Calibration Chain Flashcards
What is basic outline of calibration chain at NPL?
- Primary standard is graphite calorimeter, measures dose to graphite. Primary standard calibrates 3 NPL reference chambers in graphite phantom
- NPL reference chamber calibrations converted from D_graphite to D_water
- NPL calibrate hospital secondary standards against NPL reference chambers in water
How is dose measured in graphite?
Don’t measure temperature rise, maintain temperature and substitute electrical heating for radiation heating
D = VIt/m
Why do graphite calorimeters need to be thermally isolated?
Thermal diffusivity in graphite much larger than water, heat in graphite distribute itself in volume very quickly, too quickly to measure.
Why do NPL use graphite?
Reduce uncertainties
Bigger temperature rise from graphite
Why is graphite calorimeter not used to measure against secondary standards?
Too time consuming to use graphite calorimeter for everything. Reference chambers calibrated once a year against calorimeter.
What method do NPL use to calibrate user secondary standard with reference chambers?
Substitution method
What beams are used to calibrate graphite calorimeter against reference chambers?
60Co and a range of x-ray qualities between 4 and 25MV
What correction factors are used to correct raw dose for the measured chamber reading?
k_elec
k_TP
k_h
k_ion
k_pol
k_vol
What is k_elec correcting for?
Charge calibration coefficient for electrometer, which is calibrated by NPL separately, can be single factor or calculated by user
What is k_TP correcting for?
Temperature and pressure, account for the difference in mass of air in the chamber active volume
What is k_h correcting for?
Humidity correction. Can be taken as unity in 20-70% relative humidity, outside of this range NPL can advise
What is k_ion correcting for?
Losses because of ion recombination at finite collection voltages. Correction essential in pulsed radiation beams and can be substantial in FFF.
What is k_pol correcting for?
Polarity effects, which is negligible for 2611 chamber and only significant for small volume chambers. NPL don’t correct for polarity so long as it is constant during calibration process.
What is k_vol correcting for?
Beam non-uniformity, accounts for volume averaging and perturbation of secondary electron fluence in non-uniform beam profile such as FFF. For 2611 can be unity.
Which correction factors are usually unity
k_h
k_pol
k_vol
How calculate dose to water from secondary standard?
D_w = M_Q . N SS _D,w,Q (QI)
Where M_Q is corrected reading
What is calibration coefficient?
N SS _D,w,Q (QI)
How many measurements are taken to calibrate field instrument against secondary standard?
Either: measure 3 times, swap, measure 3 times, swap, measure 3 times
OR: substitute each time, FFF beams
How is calibration of field instrument against secondary standard done?
Work out own calibration factor for each field instrument against secondary standard.
Take measurements
Assume dose to one is same as dose to other
Divide and multiply by measurement of field instrument and then take out a ratio as calibration factor
Dfield = Dstd = Mst . N_D,w,std(Q) . correction factors
N_D,w,field(Q) = Mst / Mfield . N_D,w,std(Q) . k_ion . k_elec
Therefore Dw,field = Mfield . N_D,w,field(Q). k_tp
What do NPL give hospital after calibration?
Dose to water factor for secondary standard chamber
D_w = M_q . N^SS _ D,w,Q (QI)
What if your field can’t produce 10 x 10 field and thus can’t get TPR 20 10?
Establish a TPR 20,10 with a field size that can be achieved.
Use an equivalent field size
Reference field needs to have LCPE, a field size greater than 1.5cm does this for 6MV beams, 2.5cm for 24MV beams.
What is an MSR field and when is it used?
Machine specific reference field
MSR field is a field that is equivalent to a 10 x 10 field, and is used when a 10x10 field cannot be established, such as on a tomotherapy unit. Used when field is not considered small.
How are TPR 20,10 (10) and TPR 20,10 (s) related?
TPR 20,10 (10) is a function of TPR 20,10 (s), d1 and d2 (which are constants), and S where s = EqSq (4*area/perimeter)
Difference with local and NPL beam spectrum
NPL graphs produced using aeam with a flattening filter, wheras FFF beams have different spectra, must correct for this
When might daisy chaining be necessary?
When detector has a strong energy dependence and the spectrum is sufficiently different between the reference and small clinical field
How is daisy chaining done?
Small field detector used in small field and intermediate field
Cross calibrated ion chamber used in both intermediate and reference field
Why are audits done?
As an independent verification of dosimetry by an external group with independent equipment.
Name some groups that do audits
IAEA
ESTRO
NPL
Regional groups and local groups
How are audits done?
Tell the centre how to expose the detector and then it is done using auditor’s calibrated chamber and electrometer (NPL, regional, interdepartmental)
Dose with anthropomortphic phantoms can be compared with results from TPS
