Sodium Iodide I-131

11 DESCRIPTION 11.1 Chemical Characteristics Sodium Iodide I-131 Solution, a radioactive therapeutic agent, provides a concentrated solution of sodium iodide I-131 with a radioconcentration of 37,000 MBq/mL (1,000 mCi/mL).​‍​​​​​‍​‍‍‍‍​‍‍ Each mL of the concentrated solution contains 37,000 MBq of no-carrier-added sodium iodide I-131, disodium edetate dihydrate USP as a stabilizer, sodium thiosulfate pentahydrate USP as a reducing agent, and dibasic sodium phosphate anhydrous USP. The pH of the concentrated solution is between 7.5 and 10. The concentrated solution provided with Sodium Iodide I-131 Solution is used for the preparation of sodium iodide I-131 capsules or sodium iodide I-131 solution of varying strengths for oral administration for therapy. Sodium iodide I-131 solution is designated chemically as Na 131 I and has a molecular weight of 153.99 g/mol. Hard gelatin capsules, provided for the preparation of the sodium iodide I-131 capsules final dosage form, contain approximately 300 mg of dibasic sodium phosphate anhydrous USP as the absorbing buffer. 11.2 Physical Characteristics Iodine I-131 decays by beta emission and associated gamma emission with a physical half-life of 8.02 days. The principal radiation emissions are listed in Table 5. Table 5 Principal Radiation Emission Data from Decay of Sodium Iodide I 131 Radiation Mean % per Disintegration Mean Energy (keV) Beta-1 2.1% 69.4 Beta-3 7.2% 96.6 Beta-4 89.4% 191.6 Gamma-7 6.1% 284.3 Gamma-14 81.2% 364.5 Gamma-18 7.1% 637.0 11.3 External Radiation The specific gamma-ray constant for iodide I 131 is 4.26 × 10 -13 C·m 2 ·kg -1 ·MBq -1 ·s -1 (2.2 R·cm 2 /mCi·hr). The first half-value thickness of lead (Pb) for iodide I 131 is 0.27 cm. A range of values for the relative attenuation of the radiation emitted by iodide I 131 that results from interposition of various thicknesses of Pb is shown in Table 6. For example, the use of 2.59 cm of Pb will decrease the external radiation exposure by a factor of about 1,000. Table 6 Radiation Attenuation of Iodine I 131 by Lead Shielding Shield Thickness (Pb) cm Coefficient of Attenuation 0.27 0.5 0.56 0.25 0.99 10 -1 2.59 10 -2 4.53 10 -3 To correct for physical decay of iodine I 131, the fractions that remain at selected intervals after the time of calibration are shown in Table 7. Table 7 Physical Decay Chart: Iodine I-131 Half-Life 8.02 days *Calibration time Days Fraction Remaining Days Fraction Remaining Days Fraction Remaining 0* 1.000 11 .386 22 .149 1 .917 12 .354 23 .137 2 .841 13 .325 24 .126 3 .772 14 .298 25 .115 4 .708 15 .274 26 .106 5 .649 16 .251 27 .097 6 .595 17 .230 28 .089 7 .546 18 .211 29 .082 8 .501 19 .194 30 .075 9 .459 20 .178 10 .421 21 .163