The 18% Discrepancy at 07:10
At 07:10 on a Tuesday, a 58-year-old woman with papillary thyroid carcinoma is scheduled to receive 150 mCi (5.55 GBq) of I-131 for remnant ablation. The vendor certificate states 200 mCi (7.4 GBq) at 12:00 the previous day, which decays to about 170 mCi at 07:10. The technologist draws the dose into a 10 mL plastic syringe and measures it in the department's only dose calibrator on the I-131 setting: 139 mCi. The daily Cs-137 constancy check that morning was +1.6%. The calibrator's syringe holder was removed for cleaning and has not been replaced; the syringe is resting on the bottom of the well. The physician, who is in the room, says, 'Just draw more until it reads 150.' The patient's pregnancy test is negative and she has completed a low-iodine diet. No I-131 standard source is available.
1. What is the most likely cause of the 18% discrepancy, and what must you do before drawing additional activity?
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The discrepancy is most consistent with a geometry and volume effect, not a true activity deficit. The syringe is being counted without its holder, resting at the bottom of the well, which changes the effective distance and attenuation relative to the calibrator's calibration geometry. A 10 mL plastic syringe also differs from the glass vial used on the certificate. The Cs-137 constancy check validates only the Cs-137 setting and the instrument's short-term stability; it does not validate the I-131 dial setting or the syringe geometry. Before drawing more activity, you should: (1) replace the holder and remeasure the original vial; (2) confirm the I-131 dial setting against the manufacturer's recommended setting; (3) decay-correct the certificate independently; (4) if available, cross-check with a second instrument or a traceable I-131 standard. Do not adjust the administered volume on the basis of an unverified reading.
2. The original vial now reads 171 mCi with the holder in place, and the syringe containing the drawn dose reads 149 mCi by the corrected geometry. The physician still wants to use the earlier 139 mCi value 'to be safe.' How do you respond, and what do you document?
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Using 139 mCi would under-treat the patient by about 7% and would be based on a measurement known to be invalid. The correct activity is approximately 150 mCi, confirmed by two consistent measurements in proper geometry. The earlier 139 mCi reading should not be used for release or for dosimetry. You should explain that the calibrator's accuracy specification (typically ±10%) applies only under validated geometry and for a calibrated radionuclide setting; the corrected measurement is the defensible one. Document the initial geometry error, the corrective action, the repeat vial and syringe measurements, the certificate decay calculation, and the final administered activity. If the corrected readings still differed by more than 10% from the certificate, you would need to investigate the I-131 setting or the instrument before release, not simply choose a value.
3. Suppose the repeat measurement with the holder still reads 139 mCi. What is your next step, and can the vendor certificate be used to define the administered activity?
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If proper geometry still gives 139 mCi against a certificate-derived 170 mCi, the problem is no longer geometry. You must suspect an incorrect I-131 dial setting, a calibration drift, or a certificate error. Do not administer until this is resolved. Options include: measuring a traceable I-131 standard, cross-calibrating with another dose calibrator, or contacting the vendor to verify the certificate. A vendor certificate is traceable and can support the activity, but it should not be the sole basis for release when the on-site measurement disagrees by 18%; an independent verification is needed. If no standard or second instrument is available, postpone the therapy and arrange service. Document the discrepancy, the decision, and the communication with the patient and physician.