title: Accuracy, reproducibility, and uncertainty analysis of thyroid-probe-based activity measurements for determination of dose calibrator settings.
abstract: OBJECTIVE
  In the nuclear medicine department, the activity of radiopharmaceuticals is measured using dose calibrators (DCs) prior to patient injection. The DC consists of an ionization chamber that measures current generated by ionizing radiation (emitted from the radiotracer). In order to obtain an activity reading, the current is converted into units of activity by applying an appropriate calibration factor (also referred to as DC dial setting). Accurate determination of DC dial settings is crucial to ensure that patients receive the appropriate dose in diagnostic scans or radionuclide therapies. The goals of this study were (1) to describe a practical method to experimentally determine dose calibrator settings using a thyroid-probe (TP) and (2) to investigate the accuracy, reproducibility, and uncertainties of the method. As an illustration, the TP method was applied to determine188Re dial settings for two dose calibrator models: Atomlab 100plus and Capintec CRC-55tR.
  
  
  METHODS
  Using the TP to determine dose calibrator settings involved three measurements. First, the energy-dependent efficiency of the TP was determined from energy spectra measurements of two calibration sources (152Eu and22Na). Second, the gamma emissions from the investigated isotope (188Re) were measured using the TP and its activity was determined using γ-ray spectroscopy methods. Ambient background, scatter, and source-geometry corrections were applied during the efficiency and activity determination steps. Third, the TP-based188Re activity was used to determine the dose calibrator settings following the calibration curve method [B. E. Zimmerman et al., J. Nucl. Med. 40, 1508-1516 (1999)]. The interobserver reproducibility of TP measurements was determined by the coefficient of variation (COV) and uncertainties associated to each step of the measuring process were estimated. The accuracy of activity measurements using the proposed method was evaluated by comparing the TP activity estimates of99mTc,188Re,131I, and57Co samples to high purity Ge (HPGe) γ-ray spectroscopy measurements.
  
  
  RESULTS
  The experimental188Re dial settings determined with the TP were 76.5 ± 4.8 and 646 ± 43 for Atomlab 100plus and Capintec CRC-55tR, respectively. In the case of Atomlab 100plus, the TP-based dial settings improved the accuracy of188Re activity measurements (confirmed by HPGe measurements) as compared to manufacturer-recommended settings. For Capintec CRC-55tR, the TP-based settings were in agreement with previous results [B. E. Zimmerman et al., J. Nucl. Med. 40, 1508-1516 (1999)] which demonstrated that manufacturer-recommended settings overestimate188Re activity by more than 20%. The largest source of uncertainty in the experimentally determined dial settings was due to the application of a geometry correction factor, followed by the uncertainty of the scatter-corrected photopeak counts and the uncertainty of the TP efficiency calibration experiment. When using the most intense photopeak of the sample's emissions, the TP method yielded accurate (within 5% errors) and reproducible (COV = 2%) measurements of sample's activity. The relative uncertainties associated with such measurements ranged from 6% to 8% (expanded uncertainty at 95% confidence interval, k = 2).
  
  
  CONCLUSIONS
  Accurate determination/verification of dose calibrator dial settings can be performed using a thyroid-probe in the nuclear medicine department.
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
authors.forename: Pedro L
authors.initials: PL
authors.lastname: Esquinas
authors.affiliation: Department of Physics, IK Barber School of Arts and Sciences, University of British Columbia, Kelowna, British Columbia V1V 1V7, Canada.
authors.forename: Jesse
authors.initials: J
authors.lastname: Tanguay
authors.affiliation: Vancouver Coastal Health Authority, Vancouver, British Columbia V5Z 4C2, Canada.
authors.forename: Marjorie
authors.initials: M
authors.lastname: Gonzalez
authors.affiliation: British Columbia Cancer Agency, Vancouver, British Columbia V5Z 4E6, Canada.
authors.forename: Milan
authors.initials: M
authors.lastname: Vuckovic
authors.affiliation: Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada and Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
authors.forename: Cristina
authors.initials: C
authors.lastname: Rodríguez-Rodríguez
authors.affiliation: Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
authors.forename: Urs O
authors.initials: UO
authors.lastname: Häfeli
authors.affiliation: Department of Radiology, University of British Columbia, Vancouver, British Columbia V5Z 1M9, Canada.
authors.forename: Anna
authors.initials: A
authors.lastname: Celler
mesh_terms: D002138:Calibration; D006801:Humans; D015588:Observer Variation; D011874:Radiometry / Q000379:methods* / Q000592:standards; D015203:Reproducibility of Results; D012211:Rhenium; D013961:Thyroid Gland / Q000528:radiation effects*; D035501:Uncertainty
chemical_list: D012211:Rhenium