I-131 Dose Calculation for Graves’ Disease
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Question
A patient with Graves' disease has a 24-hour RAIU of 65% and a thyroid gland weight estimated at 40 grams. I-131 therapy is planned with a target delivered dose of 150 µCi/gram. Using the basic dosimetry formula: Activity (µCi) = [Target dose × Gland weight] / RAIU, what is the approximate prescribed I-131 activity?
- AApproximately 9.2 mCi (9,230 µCi)
- BApproximately 3.9 mCi
- CApproximately 15.0 mCi
- DApproximately 0.9 mCi
Show the answer and explanation
A. Approximately 9.2 mCi (9,230 µCi)
Using the standard Quimby-Marinelli dosimetry formula: Activity (µCi) = [Target dose (µCi/g) × Gland weight (g)] / RAIU fraction = [150 µCi/g × 40 g] / 0.65 = 6,000 / 0.65 ≈ 9,230 µCi ≈ 9.2 mCi. The formula accounts for the fact that not all administered I-131 is retained in the gland — only the fraction equal to the RAIU is captured. Higher RAIU means more efficient uptake, so less activity is needed for the same delivered dose. Higher gland weight requires more activity for the same dose per gram. This calculation is a fundamental skill for nuclear medicine technologists.
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