annual linearity - gpg93 · annual linearity - gpg93 ... datalogging software for isocals, ......

Annual Linearity - GPG93

• Do a full acceptance on every calibrator annually? Or quick checks?

.

Annual Linearity - RSCH

• RSCH has 7 clinically used calibrators

• We also look after another 8 calibrators

• Makes annual linearity quite a chore !

(so we generally don’t do it.)

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RSCH

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GPG93 Methodology

• Should measure over full range

• From maximum used (200GBq radiopharmacy, 20GBq clinical)

• To 0.1 MBq (approx 10x background)

• GPG93 recommends

decaying source

• Take reading every

2 hours

• R2 will always be 1.000

• Needs further analysis

• Calculate difference from expected measurements

• Tolerance 1% (reference) or 5% (field)

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GPG93 Methodology

• Cons:

• Needs a tonne of 99mTc (cost, finger doses)

• Not sensitive – deviations explained by systematic errors

• Difficult to commit/remember to do the readings

• Takes a week of occasional measurements

• Pros:

• Ticks the box

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GPG93 Methodology

Automatic Logging – 20mins

• Logging data every

20 minutes

• 99Mo tail

• More complete dataset

.

• Sensible amount of data – 500 data points over 5 days.

• Red curve is after 99Mo correction

• Counting errors at low activities

• Sensitive to jumps / range changes

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Automatic Logging – 20mins

Range Changes

• Fidelis and Capintecs have 3 sets of electronics to enable them to

accurately measure activities and be linear across a 10^8 range.

• Sometimes the 3 ranges don’t quite match up, and sometimes you get

increasing errors towards range changes

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Range Changes

Fidelis and Capintec range changes

for decaying sources occur at around

~18.6pA and ~1860pA –

(Approx 15MBq and 1500 MBq 99mT) (actual point varies with load/impedance)

Range changes upwards occur at

currents a little higher than these so

there are no sweet spots with rapid

switching

(Although I did try to find one once)

Range Changes

Fidelis has built in linearity

correction which gives near

perfect range change handling

But can still get infinite errors

if you log every second!

</pedant>

Range steps down at ~18.6pA

Range steps up (!) at ~19.9pA

.

Would you fail this?

• Picking instantaneous readings every 20 minutes

• Strange behaviour from 8MBq to 4MBq

• Pass? Fail? Repeat?

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• Averaging over every couple of minutes reduces counting errors

• More frequent sampling = more sensitive to faults

• Not the kind of test you want to repeat!

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Automatic Logging – 1 second

Our Current Practice

• Acceptance

• We do a full decaying source with automatic data logging

• Annual checks

• Full decaying source but only when we have a trainee. (but we have about 15 calibrators and 2 or 3 trainees)

• Quicker/easier alternatives?

• What errors are we expecting to find?

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Alternatives for Annual QC – 3 Sources

3 Source Method 10GBq, 1GBq and 10MBq

1. Measure the 3 sources at the start and end of the day

2. Calculate half life of each source. With the above activities, each source will cover one of the 3 ranges.

Pros:

• Low dose / cost / effort

• Over in one day.

• Analysis takes 2 seconds

Cons:

• Not very scientific (no graphs!)

• Not quite as sensitive even to large non-linearities

• 99Mo might affect low range

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Alternatives for Annual QC - Calicheck

Involves calculating the attenuation factor of the sleeves and

multiplying a very small number by a very large factor.

Manual says make limits the mean of all corrected readings +-5% (!!!)

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Calicheck Limits

Pros:

• Very quick – 10 minutes!

• More scientific – you get a nice graph.

Cons:

• Lead may get damaged

• Potential for calibrator damage

• Set costs <£650

Discussion

Method Time Materials needed

Number of measurements

Sensitivity to small

non-linearities

Sensitivity to large non-linearities

Automatic logging

1 week (downtime!)

200GBq 500 to 500,000

Very good Very good

GPG93 occasional measurements

1 week 200GBq 20 Poor Good

3 source method

1 day <20GBq 6 Very poor OK

Calicheck 10 mins Lead set <20GBq

12 Very poor Good

Discussion

1. Does anyone actually do annual linearity QC (as in... every year...)?

2. Has anyone had a calibrator pass acceptance but fail annual checks?

3. Any other methods in use that I have overlooked?

4. Datalogging software for Isocals, Veenstras, Atomlabs etc.

Alexander Smout

Medical Physicist

a.smout@nhs.net

Practical Guidance