a:
[1] Three Month usage/disposal or
[2] Maximum activity handled at one time.
Internal Exposure Monitoring
Reference: (Adapted)
U.S. Nuclear Regulatory Commission Regulatory Guide 8.20, “Applications
of Bioassay for Radioiodine,” September, 2014, Revision 2.8
Types of Operations Volatile or Dispersible Bound to Nonvolatile
Agent
uses in open room or
bench, where iodine
could escape from
vessels
1 mCi (37 MBq) 10 mCi (370 MBq)
Processes in well-
ventilated and controlled
areas (i.e., fume hood)
10 mCi (370 MBq) 100 mCi (3.7 GBq)
Processes in gloveboxes 100 mCi (3.7 GBq) 1 Ci (37 GBq)
ICRP 56 Model
• Three Compartments
• Blood -0.25 day retention
• Rest of body tissues -12 day retention
• Thyroid – 80 day retention
– This model is not suitable for someone with ablated or removed thyroid
– Pseudo Uptake Retention Function is needed then,
– Determine a dose coefficient for calculating a committed effective dose (CEDE)
9
This Photo by Unknown Author is licensed under CC BY-NC-ND
Method (Potter and Culp, 2002)
• Pseudo URF (Uptake Retention Function) was
developed from three sets of urine excretion data from
I-131 administrations provided by Rodriquez, 1997
• Assumed both I-125 and I-131 metabolism are
identical –all data sets used normalized to a 24 hour
sampling period
• The Pseudo URF was then used to develop a
bioassay program with Intake Retention Function (IRF)
• The IRF is used to develop intake retention fractions
for various bioassay frequencies from weekly up to a
period of one year for both acute and continuous
intakes (table of these values available in Potter
and Culp, 2002 paper)10
Method (Potter and Culp, 2002)
• Organ dose coefficients data (DC) were used from the ICRP
database of dose coefficients: Workers and Members of the
Public (ICRP 1998):
• These DCs were developed from the iodine model presented in
ICRP 56
• Weighting factors from 10 CFR 835.2 were multiplied by these
dose coefficients for all target organs/tissues and summed to
produce a DC for CEDE (the thyroid weighting factor = 0). The
conservative estimated DC = 7.63E-11 Sv/Bq (0.282 mrem/uCi)
Note: the CEDE DC for I-125 for remainder of body (minus
gonad, breast, lung, marrow, surface, thyroid) from EPA FGR
No. 11 = 3.33E-11 Sv/Bq (0.123 mrem/uCi)
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CEDE Determination
• Minimum detectable doses were calculated as a
CEDE as:
• MDD = MDA/IRF (DC)
• MDD =minimum detectable dose
• MDA = minimum detectable activity
• IRF = intake retention fraction
• DC = dose coefficient
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Some
Assumptions
or Issues
with this
method
• Only three sets of urine excretion data from I-131 therapeutic doses was used to derive the pseudo URF
• To which organs/tissues iodine translocates to and at what rate is unknown
• Assume I-125 is in gaseous as opposed to particulate or aerosol form (ICRP 66 allows for iodine to be in either physical state)
• ICRP 56 based on functioning thyroid
• Assume identical metabolism with I-125 and I-131
• Half-Life differences between I-131 and I-125 relative to sampling times (beyond 72 day sampling, how much I-131 is left?)
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I-125 – physical HL = 60.2 D; Bio HL =
120-138 D: Effective HL = 42 D
I-131 – physical HL = 8.04 D; Bio HL =
120-138 D; Effective HL = 7.6 D
The Screening Method (EPA-RTP)
Calculate MDA from Control Sample
𝑀𝐷𝐴 𝑑𝑝𝑚/𝑚𝑙 =2.71
𝐵𝑇+ 4.65 𝑆𝑄𝑅𝑇(𝐵𝐶𝑅/𝐵𝑇)/eff./2
Where:
BT =background sample count time (t)
BCR = background count rate (cm)
Eff. = counting efficiency for 125I (.78)
1. For each samples determine the net activity (Q) as follows:
Q (dpm/ml) = (sample dpm – control dpm)/2 ml
1. If Q is greater than MDA, an intake and subsequent dose must be determined.
2. If Q is less than the MDA value, then record the results as < MDA with ND
(non-detectable).
Calculate Intake and dose:
• Intake (I) = Current Activity (CA)/IRF, where:
• I = initial intake (µCi) of activity during the weekly time period when CF 125I was performed
• CA = current activity (in µCi) as determined by the weekly urine sample taken
• IRF = weekly Intake Retention Fraction = 2.6 E-4
• Note: the IRF is a unitless number representing the fraction of radioactive material
remaining in a given organ(s) at a specific time after the initial uptake occurs
• Further, determining CA from the grab sample requires the inclusion of the normal
excretion rate for standard Reference Woman which is one liter per day (1000 mls) in order
to determine the total activity over a 24 hour period. As such, this is calculated as follows:
• CA = Q (dpm/ml) x 1000/2.22E+6 (dpm/ µCi)
• To calculate the estimated acute committed effective dose to the individual, the following
method is shown below.
• Dose (mrem) = I x DCF, where:
• DCF = estimated dose conversion factor = 7.63 E-11 Sv/Bq (0.282 mrem/µCi)
Ref. – Charles A. Potter and Todd A. Culp, Development of a Routine I-125 Bioassay Program for
Athyrotic Individuals Using a Pseudo Uptake Retention Function, Health Physics, Vol 82 (4), April
2002
The Screening method (contd.)
Problems with EPA Procedure
➢Grab Sample is used when a sample should be pulled
from a 24 hour void sample
➢Dual samples are counted for 10 minutes, could count
them for a longer period for better detection capability
➢Assume acute intake rather than multiple (continuous)
uptakes from more than a single administration during
the week
➢Use of a Liquid Scintillation Counter (LSC) instead of
using a gamma counter or other NaI crystal to count the
samples
➢LSC can detect conversion or auger electrons as I-
125 decays via electron capture process
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Conclusions
• Pseudo URF based on modeling organs, tissues, body fluids
other than the Thyroid
• DC’s used from ICRP 56 which assumes a functioning thyroid
• Half-Life of I-125 much longer than I-131; sampling times varied
from 37-72 days after intake; assumed both isotopes metabolize
the same in humans
• IRF’s derived may be adequate over a short period of time but
could possibly break down over periods of quarterly or longer
sampling periods post intake
• Translocation (and at what metabolic rates) of radioiodine in
various body tissues/fluids is not very well understood
• Conservative estimate of dose coefficient and CEDE estimation
method for radioiodine in athyrotic individuals is the best available
at the present time
• Need more research on this subject matter; only three sets of I-
131 post administration bioassay data was used to derive this
method17
Acknowledgements
• Support and feedback provided by:
• John Mclamb – RSO, NIEHS and NCHPS member
• Chris Tate – Cone Health and NCHPS member
18
References• 1. Potter, Charles A. and Todd A. Culp, 2002, ‘Development of a Routine
125I Bioassay Program for Athyrotic Individuals Using a Pseudo Uptake
Retention Function’. Health Physics, 82, 4, 533-538
• 2. Rodriquez M., Development of a Kinetic Model and Calculation of
Radiation Dose Estimates for Sodium Iodide-131I in Athyroid Individuals,
masters project. Fort Collins, CO: Colorado State University; 1997
• 3. U.S. Nuclear Regulatory Commission Regulatory Guide 8.20,
“Applications of Bioassay for Radioiodine,” September, 2014, Revision 2.
• 4. Baker, Todd W. “Calibration & Use of a Capintec CAPTUS 3000
Portable Thyroid Uptake System for Iodine-125 Bioassay
Measurements”, presented at the Spring 2011 North Carolina Health
Physics Society, Chapel Hill, NC.
• U.S. Environmental Protection Agency Federal Guidance Report No. 11,
“Limiting Values of Radionuclide Intake and Air Concentration and Dose
Conversion Factors for Inhalation, Submersion, and Ingestion”, EPA-
520/1-88-020, September 1988
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• Any Interests in future pursuits for refining this method
contact:
• Ritchie Buschow, U.S. EPA/ORD/ORM/SHEMB -
[email protected], 919-542-0550
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Be Safe!!
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