Radioiodine Bioassays

QA Programs for Uptake Probes and Well Counters

Radioiodinated compounds and solutions undergo decomposition that may result in the volatilization of radioiodine. If this occurs, individuals working with these materials have a potential for accidental ingestion or inhalation of radioactive iodine. Once inside the body, the iodine concentrates in the thyroid and irradiates that organ. A bioassay program will enable the Radiation Safety staff to monitor and determine the radioiodine burden in an individual’s thyroid.

Regulatory Guide 8.20 Application of Bioassay for I-125 and I-131(dated September 1979) and DRAFT Regulatory Guide DG-8050 (dated September 2011), provide criteria acceptable to the NRC staff for the development and implementation of a bioassay program. It provides guidance on the selection of workers who should participate in the program, frequencies of bioassay, actions to take based on bioassay results and the particular results that should initiate such actions.

Below are excerpts from the NRC Reg Guide 8.20.

1. Conditions under Which Bioassay Is Necessary

a. Routine bioassay is necessary when an individual handles unsealed quantities of radioactive iodine in open form that exceed those shown in Table 1 of Regulatory guide 8.20. Table 1 applies to both the quantity handled at any one time or integrated as the total amount of activity introduced into a process by an employee over any 3-month period (i.e., 90 days).

b. When quantities handled in unsealed form are greater than 10 percent of Table 1 values, routine bioassay may be necessary under certain circumstances. A written justification for not performing such bioassay measurements should be prepared and documented for subsequent review during NRC inspections whenever bioassay is not performed and the quantities handled exceed 10 percent of Table 1 values.

c. Except as noted above, bioassay is not required when process quantities handled by a worker are less than 10 percent of those values in columns 1 and 2 of Table 1.

Table 1

2. Participation

All workers who handle radioiodine substances or are sufficiently close to the process so that intake is possible (e.g., within a few meters and in the same room as the worker handling the material) should participate in the bioassay programs.

3. Type of Bioassay That Should Be Performed

a. Baseline Bioassay. (pre-employment or pre-operational). Before beginning work with radioiodine materials in quantities as specified in regulations.

b. Routine Bioassay. At the frequency specified below in section 4.

c. Special Bioassay. As soon as possible after any incident that might cause radioiodine uptake to exceed thyroid burdens given in regulations.

d. Post-operational and Termination. Within 2 weeks of the last possible exposure to I-125 or/and I-131 when operations are being discontinued or terminated.

e. Diagnostic. Follow up bioassay within 2 weeks of any measurements exceeding activities given as action levels to allow an estimate of the effective half-life of radioiodine in the thyroid or the body.

4. Frequency

a. Initial Routine. Except in situations where thyroid contents may exceed quantities specified in, a bioassay sample or monitoring measurement should be obtained within 72 hours (3 days) following entry of an individual into an area where bioassay is performed. When work with radioiodine is on an infrequent basis (less frequently than every 2 weeks), bioassay could be performed within 10 days of the end of the work period during which radioiodine was handled (but not sooner than 6 hours unless emergency actions to obtain an early prognosis and thyroid blocking treatment are appropriate).

b. After 3 Months. Frequency may be changed to quarterly monitoring if certain conditions are met.


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