A Thyroid-Therapy Vial That Does Not Decay as Labeled
A hospital's nuclear medicine department receives a vial labeled I-131 (t1/2 = 8.02 d) for thyroid therapy. The supplier's certificate states a calibration activity of 3.70 GBq in 2.0 mL at 08:00 six days ago. The radiation safety officer measures the vial today and finds 3.55 GBq, within the 10% acceptance range. A gamma spectrometer shows a strong peak at 364 keV and a smaller peak at 637 keV. A resident calculates that the activity should have fallen to 2.20 GBq and argues the vial is mislabeled. The department must decide whether to use the vial for a patient scheduled tomorrow morning or quarantine it. The patient's thyroid uptake test is not urgent, but the referring physician is pressing for treatment.
1. Calculate the activity expected today if the vial is I-131 and has decayed from the stated calibration activity. Then calculate the half-life implied by the measured 3.55 GBq. What does the comparison suggest?
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For I-131, A = A0 e^(−ln2·t/t1/2) = 3.70 GBq × e^(−0.693×6.00/8.02) = 3.70 × e^(−0.5185) = 3.70 × 0.595 = 2.20 GBq. The measured 3.55 GBq is 1.6 times higher than expected.
If the certificate time and measured activity are both correct, the implied half-life is t1/2 = t ln2 / ln(A0/A) = 6.00×0.693/ln(3.70/3.55) = 4.16/0.0414 = 101 d, which is not I-131. The comparison therefore indicates a bookkeeping or measurement problem (wrong calibration time or time zone, wrong isotope setting on the dose calibrator, or a mislabeled vial), not a normal decay correction.
2. The gamma spectrum shows peaks at 364 keV and 637 keV. How should this information affect the identity question, and what is the most likely explanation for the activity discrepancy?
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Both 364 keV and 637 keV are gamma emissions associated with I-131, so the spectrum supports the presence of I-131 rather than a different iodine isotope or a pure beta emitter. That makes a wholesale mislabeling less likely and shifts attention to the activity measurement.
The most likely explanation is a dose-calibrator problem: the instrument may be set to the wrong isotope or have a drifted calibration, or the certificate's calibration time may have been misread (for example, local time vs supplier time). A 101 d apparent half-life is far too long for I-131 decay, so it cannot be explained by normal decay; it is consistent with measuring the vial on an incorrect setting or comparing to the wrong reference time. The spectrum does not prove the activity, only the identity.
3. Recommend whether to use or quarantine the vial, and outline the specific checks needed before a patient dose.
Read the worked answer
Quarantine the vial until the discrepancy is resolved. The patient's test is not urgent, so there is no clinical justification for using an assay that is inconsistent by 60%.
Specific checks: (1) verify the certificate's calibration time and time zone and recalculate; (2) confirm the dose calibrator isotope setting and perform a daily constancy check with a long-lived standard such as Cs-137; (3) re-measure the vial in a second calibrator or geometry, and repeat the measurement after 24 h to estimate the half-life; (4) review the gamma spectrum against an I-131 reference and check for contaminant peaks; (5) confirm the supplier's lot and certificate. If the activity and identity are confirmed, recalculate the patient dose from the current assay; if not, reject the vial and notify the supplier and radiation safety officer. Document the investigation.