Derivation of Calibration Factors and Dial Settings

Michaela Baker

29 November 2012

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

NPL Calibration factors (pA/BMq):

Defined for specific: - 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)

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

Volume correction: I0 / Im = a2(m- m0)2 + a1(m- m0) + 1

where: I0 = current expected at the nominal mass of “m0”

Im = measured current at an individual mass “m”

NPL Secondary standard ion chamber

11.5

11.55

11.6

11.65

11.7

11.75

11.8

11.85

11.9

1.4 2.4 3.4 4.4 5.4 6.4 7.4 8.4

I m (

pA

)

Mass (g)

I123 10R Schott vial

y = -0.000299x2 + 0.004444x + 1.000026 R² = 0.998322

0.984

0.989

0.994

0.999

1.004

1.009

1.014

1.019

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Io/Im

(m-4)

I123 10R Schott vial

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: www.npl.co.uk/fidelis

Fidelis Calibration Factors

Calibrated directly using NIST standards of 60Co and 57Co

Expressed as: Calibration setting number (Dial factor)

Defined for specific: - Radionuclide: Relative to 60Co 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 <60Co Dial Factor <ICh response

Capintec Dial Setting numbers

123I UK comparison:

Container type/size:

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0% 30% 60% 90% 120%

Sample volume as % of total container volume

Ca

pin

tec

ac

tiv

ity/N

PL

ac

tiv

ity

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 ampoule0

5

1015

20

25

3035

40

45

5055

60

65

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Reported value / NPL value

Number of

resultsReported P6 vial value/NPL value

Reported syringe value/NPL value

Why recalibrate?

10R Schott vial

(1.0 mm)

P6 Vial

( 1.2mm)

Difference

(0.2mm)

123I 1.721 pA/MBq 1.685 pA/MBq 2 %

90Y 0.0734 pA/MBq 0.0682 pA/MBq 7 %

99mTc 1.240 pA/MBq 1.227 pA/MBq 1 %

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 (99Mo breakthrough kit, 123I and 90Y Copper insert)

Why recalibrate?

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)

- 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

Calibration with standard source

NPL calibration of hospital-supplied sources

Advantages:

Cost effective

Direct traceability

Prior to calibration: Measured MBq

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

NPL calibration of hospital-supplied sources

Calibration transfer to other containers

EXTENSION OF CALIBRATION FACTORS

TO OTHER CONTAINERS

Stock Solution

Ionisation Ionisation

Chamber Chamber precalibrated uncalibrated

geometry geometry

MBq/g pA/g

Calibration Figure

pA/MBq (well-defined geometry)

Accurate

weighing

Alternative method without weighing

Calibration transfer to other containers

Vial active

solution full: (known calibration factor)

Measure: Vial MBq

Transfer

active

solution

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)

Syringe

active solution

Why?

- Correct Activity Assay prior to administration

- Improved diagnostic and treatment

- Waste disposal (123I)

Where from?

- Impurity check – gamma spectrometry

- Information available from the supplier

Impurities correction

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)

123I 0.55098 1.685

121Te 19.16 5.932

125I 59.388 0.3706

NPL ion chamber

Half life

(days)

Cal. Factor

(pA/MBq)

99mTc 0.25028 1.227

99Mo 2.7479 2.700

Impurities correction

89Sr 85Sr Ro/Ri % Impurity Corr

Half-lives: 50.57 days 64.850 days

NPL Chamber (P6): 0.0279 pA/MBq 5.258 pA/MBq 186 18 %

Capintec: 40.0 5 %

Xi (at measurement time) 0.12 %

89Sr correction

for 85Sr impurity

GPG93

89Sr Activity = Corr. factor A * Indicated Activity

“NPL Report DQL-RN 012

Comparison of Strontium-89

Solution Sources in UK Hospitals, 2003”

Available from:

www.npl.co.uk/rcuf see “publications”

Impurities correction

99Mo breakthrough kit

99Mo kit – dimensions

Lead walls thickness: 0.7 cm

Lead Density: 11.34 g cm-3

Height: 9.4 cm

Inner diameter: 4 cm

Calibrator

type

99mTc

Attenuation factor

99Mo

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

Thank you

Title of Presentation

Name of Speaker

Date

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).

The National Measurement System delivers world-class

measurement science & technology through these organisations