Sodium Iodide I 131

DRUG DESCRIPTION

Sodium Iodide 131 (Na ¹³¹I) for diagnostic use is
supplied for oral administration in opaque white gelatin capsules. The capsules
are available in a strength of 3.7 megabecquerels (100 microcuries) iodine
I-131 at the time of calibration.

Sodium Iodide I 131 (sodium iodide i 131 (sodium iodide i 131 capsules) capsules) Capsules are prepared by absorbing a
solution of carrier-free sodium iodide I-131 into inert fillers. The iodine I-131
utilized in the preparation of the capsules contains not less than 99% iodine
I-131 at the time of calibration.

Physical Characteristics

Iodine I-131 decays by beta and associated gamma emissions
with a physical half-life of 8.04 days.¹ The principle beta
emissions and gamma photons are listed in Table 1.

Table 1. Principal Radiation Emission Data

Radiation	Mean Percent Per Disintegration	Energy (keV)
Beta-1	2.12	69.4 Avg.
Beta-3	7.36	96.6 Avg.
Beta-4	89.3	191.6 Avg.
Gamma-7	6.05	284.3
Gamma-14	81.2	364.5
Gamma-17	7.26	637.0

External Radiation

The specific gamma ray constant for iodine I-131 is 2.27
R/hr-mCi at 1 cm. The first half-value thickness of lead (Pb) for iodine I-131 is
0.24 cm. A range of values for the relative attenuation of the radiation
emitted by this radionuclide that results from interposition of various
thicknesses of Pb is shown in Table 2. For example, the use of 4.6 cm of Pb
will decrease the external radiation exposure by a factor of about 1,000.

Table 2. Radiation Attenuation by Lead Shielding*

Shield Thickness (Pb), cm	Coefficient of Attenuation
0.24	0.5
0.95	10-1
2.6	10-2
4.6	10-3
6.5	10-4
*Data supplied by Oak Ridge Associated Universities, Radiopharmaceutical Internal Dose Information Center, Oak Ridge, TN, 1987.

To correct for physical decay of this radionuclide, the
fractions that remain at selected time intervals after the date of calibration
are shown in Table 3.

Table 3. Physical Decay Chart, Iodine I-131, Half Life 8.04 Days

Days	Fraction Remaining	Days	Fraction Remaining
0*	1.000	16	0.252
1	0.917	17	0.231
2	0.842	18	0.212
3	0.772	19	0.194
4	0.708	20	0.178
5	0.650	21	0.164
6	0.596	22	0.150
7	0.547	23	0.138
8	0.502	24	0.126
9	0.460	25	0.116
10	0.422	26	0.106
11	0.387	27	0.098
12	0.355	28	0.089
13	0.326	29	0.082
14	0.299	30	0.075
15	0.274
*Calibration Day