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Annual Linearity - GPG93
• Do a full acceptance on every calibrator annually? Or quick checks?
.
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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.)
.
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
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• Calculate difference from expected measurements
• Tolerance 1% (reference) or 5% (field)
.
GPG93 Methodology
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• 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
.
GPG93 Methodology
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Automatic Logging – 20mins
• Logging data every
20 minutes
• 99Mo tail
• More complete dataset
.
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• 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
.
Automatic Logging – 20mins
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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)
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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
.
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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
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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
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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
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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
Practical Guidance