Radiotherapy Standards Users Group meeting NPL, 5 th 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 Practice 2. The UK Primary Standard Therapy Level Electron Calorimeter 3. Redesign of calorimeter body and electronics 4. Thermal modelling and new analysis procedure 5. 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 3009 Up to 2005 14.4 Calibration factor, Gy/C x 10 7 14.2 14.0 13.8 13.6 Jan-96 Apr-96 Oct-96 Apr-97 Feb-98 Feb-99 Sep-03 Aug-04 Mean pre-2005 13.4 13.2 0 1 2 3 4 5 6 7 R 50, cm
Calibration of working standard NACP-02 chamber - 2005 NPL NACP-02 Chamber 3009 including 2005 data (2005 data excluded from mean) Calibration factor, Gy/C x 10 7 14.4 14.2 14.0 13.8 13.6 Jan-96 Apr-96 Oct-96 Apr-97 Feb-98 Feb-99 Sep-03 Aug-04 Mean pre-2005 Sep-05 13.4 13.2 0 1 2 3 4 5 6 7 R 50, cm
Calorimeter/beam monitor ion chamber response Calorimeter/monitor ratios for data 2003-2005 (normalised against 2006/7) 1.15 1.10 1.05 Uncertainties much larger in 2004 and 2005 compared with 2003 MathCad polynomials 2003 Gy/microcoulomb 1.00 0.95 MathCad polynomials 2004 MatLab polynomials 2005 0.90 0.85 0.80 0 1 2 3 4 5 6 7 8 Beam quality: R50_D, cm
Electron calorimeter general layout Absorber: 2mm thick, 5cm diameter graphite disc. 22kΩ NTC thermistors embedded within disc to measure temperature.
Principle of operation 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 Temperature ( o C) 27.9615 27.9610 27.9605 27.9600 Temperature rise A B Linear extrapolation of curve to irradiation mid-point C Suitable curve fit on CD D 27.9595 27.9590 0 50 100 150 200 Linear fit on AB time (s)
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 Insulating covers Al beam shield for cables Calorimeter behind Calorimeter and ion chamber phantom in fixed geometry Ion chamber phantom
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 20.8205 20.8200 20.8195 Temperature, deg C 20.8190 20.8185 20.8180 20.8175 0 50 100 150 200 250 300 Time, s
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 3.2 3.0 2.8 3.5 3.0 2.5 Temperature curves for core (dark blue, solid), jackets (pastel, solid) and buildup/backscatter plates Simulated core (dark blue) and jacket (pastel shades) temperatures with time (dashed) Simulated core (dark blue) and jacket (pastel shades) temperatures with time Only have measured temperature for core Temp rise, mk 2.6 Temp rise, rise, mk mk 2.0 2.0 1.5 1.5 2.4 2.2 2.0 1.0 1.0 0.5 0.5 Vital that core be in a stable thermal environment for any guess on thermal behaviour used in extrapolation to be valid 0.0 0.0 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 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 0 10 20 30 40 50 60 70 80 90 100 110 Time, 120 seconds 130 140 150 160 170 180 190 200 210 220 230 240 Time, seconds
New analysis technique MatLab script takes text output from calorimeter runs, fits polynomials and exponential-based functions as follows Quadratic polynomial, A + Bt + Ct 2 Cubic polynomial, A + Bt + Ct 2 + Dt 3 Quartic polynomial, A + Bt + Ct 2 + Dt 3 + Et 4 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 3.03E-03 3.02E-03 3.01E-03 Temperature rise, K 3.00E-03 2.99E-03 2.98E-03 Clearly expect quadratic to be of little value in analysis of real data, due to rapid trend to underestimation Double exponential Cubic expected to be valid in limited range Treble exponential Quadratic Quartic polynomial and the two exponential-based Cubic functions should be more Quartic successful 2.97E-03 80 120 160 200 240 280 Length of data used, seconds
Analysis: Results from real data 10 MeV, 15 Sept 06, Run 4 2.16E-03 ~0.3% 2.14E-03 Temperature rise, K 2.12E-03 2.10E-03 2.08E-03 2.06E-03 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% Double exponential All -0.9778 0.0018 Treble exponential 0.18% 0.0006 0.06% Quadratic Dble-Exp Trble-Exp Cubic Quartic Average Cubic Mean -0.9773-0.9776-0.9768-0.9777-0.9773 Quartic SD 0.0005 0.0008 0.0013 0.0017 0.0012 SD, % 0.054% 0.080% 0.13% 0.18% 0.12% 80 120 160 200 240 280 Length of data used, seconds
More real data 10 MeV, 15 Sept 06, Run 10 3.00E-03 ~0.3% 2.98E-03 Temperature rise, K 2.96E-03 2.94E-03 2.92E-03 2.90E-03 Double exponential Treble exponential Dble-Exp Trble-Exp Quadratic Cubic Cubic Quartic Average Mean -0.9790-0.9798-0.9783-0.9782-0.9787 SD 0.0006 Quartic 0.0006 0.0006 0.0007 0.0008 SD, % 0.060% 0.066% 0.057% 0.068% 0.087% 80 120 160 200 240 280 Length of data used, seconds
The real data 20.8205 20.8180 Temperature (deg C) 20.8175 20.8200 20.8170 20.8195 20.8165 20.8190 20.8160 20.8185 20.8155 20.8180 20.8150 Bad data! Good data. How can we tell? 20.8175 20.8145 0 50 100 150 200 250 300 Time (s)
Analysis: Results from bad data 10 MeV, 15 Sept 06, Run 3 2.20E-03 2.18E-03 ~4% Temperature rise, K 2.16E-03 2.14E-03 2.12E-03 Using a completely automated analysis Double exponential technique Treble can exponential give unreliable results. Quadratic Cubic Quartic 2.10E-03 80 120 160 200 240 280 Length of data used, seconds
Results: Refurbished and with new analysis method 1.15 Calorimeter/monitor ratios Sept 06 - Feb 07 with normalised 2003 data 1.10 Gy/microcoulomb 1.05 1.00 0.95 Double exponential Triple exponential Cubic polynomial Quartic polynomial Average 2006-07 MathCad polynomials 2003 0.90 0.85 NPL Linac installed 1975 0.80 0 1 2 3 4 5 6 7 8 Beam quality: R50_D, cm
New NPL Clinical Linac facility
Calibration of working standard NACP-02 chamber NPL NACP-02 Chamber 3009 Including 2006 and 2007 data (2005 data excluded) Calibration factor, Gy/C x 10 7 14.4 14.2 14.0 13.8 13.6 Jan-96 Apr-96 Oct-96 Apr-97 Feb-98 Feb-99 Sep-03 Aug-04 Mean pre-2005 Sep-06 Feb-07 Mean including 2006/7 13.4 13.2 0 1 2 3 4 5 6 7 R 50, cm
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