Federal emergency management agency fema rep-2, rev. 2 / June 1990

5.6Thyroid Dose Commitment from Iodine Inhalation

The Food and Drug Administration^lxii (FDA) has stated that the administration of 100 mg of iodide (130 mg of potassium iodide) prior to radioiodine exposure results in blocking over 90 percent of the peak radioactive iodine uptake in the thyroid.^lxiii A block of up to 50 percent is attainable if the dose is given within 3-4 hours after acute exposure and some benefit is obtained even if the drug is administered up to 12 hours after exposure.

A daily dose of potassium iodide would be required to maintain the blocked state and should be continued 3 to 7 days to prevent thyroidal uptake of any radioiodine still circulating in the body. Use is not expected to exceed 10 days. All emergency workers should be medically checked for sensitivity to potassium iodide to assure that there will be no undesirable medical effects.

Respiratory protection equipment, half and full-face charcoal filtered masks, and particularly air-supplied masks, could be used, if available, to reduce the inhalation dose to the thyroid. However, the use of potassium iodide is a much more effective procedure for reducing thyroid dose. If respiratory, protection equipment is to be used by emergency response personnel, each individual wearing the respiratory protection equipment must be qualitatively fit tested in accordance with ANSI Z88.2-1980^lxiv and the individual must meet the physical qualifications specified in ANSI Z88.6-1984.^lxv

Thyroid dose can be estimated with a fair degree of accuracy by measurement with a gamma radiation detector held horizontally adjacent to an individual’s thyroid, which is immediately below the Adam's apple. From the count rate obtained, an approximation of the thyroid uptake can be obtained from data given in Table 6.^lxvi The data presented in Table 6 are averages of ten measurements made on thyroid phantoms. Each instrument's detector was positioned to give maximum responses, i.e., horizontally adjacent to the neck, with the detector's active area centered between the thyroid lobes. From the above data, the projected dose to the thyroid from either a single or continuous uptake of radioiodine can be obtained using the following equation:

^DTh = ^kD x μ
where ^DTh = projected thyroid dose in rem

^kD = dose conversion factor in rem/microcurie in the thyroid

μ = uptake in microcuries

where ^DTh = projected thyroid dose in rem

μ = uptake in microcuries

The sensitivity of this monitoring method can be determined by calculating the minimum detectable levels (MDL) for the monitoring instruments. The MDL is a function of the uncertainty of instrument backgrounds and as such it will vary among instrument types as well as within an individual instrument based on the sample measurement location, e.g., potentially different backgrounds for different locations. For instruments with analog readouts, the MDL in counts per minute can be calculated from the equation:

MDL= 2 √B/2RC
where B = background count rate in counts per minute

RC = meter time constant in minutes, which can be determined from information normally found in the manufacturer's specifications

MDL = 2 √B

*See attachment for image – Figure 3 and Table 7*

Table 7. Minimum Detectable Levels – CPM^a

*See attached image – Figure 3 and Table 7*

Table 8. Minimum Detectable Dose Commitment for Thyroids^a,b REM Infinity ^c

Scintillation detectors have the most sensitive detection level for ¹³¹ I in human thyroids. The 5 x 5 cm (2x2 in.) Nal(Tl) crystal detector is able to detect radioactivity levels approximately equivalent to a 0.03 rem dose commitment for two year olds under the test background conditions. The smaller 3.2 x 3.8 cm (1.25 x 1.5 in.) Nal(Tl) crystal detector is able to detect radioactivity levels equivalent to a 0.14 rem dose commitment for two year olds. The GM detectors are less sensitive than the scintillation detectors. The least sensitive of the GM detectors tested is able to detect radioactivity levels equivalent to a 4.3 rem dose commitment for two year olds.

This thyroid monitoring procedure is recommended only for screening all emergency personnel at the completion of their final mission involving direct exposure to the plume or evacuees who are known to have been exposed to the plunge for a significant period of time prior to evacuation (see footnote (b) to Table 8). If this procedure indicates a projected dose of more than 10 rem to the thyroid, the individual should be sent to a hospital or laboratory where an accurate determination of the radio-iodine uptake can be made. Ten rem to the adult thyroid would result in a reading of approximately 1200 cpm on the CD V-700 with a standard D-103 detector, i.e., just a little more than 1/3 of full scale on the Xl range.

The minimum detectable dose commitment values found in Table 8 of this document are different from the values contained in Table 7 of Reference 31, because the dose conversion factors used in Reference 31 are incorrect, e.g., dose conversion factors for radioactivity inhaled into the body were used rather than dose conversion factors for radioactivity contained in the thyroid.