Derivation of Calibration Factors and Dial Settings Michaela Baker

Transcription

1 Derivation of Calibration Factors and Dial Settings Michaela Baker 29 November 2012

2 NPL Secondary standard ion chamber Direct calibration with NPL primary standards: - Photon emitting radionuclides - High energy beta emitting isotopes Excellent stability over several decades (stdev: 0.1%) - Accurate weighing - Dilution - Same reference time

3 NPL Calibration factors (pa/bmq): Defined for specific: NPL Secondary standard ion chamber - Radionuclide (Individually derived from NPL primary standards) - Dedicated holder - Glass containers (vials, ampoules) - Volume of liquid Direct correlation between: Sample Activity (MBq) Current output (pa)

4 I m (pa) Io/Im NPL Secondary standard ion chamber Volume correction Factors: Radionuclide and Container dependent - Minimal volume (mass) of active solution + Activity Assay - Gradual top up with inactive carrier + Activity assay at each stage Calibrator Response vs. Sample Volume: - graph (normalise to nominal volume) - estimate % difference I123 10R Schott vial Mass (g) I123 10R Schott vial y = x x R² = (m-4) Volume correction: I 0 / I m = a 2 (m- m 0 ) 2 + a 1 (m- m 0 ) + 1 where: I 0 = current expected at the nominal mass of m 0 I m = measured current at an individual mass m

5 Fidelis Calibration Factors Fidelis Secondary standard radionuclide calibrator: (Vinten, Isocal IV, NPL-CRC) NPL-designed ion chamber: - same as the NPL SS ion chamber - tested by NPL Electrometer and user interface unit Fidelis Calibration factors and volume correction factors: - Transferable from the NPL SS ion chamber - Calibrator independent - Direct traceability to NPL standards - Continuously updated:

6 Calibrated directly using NIST standards of 60 Co and 57 Co Expressed as: Calibration setting number (Dial factor) Defined for specific: Capintec Dial Setting numbers - Radionuclide: Relative to 60 Co dial factor - Container: glass ampoule with 0.6mm wall thickness good approximation to syringes - corrections given - Volume of liquid: 5 ml Linear relation: Sample Activity (MBq) Dial factor < 60 Co Dial Factor <ICh response

7 Capintec activity/npl activity Why recalibrate? 123 I UK comparison: Number of results Reported P6 vial value/npl value Reported syringe value/npl value % 30% 60% 90% 120% Sample volume as % of total container volume 5 ml syringe (A) 1 ml syringe 5 ml syringe (B) 10 ml syringe 2 ml BS ampoule 5 ml BS ampoule P6 vial 5 ml NBS ampoule Reported value / NPL value Container type/size: 10R Schott vial P6 Vial Difference (1.0 mm) ( 1.2mm) (0.2mm) 123 I pa/mbq pa/mbq 2 % 90 Y pa/mbq pa/mbq 7 % 99m Tc pa/mbq pa/mbq 1 %

8 Why recalibrate? Confirm the accuracy of the calibration factors Recalibrate/derive new factors for the preferred measurement format: - radionuclide - container type: vials, syringes - sample volume - shielding - attenuating holders ( 99 Mo breakthrough kit, 123 I and 90 Y Copper insert) Methods of calibration: Standardised source NPL calibration of hospital-supplied sources (comparison exercise) Transfer of calibration factors/dial settings to other containers Theoretically derived factors from photon energy response curves (needs validated by other means)

9 Calibration with standard source - Standard MBq Assay standard source: - Measured MBq Fidelis: Increase/decrease calibration factor by % difference between Standard MBq and Measured MBq => NEW Calibration Factor (pa/mbq) Capintec: Gradually increase/decrease Dial setting until Measured MBq = Standard MBq => NEW Dial Factor Issues? Limited availability (Half life) High cost

10 NPL calibration of hospital-supplied sources Advantages: Cost effective Direct traceability

11 Prior to calibration: Measured MBq NPL calibration of hospital-supplied sources From Calibration: NPL MBq % difference: Measured MBq and NPL MBq Post calibration: Same matrix source Activity assay: Measured MBq Fidelis: Increase/decrease Calibration factor by % difference from NPL calibration => NEW Calibration Factor (pa/mbq) Capintec: Adjust Dial factor until Measured MBq is increased/decreased by % difference from NPL calibration => NEW Dial Factor Or: Prior to calibration: Measured Activities over a range of Dial factors Post calibration: Select Dial factor for which Measured MBq = NPL MBq

12 Calibration transfer to other containers Accurate weighing EXTENSION OF CALIBRATION FACTORS TO OTHER CONTAINERS Stock Solution Ionisation Chamber precalibrated geometry MBq/g pa/g Ionisation Chamber uncalibrated geometry Calibration Figure pa/mbq (well-defined geometry)

13 Alternative method without weighing Calibration transfer to other containers Syringe active solution Transfer active solution Vial active solution full: (known calibration factor) Measure: Vial MBq Vial residue active solution: (refill with carrier) Measure: Vial res MBq Syringe activity = Vial full activity Vial residue activity Advantages: No weighing involved Not dependent of the type of syringe/needle or volume of solution in the syringe Limitations: - The accuracy of the vial residue activity measurement (beta emitters)

14 Impurities correction Why? - Correct Activity Assay prior to administration - Improved diagnostic and treatment - Waste disposal ( 123 I) Where from? - Impurity check gamma spectrometry - Information available from the supplier How? - Relative decay rate of impurities to that of the main radionuclide and/or - Relative response of impurities to that of the main radionuclide NPL ion chamber Half life (days) Cal. Factor (pa/mbq) 99m Tc Mo NPL ion chamber Half life (days) Cal. Factor (pa/mbq) 123 I Te I

15 Impurities correction 89 Sr correction for 85 Sr impurity GPG93 89 Sr 85 Sr R o /R i % Impurity Corr Half-lives: days days NPL Chamber (P6): pa/mbq pa/mbq % Capintec: % X i (at measurement time) 0.12 %

16 Impurities correction 89 Sr Activity = Corr. factor A * Indicated Activity NPL Report DQL-RN 012 Comparison of Strontium-89 Solution Sources in UK Hospitals, 2003 Available from: see publications

17 99 Mo breakthrough kit 99 Mo kit dimensions Lead walls thickness: 0.7 cm Lead Density: g cm -3 Height: 9.4 cm Inner diameter: 4 cm Calibrator type 99m Tc Attenuation factor 99 Mo Attenuation factor Lead wall thickness effect NPL ion chamber Measured: 4.5 GBq -100% Theoretical: 4 GBq 100% Measured: 4.43 Theoretical: 4.5 ± 0.05 cm: ± 6% response variation Capintec 4.5 GBq -100% 8.04

18 Thank you

19 Title of Presentation Name of Speaker Date The National Measurement System delivers world-class measurement science & technology through these organisations The National Measurement System is the UK s national infrastructure of measurement Laboratories, which deliver world-class measurement science and technology through four National Measurement Institutes (NMIs): LGC, NPL the National Physical Laboratory, TUV NEL The former National Engineering Laboratory, and the National Measurement Office (NMO).