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

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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)

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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)

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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

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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 -

buschow.ritchie@epa.gov, 919-542-0550

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Be Safe!!

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