Radiotherapy Standards Users’ Group meeting NPL, 5th June 2007

Improvements to the UK Primary Standard Therapy Level Electron Beam Calorimeter

Mark Bailey, G A Bass, N D Lee, G A Mapp

Contents

1. History – the Codes of Practice2. The UK Primary Standard Therapy Level

Electron Calorimeter3. Redesign of calorimeter body and electronics4. Thermal modelling and new analysis procedure5. Results obtained

6. The UK Therapy Level Primary Standard Absorbed Dose Calorimeter – The Next Generation

History – Codes of Practice, calorimeter development

• Dosimetry in UK based on absorbed dose to water using graphite calorimeters, in Codes of Practice dating from 1990 (photons) and 2003 (electrons)

• 2003 IPEM Electron therapy-level dosimetry Code of Practice published, based on calibrations using the primary standard therapy-level electron beam graphite calorimeter

• 2005: Problems with noise, interference, and ageing materials and electronics on the electron calorimeter made it impossible to carry out a good series of measurements. Refurbishment necessary

• 2006: Improvements made to housing, electronics, analysis techniques for the electron calorimeter to reduce uncertainties

• 2007: Experience from this fed back into design for next generation primary standard graphite calorimeter at NPL

Calibration of working standard NACP-02 chamber – up to 2004

NPL NACP-02 Chamber 3009Up to 2005

13.2

13.4

13.6

13.8

14.0

14.2

14.4

0 1 2 3 4 5 6 7

R50, cm

Cal

ibra

tion

fact

or, G

y/C

x 1

07

Jan-96

Apr-96

Oct-96

Apr-97

Feb-98

Feb-99

Sep-03

Aug-04

Mean pre-2005

Calibration of working standard NACP-02 chamber - 2005

NPL NACP-02 Chamber 3009including 2005 data

(2005 data excluded from mean)

13.2

13.4

13.6

13.8

14.0

14.2

14.4

0 1 2 3 4 5 6 7

R50, cm

Cal

ibra

tion

fact

or, G

y/C

x 1

07

Jan-96

Apr-96

Oct-96

Apr-97

Feb-98

Feb-99

Sep-03

Aug-04

Mean pre-2005

Sep-05

Calorimeter/beam monitor ion chamber response

Calorimeter/monitor ratios for data 2003 - 2005 (normalised against 2006/7)

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

0 1 2 3 4 5 6 7 8

Beam quality: R50_D, cm

Gy/

mic

roco

ulom

b

MathCad polynomials 2003

MathCad polynomials 2004

MatLab polynomials 2005

Uncertainties much larger in 2004 and 2005 compared with 2003

Electron calorimeter – general layout

Absorber: 2mm thick, 5cm diameter graphite disc.

22kΩ NTC thermistors embedded within disc to measure temperature.

Principle of operation

time (s)

0 50 100 150 200

Tem

pera

ture

(o C)

27.9590

27.9595

27.9600

27.9605

27.9610

27.9615

A B

CD

• Quasi-adiabatic measurement of temperature rise during irradiation

• Requires good temperature calibrations and good knowledge of heat capacity of absorber, and very good resistance to noise

Linear extrapolation of curve to irradiation mid-point

Temperaturerise

Temperaturerise

Linear fit on AB

Suitable curve fit on CD

Calorimeter up to 2005

Thermistor wires

New physical arrangements

• Original core supports had been glued-on beads of expanded polystyrene– Imprecise location– Aged due to accumulated dose

• New supports: Sprung, thin polystyrene strips– Hold core in precise location– No glue involved, minimises

impurity uncertainties– Contact area with core actually

less than with original supports

New physical arrangements (continued)

• New mountings for calorimeter core and chamber phantom– Ensures measurement

depths correctly set– Ensures cables properly

protected– Cables well-wrapped and

sent to back from beneath calorimeter body

Calorimeter

Ion chamber phantom

Calorimeter and ion

chamber phantom in

fixed geometry

Insulating covers

Al beam shield for

cables behind

New electronics

• New Keithley 2182A nanovoltmeters on Wheatstone bridges– Low noise

• New Wheatstone bridges with Vishay sealed high-precision resistors, with built-in preamp stages for input to nanovoltmeters– Higher signal-to-noise ratio than previously

• Care taken with thermistor cables including:– Taping all sharp edges inside graphite assembly– Protecting thermistor cables in screened insulating sleeves– Using twisted-pair screened cables back to control room– New signal earth arrangements, careful avoidance of earth loops etc.

A real run with the new setup

10 MeV Run 4 15 Sept 2006

20.8175

20.8180

20.8185

20.8190

20.8195

20.8200

20.8205

0 50 100 150 200 250 300

Time, s

Tem

pera

ture

, deg

C

Analysis techniques

• Historically, MathCAD and then MatLab used to fit polynomial functions on interval CD after irradiation, which are then extrapolated back linearly (which allows correctly for increase of heat flow from zero during the irradiation) to irradiation midpoint– Drawback: Fixed intervals and weightings, mean no account taken of

goodness of data (…fine if data are good!)– May contribute significantly to uncertainty

• New: MatLab used to provide fits based on polynomials and sums of exponential functions, then Excel used to analyse output and select good data. Functions chosen based on results from simple thermal modelling.– Criterion: Convergence in extrapolation to mid-time of temperature rise

• Advantages: – Inclusion of exponential functions, based on the physics of heat transfer– Can exercise control over data included in final estimate– Reduced statistical contribution to uncertainty

Analysis: Thermal modelling

• Thermal modelling assumptions:– Thermal conductivity of graphite much greater than that of air– Thermal behaviour dominated by heat transfer between components

• Model developed using Excel to simulate core behaviour– Using Excel allows easy change of parameters such as component size, initial

conditions, air gaps between buildup plates etc.– Model used to check behaviour of analysis methods

• The great disadvantage of course, is that the analysis process involves an extrapolation!

• Therefore:– Expect polynomial-based solutions particularly, to have a significant

uncertainty– Motivation to use a more physics-based approach as far as possible– Motivated to use a variety of functions to avoid model-dependency in the final

result

Model behaviour

Simulated core (dark blue) and jacket (pastel shades) temperatures with time

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240

Time, seconds

Tem

p ris

e, m

K

Simulated core (dark blue) and jacket (pastel shades) temperatures with time

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240Time, seconds

Tem

p ris

e, m

K

Temperature curves for core (dark blue, solid), jackets (pastel, solid) and buildup/backscatter plates (dashed)

2.0

2.2

2.4

2.6

2.8

3.0

3.2

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240

Time, seconds

Tem

p ris

e, m

K

Only have measured temperature for core

Vital that core be in a stable thermal environment for any guess on thermal behaviour used in extrapolation to be valid

New analysis technique

• MatLab script takes text output from calorimeter runs, fits polynomials and exponential-based functions as follows– Quadratic polynomial, A + Bt + Ct2

– Cubic polynomial, A + Bt + Ct2 + Dt3

– Quartic polynomial, A + Bt + Ct2 + Dt3 + Et4

– Function based on two exponentials, Ae-Bt + C(1 – e-Dt), with pre-heat linear fit subtracted

– Function based on three exponentials, Ae-Bt + Ce-Dt + E(1 – e-Ft), with pre-heat linear fit subtracted

• Linear extrapolations of temperature rise at irradiation mid-time calculated

• Excel then used to examine and analyse the results from these fits, giving final value of absorbed dose to monitor ion chamber ratio with estimate of uncertainty

Analysis: Results from simulated data

10 MeV, simulation

2.97E-03

2.98E-03

2.99E-03

3.00E-03

3.01E-03

3.02E-03

3.03E-03

80 120 160 200 240 280

Length of data used, seconds

Tem

pera

ture

rise

, K

Double exponentialTreble exponentialQuadraticCubicQuartic

Clearly expect quadratic to be of little value in analysis of real data, due to rapid trend to underestimation

Cubic expected to be valid in limited range

Quartic polynomial and the two exponential-based functions should be more successful

Analysis: Results from real data

10 MeV, 15 Sept 06, Run 4

2.06E-03

2.08E-03

2.10E-03

2.12E-03

2.14E-03

2.16E-03

80 120 160 200 240 280

Length of data used, seconds

Tem

pera

ture

rise

, K

Double exponentialTreble exponentialQuadraticCubicQuartic

~0.3%

Dble-Exp Trble-Exp Cubic Quartic AverageMean -0.9773 -0.9776 -0.9768 -0.9777 -0.9773SD 0.0005 0.0008 0.0013 0.0017 0.0012SD, % 0.054% 0.080% 0.13% 0.18% 0.12%

Weighted: Mean SD % SEOM % Dble-Exp -0.9775 0.0015 0.15% 0.0005 0.05% Trble-Exp -0.9793 0.0016 0.16% 0.0006 0.06% Cubic -0.9771 0.0017 0.17% 0.0006 0.06% Quartic -0.9785 0.0015 0.15% 0.0006 0.06%

All -0.9778 0.0018 0.18% 0.0006 0.06%

More real data

10 MeV, 15 Sept 06, Run 10

2.90E-03

2.92E-03

2.94E-03

2.96E-03

2.98E-03

3.00E-03

80 120 160 200 240 280

Length of data used, seconds

Tem

pera

ture

rise

, K

Double exponentialTreble exponentialQuadraticCubicQuartic

~0.3%

Dble-Exp Trble-Exp Cubic Quartic AverageMean -0.9790 -0.9798 -0.9783 -0.9782 -0.9787SD 0.0006 0.0006 0.0006 0.0007 0.0008SD, % 0.060% 0.066% 0.057% 0.068% 0.087%

The real data

20.8175

20.8180

20.8185

20.8190

20.8195

20.8200

20.8205

0 50 100 150 200 250 300

Time (s)

Tem

pera

ture

(deg

C)

Good data.

20.8145

20.8150

20.8155

20.8160

20.8165

20.8170

20.8175

20.8180

0 50 100 150 200 250 300

Time (s)

Tem

pera

ture

(deg

C)

Bad data!

How can we tell?

Analysis: Results from bad data

10 MeV, 15 Sept 06, Run 3

2.10E-03

2.12E-03

2.14E-03

2.16E-03

2.18E-03

2.20E-03

80 120 160 200 240 280

Length of data used, seconds

Tem

pera

ture

rise

, K

Double exponentialTreble exponentialQuadraticCubicQuartic

…Using a completely automated analysis technique can give unreliable results.

~4%

Results: Refurbished and with new analysis method

Calorimeter/monitor ratios Sept 06 - Feb 07 with normalised 2003 data

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

0 1 2 3 4 5 6 7 8

Beam quality: R50_D, cm

Gy/

mic

roco

ulom

b

Double exponentialTriple exponentialCubic polynomialQuartic polynomialAverage 2006-07MathCad polynomials 2003

NPL Linac installed 1975…

New NPL Clinical Linac facility

Calibration of working standard NACP-02 chamber

NPL NACP-02 Chamber 3009Including 2006 and 2007 data

(2005 data excluded)

13.2

13.4

13.6

13.8

14.0

14.2

14.4

0 1 2 3 4 5 6 7

R50, cm

Cal

ibra

tion

fact

or, G

y/C

x 1

07

Jan-96Apr-96Oct-96Apr-97Feb-98Feb-99Sep-03Aug-04Mean pre-2005Sep-06Feb-07Mean including 2006/7

Conclusions

• The UK Primary Standard Electron Beam Therapy Level graphite calorimeter has been refurbished and improved

• Now delivers a smaller, measurable uncertainty in the calibration of NPL’s transfer standard ionisation chambers

• Core support system and new electronics, are being adapted for inclusion in the new design for a replacement graphite calorimeter to be the UK Primary Standard for electron and photon beams

Replacement electron/photon calorimeter

• NPL Currently building a new graphite calorimeter to replace the ageing photon microcalorimeter and electron calorimeter

• Designed to operate chiefly at therapy levels• Incorporating design ideas from LNHB’s GR8 and GR9 calorimeters as well as

improvements in electronics from the current electron calorimeter• Gap calculation techniques using EGSnrc user codes, on the UD Grid running on

the NPL PC network, have been successfully trialled for the electron calorimeter and will be used for the new Primary Standard calorimeter (see Poster by E Kirby, M Bailey, D Shipley)

• Core, first jacket (“shield”) and second jacket (“mantle”) will be operated at constant temperature, with DC or switched DC Wheatstone bridge networks maintained close to balance– Experience with AC bridges and control systems strongly influenced this

decision!– Measurement of absorbed dose by electrical substitution, using thermistors

throughout inner components as sensors and heaters

New calorimeter illustrations