Survey of the Performance of Commercial Dose Calibrators for Measurement of 123I Activity

The accuracy with which 123I activity is measured by a dose calibrator depends on the composition and geometry of the source. The present study assessed the variability of current commercial dose calibrators in assaying liquid 123I samples. Methods: A calibration procedure for 123I measurement was performed on 177 dose calibrators (11 manufacturers) at 138 sites in North America and Europe. Using the standard 123I push-button or dial setting, activity in a 5-mL 123I calibration source in a 10-mL U.S. Pharmacopeia type 1 glass vial (actual activity previously determined using a National Institute of Standards and Technology–traceable standard metrology chamber) was measured. A portion of the source was then transferred to a plastic syringe (10-, 5-, and 3-mL sizes at different sites), and the activity in the syringe was measured. Calibration factors (CFs) for converting the dose calibrator readings to the reference activities were then determined for the vial and the syringes. Data were analyzed for all calibrators combined and based on device manufacturers. Measurements using a copper attenuator (sleeve) were made for a subset of 10 dose calibrators at sites that used these devices in clinical practice. Results: Mean CFs for the different measurements were as follows: 10-mL vial, 1.278; 10-mL syringe, 0.811; 5-mL syringe, 0.815; 3-mL syringe, 0.792. Almost half of the dose calibrators had vial CFs between 1.2 and 1.3 and 10-mL syringe CFs between 0.7 and 0.8, whereas less than 16% of the instruments had uncorrected readings within 610% of the reference activities. Although there was a wide range of CFs for different calibrators using the copper sleeve, for each unit the CFs for the glass vial and the 10-mL plastic syringe were virtually identical. Conclusion: On most commercial dose calibrators, the standard 123I settings result in significant errors in activity measurements for sources in glass vials and plastic syringes. The difference in ionization chamber detection caused by the container composition (glass vs. plastic) is a much larger source of measurement variation than source volume or geometry.