Calibration Chain

Question 1

What is basic outline of calibration chain at NPL?

Answer 1

1. Primary standard is graphite calorimeter, measures dose to graphite. Primary standard calibrates 3 NPL reference chambers in graphite phantom
2. NPL reference chamber calibrations converted from D_graphite to D_water
3. NPL calibrate hospital secondary standards against NPL reference chambers in water

Question 2

How is dose measured in graphite?

Answer 2

Don’t measure temperature rise, maintain temperature and substitute electrical heating for radiation heating
D = VIt/m

Question 3

Why do graphite calorimeters need to be thermally isolated?

Answer 3

Thermal diffusivity in graphite much larger than water, heat in graphite distribute itself in volume very quickly, too quickly to measure.

Question 4

Why do NPL use graphite?

Answer 4

Reduce uncertainties
Bigger temperature rise from graphite

Question 5

Why is graphite calorimeter not used to measure against secondary standards?

Answer 5

Too time consuming to use graphite calorimeter for everything. Reference chambers calibrated once a year against calorimeter.

Question 6

What method do NPL use to calibrate user secondary standard with reference chambers?

Answer 6

Substitution method

Question 7

What beams are used to calibrate graphite calorimeter against reference chambers?

Answer 7

60Co and a range of x-ray qualities between 4 and 25MV

Question 8

What correction factors are used to correct raw dose for the measured chamber reading?

Answer 8

k_elec
k_TP
k_h
k_ion
k_pol
k_vol

Question 9

What is k_elec correcting for?

Answer 9

Charge calibration coefficient for electrometer, which is calibrated by NPL separately, can be single factor or calculated by user

Question 10

What is k_TP correcting for?

Answer 10

Temperature and pressure, account for the difference in mass of air in the chamber active volume

Question 11

What is k_h correcting for?

Answer 11

Humidity correction. Can be taken as unity in 20-70% relative humidity, outside of this range NPL can advise

Question 12

What is k_ion correcting for?

Answer 12

Losses because of ion recombination at finite collection voltages. Correction essential in pulsed radiation beams and can be substantial in FFF.

Question 13

What is k_pol correcting for?

Answer 13

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.

Question 14

What is k_vol correcting for?

Answer 14

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.

Question 15

Which correction factors are usually unity

Answer 15

k_h
k_pol
k_vol

Question 16

How calculate dose to water from secondary standard?

Answer 16

D_w = M_Q . N SS _D,w,Q (QI)

Where M_Q is corrected reading

Question 17

What is calibration coefficient?

Answer 17

N SS _D,w,Q (QI)

Question 18

How many measurements are taken to calibrate field instrument against secondary standard?

Answer 18

Either: measure 3 times, swap, measure 3 times, swap, measure 3 times
OR: substitute each time, FFF beams

Question 19

How is calibration of field instrument against secondary standard done?

Answer 19

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

Question 20

What do NPL give hospital after calibration?

Answer 20

Dose to water factor for secondary standard chamber
D_w = M_q . N^SS _ D,w,Q (QI)

Question 21

What if your field can’t produce 10 x 10 field and thus can’t get TPR 20 10?

Answer 21

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.

Question 22

What is an MSR field and when is it used?

Answer 22

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.

Question 23

How are TPR 20,10 (10) and TPR 20,10 (s) related?

Answer 23

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)

Question 24

Difference with local and NPL beam spectrum

Answer 24

NPL graphs produced using aeam with a flattening filter, wheras FFF beams have different spectra, must correct for this

Question 25

When might daisy chaining be necessary?

Answer 25

When detector has a strong energy dependence and the spectrum is sufficiently different between the reference and small clinical field

Question 26

How is daisy chaining done?

Answer 26

Small field detector used in small field and intermediate field
Cross calibrated ion chamber used in both intermediate and reference field

Question 27

Why are audits done?

Answer 27

As an independent verification of dosimetry by an external group with independent equipment.

Question 28

Name some groups that do audits

Answer 28

IAEA
ESTRO
NPL
Regional groups and local groups

Question 29

How are audits done?

Answer 29

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