Therapy Educational Course - Impact of the National Institute of Standards and Technology (NIST) on Radiation Dosimetry in Medical Physics M Mitch1, M McEwen2, R Tosh1 (1) National Institute of Standards and Technology, Gaithersburg, MD (2) National Research Council, Ottawa, ON, CA Tuesday 9:00:00 AM - 9:55:00 AM TU-B-224-1 Room: 224 The Calibration Chain: Role of BIPM, PSDLs and ADCLs M. McEwen National Research Council Canada 2011 COMP/AAPM Joint Annual Meeting Calibration Chain PSDL (NIST or NRCC) Calibration laboratory (SSDL/ADCL) Clinical beam Dw ND,w cal lab Q ND,w clinic Q Dw clinic What is a PSDL? • Primary Standards Dosimetry Laboratory • The national laboratory designated by the government for the purpose of developing, maintaining and improving primary radiation standards • North America: NIST (Gaithersburg) or NRCC (Ottawa) • In some countries there is no PSDL Primary Standard • Instrument that allows the determination of a dosimetric quantity according to its definition • Preferably with a direct path to SI quantities not involved with ionizing radiation • SI base unit: meter, kilogram, second, ampere, kelvin, mole, and candela • SI derived units: J, Gy, etc. Primary Standard - Example Exposure X Q m V dQ dm Free air ionization chamber or cavity ionization chamber Measurement of current, time air Length, density Density = mass / length3 No ionizing radiation needed to determine the volume Green: SI base unit orange: SI derived unit Air Kerma Does require the use of ionizing radiation K air Q m V W 1 dQ Free air ionization chamber e 1 g dm or cavity ionization chamber Measurement of current, time air Length, density Density = mass / length3 W/e is not a fundamental constant but has “special” status with agreed value – 33.97 J/C The “Bureau International des Poids et Mesures” (BIPM) • BIPM = International laboratory created by the metre convention; has an Ionizing Radiation Division • Role: “The task of the BIPM is to ensure world-wide uniformity of measurements and their traceability to the International System of Units (SI).” • www.bipm.org • Through the BIPM intercomparison program the NMI can declare its calibration and measurement capabilities (CMCs) • Key comparisons and database (http://kcdb.bipm.org) • Mutual recognition arrangement (MRA) What is an SSDL? • Secondary Standards Dosimetry Laboratory • SSDL = Laboratory designated by a competent national authorities to provide the necessary link in traceability of radiation dosimetry to national/international standards for users within that country SSDL network IAEA has direct traceability to BIPM but linked to national PSDLs through regular comparison programs IAEA lab calibrates some SSDLs (others have direct traceability to BIPM) IAEA also operates TLD audit service for users What is an ADCL? • Accredited Dosimetry Calibration Laboratory (the SSDL equivalent in the US but different…) • Accredited by the AAPM • Provides calibrations to users for instruments and radioactive sources for dosimetry in radiotherapy and diagnostic imaging • ADCL network links the ADCLs to the NIST with oversight by the Calibration Laboratory Accreditation (CLA) sub-committee 11 The AAPM CLA subcommittee: what does it do? “The Subcommittee’s task is to accredit, supervise and maintain the highest level of confidence in the quality of the ADCL system, with sufficient capacity in the system to prevent undue delays in satisfying the membership’s calibration needs.” Main forum for discussion of issues relating to calibration of ion chambers and brachytherapy sources NIST has permanent membership of the CLA 12 The CLA: Develops criteria Accredits laboratories Carries out assessment visits Monitors performance Makes recommendations 13 Summary • Clinical dosimetry in NA is traceable to national standards through a chain connecting clinics to ADCLs and PSDLs (NIST and NRCC). • National standards are declared equivalent via key-comparisons organized through the BIPM • In the, ADCLs are monitored by the CLA subcommittee of the AAPM Thank You Standards in Radiation Dosimetry for Medical Physics Michael G. Mitch, Ph.D. Leader, Radiation Interactions and Dosimetry Group Ionizing Radiation Division Physical Measurement Laboratory National Institute of Standards and Technology (NIST) Materials Measurement Laboratory (MML) Physical Measurement Laboratory (PML) Engineering Laboratory (EL) Information Technology Laboratory (ITL) Center for Nanoscale Science and Technology (CNST) NIST Center for Neutron Research (NCNR) Ionizing Radiation Division, PML Radiation Interactions and Dosimetry Group Neutron Interactions and Dosimetry Group Radioactivity Group Radiation Interactions and Dosimetry Group Staff Group Leader (Supervisory Physicist) Group Secretary 7 Physicists 1 Research Chemist 1 Physical Scientist 2 Electronics Technicians Research Associates Radiation Interactions and Dosimetry Group Strategic Element Develop dosimetric standards for x rays, gamma rays, and electrons based on the SI unit, the gray, for homeland security, medical, radiation processing, and radiation protection applications. How do we know what to do? • Council on Ionizing Radiation Measurements and Standards (CIRMS) • Consultative Committee for Ionizing Radiation (CCRI) • National Academy of Sciences review panel • Feedback from colleagues and calibration customers • Membership in professional societies and committees How do we know what to do? AAPM committees and task groups with NIST members: Calibration Laboratory Accreditation Subcommittee (CLA) Therapy Physics Committee (TPC) Brachytherapy Subcommittee (BTSC) Low-Energy Brachytherapy Source Dosimetry Work Group (WGLEBS) TG-136 (Induced Radioactivity Produced by Radiotherapy Accelerators) TG-138 (Uncertainty in Brachytherapy Dosimetry) TG-144 (Y-90 Microsphere Brachytherapy) TG-145 (Quantitative PET/CT Imaging) TG-200 (CT Dosimetry Phantoms) Calibration and Measurement Capabilities (CMCs) Quantity Parameter Reference Standard Key Calibration Comparison Service? BIPM.RI(I)- Air Kerma x ray (10 to 50) mammography x ray (50 to 300) Cs-137 Co-60 free-air chamber free-air chamber free-air chamber graphite cavity chamber graphite cavity chamber K2 K7 K3 K5 K1 Yes Yes Yes Yes Yes Absorbed Dose to Water Co-60 MV x rays Sr-90/Y-90 water calorimeter water calorimeter extrapolation chamber K4 K6 Yes No Yes Air Kerma Strength Cs-137 brachy graphite cavity chamber Ir-192 brachy graphite cavity chamber HDR Ir-192 brachy I-125 brachy WAFAC Pd-103 brachy WAFAC Cs-131 brachy WAFAC K8 Yes Yes No Yes Yes Yes Free-Air Ionization Chamber (< 300 keV) 20 keV to 100 keV K air Qair W e airV C J x 33.97 kg C Primary Standard for Mammography X Rays (≤ 50 kV) Electrometer Mo, Rh anode x-ray tubes filters V Attix free-air chamber Cavity Ionization Chamber (> 300 keV) K air Qair W e airV 1 1 g S/ S/ graphite air ( / )air ( en / )graphite en Water Calorimetry (MV photons, electrons) Dwater c T Vessel with thermistors Vessel with an ion chamber Extrapolation Ionization Chamber (electrons) Electrometer 79.54 pA Collecting electrode Insulating gap Air gap=0.40 mm Source Ionization Current, pA Water-equivalent plastic High-voltage electrode/window Air gap, mm Extrapolation Ionization Chamber (electrons) D water 1 air A d I ( x) dx x 0 W e slope of current vs. air gap • Ophthalmic applicators 1. Planar (90Sr/Y) 2. Concave (106Ru/Rh) • IVB seed and line sources (90Sr/Y, 32P) ( S / ) water ( S / ) air Wide-Angle Free-Air Chamber (WAFAC) 160 mm Al Center Electrode Al Filter Electrometer W Aperture Rotating Source V/ 2 V = - 1674 V Aluminized Mylar Electrodes Wide-Angle Free-Air Chamber (WAFAC) 43 mm Al Center Electrode Al Filter Electrometer W Aperture Rotating Source V/ 2 V = - 450 V Aluminized Mylar Electrodes Original and Automated WAFACs HPGe Spectrometer Al filter wheel Automated WAFAC seed Original WAFAC seed Air-Kerma Strength from WAFAC Measurements SK K air (Q)d 2 W e d2 K dr ( K ) M det ( K , Q) air Veff Ki i K j (Q) j 125I Value Net current, M det ( K , Q) W /e Air density, ρair Aperture distance, d Effective chamber volume, Veff Decay correction, K1 Recombination, K dr (K ) Attenuation in filter, K3(Q) Air attenuation in WAFAC, K4(Q) Source-aperture attenuation, K5(Q) Inverse-square correction, K6 Humidity, K7(Q) In-chamber photon scatter, K8(Q) Source-holder scatter, K9 Electron loss, K10 Aperture penetration, K11(Q) External photon scatter, K12(Q) Combined standard uncertainty, uc Expanded uncertainty, V sI 0.06 33.97 J / C 1.196 mg / cm3 T1/2 = 59.43 d < 1.004 1.0295 1.0042 1.0125 1.0089 0.9982 0.9966 0.9985 1.0 0.9999 1.0 Type A (%) Type B (%) s 0.11 - 0.06 0.15 0.03 0.24 0.01 0.02 0.05 0.61 0.08 0.24 0.01 0.07 0.07 0.05 0.05 0.02 0.17 (s2 + 0.7622)1/2 2uc Monte Carlo Simulations • Photon and electron source modeling • Detector response calculations • Ionization chamber correction factors • Stopping power ratios S/ S/ k wall graphite air • Mass-energy absorption coefficient ratios ( / )air ( en / )graphite en Photon and Charged-Particle Data Center 1.0E+04 104 1.0E+03 103 Water Water 1.0E+03 103 2 1.0E+02 10 2 1.0E+02 10 1 10 1.0E+01 2 (MeV cm / g) 2 (cm / g) 1 1.0E+01 10 0 Collisional S/ / 1.0E+00 10 Total -1 1.0E-01 10 0 1.0E+00 10 Total -2 1.0E-02 10 Incoherent Photoelectric -1 10 1.0E-01 Radiative Pair -3 1.0E-03 10 -4 -2 1.0E-04 10 1.0E-03 10-3 1.0E-02 10-2 1.0E-01 10-1 1.0E+00 100 1.0E+01 101 1.0E+02 102 1.0E-02 10 1.0E-02 10-2 1.0E-01 10-1 1.0E+00 100 1.0E+01 101 Electron Energy (MeV) Photon Energy (MeV) pe coh incoh pair uA trip ph.n. S S col S rad 1.0E+02 102 1.0E+03 103 Web-based Photon and Electron Databases XCOM: Photon Cross Sections Database http://www.nist.gov/pml/data/xcom/index.cfm http://www.nist.gov/pml/data/photon_cs/index.cfm http://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html “Accurate MCPT-aided brachytherapy dosimetry would not be possible without the evolution of accurate photon cross-section libraries made over the last 30 years (Hubbell 1999), an effort led by [the late] John Hubbell of NIST.” Chapter 14, Thermoluminescent Detector and Monte Carlo Techniques for Reference-Quality Brachytherapy Dosimetry by J. F. Williamson and M. J. Rivard, in Clinical Dosimetry Measurements in Radiotherapy, D.W. O. Rogers, Joanna E. Cygler, Editors, Proceedings of the American Association of Physicists in Medicine Summer School, Colorado College, Colorado Springs, Colorado, June 21–25, 2009, p. 471. CLA Criteria – Traceability to NIST The only type of laboratory accredited by the AAPM is a secondary standard laboratory with the capability of providing direct traceability to the National Institute of Standards and Technology (NIST). Such a laboratory is referred to as an Accredited Dosimetry Calibration Laboratory (ADCL). A3.6.1.6 Calibration traceability to NIST dosimetry standards shall be maintained by participation in NIST measurement quality assurance tests and in ADCL intercomparisons at intervals prescribed by the Subcommittee. CLA Criteria – Equipment Calibrations A3.6.1.2 Each reference class ionization chamber, which serves as the laboratory’s standard for accredited beam qualities, shall be calibrated by NIST. The laboratory must have standard ionization chambers calibrated at beam qualities sufficient to cover the laboratory’s accredited beam qualities. A1.5.1.11.2 At least one of the laboratory standard voltmeters and one of the capacitors used for charge measurement, shall be calibrated at least biennially at another facility. Alternatively, an electrometer with a precision and stability of 0.1% or better may be calibrated biennially by NIST. These calibrations shall be documented as traceable to NIST. CLA Criteria – Equipment Calibrations 5.5.13 The laboratory shall have at least two high-quality barometers (resolution of 0.5 mm Hg or better) and two high-quality thermometers (resolution of 0.1 C or better). At least one barometer and one thermometer shall have a calibration documented as traceable to NIST. 5.5.15 The laboratory shall have a device to measure relative humidity (RH). The device shall have a calibration traceable to NIST with an uncertainty of +/- 7% RH or better. CLA Criteria – Brachytherapy Source and Chamber Calibrations A2.4.1 Standard Source Traceability: The ADCL shall obtain all model-specific calibrations of standard sources used as reference standards for calibrations directly from NIST. A7.5.2 For chamber calibrations the laboratory shall have at least one sealed source of each radionuclide, manufacturer, model and encapsulation for which calibration will be offered…This source shall have direct traceability to NIST. Measurement Traceability for Brachytherapy Sources – New Source SK ADCL 5 sources Manufacturer Clinic Measurement Traceability for Brachytherapy Sources – New Source 3 sources ADCL1 ADCL2 ADCL3 SK secondary standard (SK / I)0 2 sources Manufacturer SK / I ? ADCL calibration date Clinic Measurement Traceability for Brachytherapy Sources - Clinics ADCL Manufacturer well-ionization chambers (SK / I)ADCL Clinic Measurement Traceability for Brachytherapy Sources - Clinics ADCL Manufacturer verification for treatment planning (SK / I)ADCL Clinic sources (SKM) SKClinic Measurement Traceability for Brachytherapy Sources – Annual QA 3 sources SK 3 sources Manufacturer ADCL1 ADCL2 ADCL3 3 sources (SK / I)t 2.00 % vs. (SK / I)0 Clinic Measurement Traceability for Brachytherapy Sources sources ADCL well-ionization chambers SK secondary standard sources Manufacturer verification for treatment planning Clinic sources SKClinic Clinical Brachytherapy Source Measurements Well-ionization chambers, calibrated by an ADCL S KClinic I Clinic SK I ADCL AAPM Clinical Protocols TG-43U1 (Brachytherapy Dosimetry) “For experimental measurement of absolute dose rates to water, at least one source should have direct traceability of SK to the 1999 NIST WAFAC calibration standard.” “For calibrating radioactive sources, the AAPM has previously recommended that users not rely on the manufacturer’s calibrations, but instead confirm the accuracy of source strength certificates themselves by making independent measurements of source-strength that are secondarily traceable to the primary standard maintained at NIST.” TG-51 (High-Energy Photon and Electron Dosimetry) “The primary purpose of this dosimetry protocol is to ensure uniformity of reference dosimetry in external beam radiation therapy with high-energy photons and electrons. To achieve this goal requires a common starting point and this is accomplished by starting with an ion chamber calibration factor which is directly traceable to national standards of absorbed dose to water maintained by Primary Standards Laboratories (National Institute of Standards and Technology, NIST, in the US, the National Research Council of Canada, NRCC, in Canada). Direct traceability is also achieved via calibration factors obtained from an Accredited Dosimetry Calibration Laboratory (ADCL).” TG-61 (40-300 kV X-Ray Beam Dosimetry) “Calibration factors NK shall be traceable to national standards, i.e., from an ADCL, NIST or NRCC, preferably for a number of x-ray beam qualities.” Current Research Areas in Medical Dosimetry at NIST • Small-field therapy dosimetry using alanine/EPR • Water calorimetry studies at room temperature and 4 oC (60Co and MV x rays) • Air kerma standard for electronic brachytherapy • CT dosimetry (ion chamber and phantom tests) • New reference standard beams for digital mammography • 3D dosimetry through ultrasonic tomography Electronic Brachytherapy Calibration Facility 1.5 kV x-ray source leaded glass shield Lamperti free-air chamber HPGe spectrometer 50 cm I Shield, free-air chamber, and spectrometer rotate around source “Although radiation calorimetry for the measurement of absorbed dose has a long history going back to the 1920s, the modern era of primary standard devices can be traced back to the graphite calorimeter designed by Domen and Lamperti (1974) at the U.S. National Bureau of Standards (now NIST). More than three decades later, this design, in modified forms, remains the primary standard for 60Co and MV photon beams at several national laboratories…” Chapter 15, Primary Standards of Air Kerma for 60Co and X-Rays and Absorbed Dose in Photon and Electron Beams by M. McEwen, in Clinical Dosimetry Measurements in Radiotherapy, D.W. O. Rogers, Joanna E. Cygler, Editors, Proceedings of the American Association of Physicists in Medicine Summer School, Colorado College, Colorado Springs, Colorado, June 21–25, 2009, p. 523. Radiation Interactions and Dosimetry Group Staff Dr. Fred Bateman, Physicist Dr. Paul Bergstrom, Physicist Diana Copeland, Secretary Dr. Marc Desrosiers, Research Chemist Dave Eardley, Electronics Technician Dr. Larry Hudson, Physicist Mel McClelland, Electronics Technician Dr. Ronnie Minniti, Physicist Michelle O’Brien, Physicist Jim Puhl, Physical Scientist Dr. Ron Tosh, Physicist Jason Walia, Physicist Water Calorimetry at NIST: Research and Applications Ronald E. Tosh, Ph.D. National Institute of Standards and Technology Physical Measurement Laboratory Ionizing Radiation Division 2011 Joint AAPM/COMP Meeting Aug 2, 2010 Water Calorimetry at NIST Source Timing Control On/Off LIA/PSD 60Co source, ~ 10 k Ci (~ 1 Gy/min at 1 m) Insulation: foam wood aluminum Height adjustment •Water phantom enclosed in a tank of (30 x 30 x 30) cm3 made with PMMA •Entrance window thinned to 3.5 mm for an area of 12 cm x 12 cm (radiation field 10 cm x 10 cm) Sealed glass core blown from Pyrex tubing •ID < 35 mm •wall thickness < 0.3 mm at the center •fitted with two threaded Teflon (PTFE) plugs Experimental uncertainties Heat defect ? ~ 1 Gy/min Heat transport … V(t) ~2 V ~ 0.239 mK t ~ 60 sec Heat Defect Causes: • Chemical reactions involving products of incident radiation and various dissolved species within the water Effect on signals: • Transient – can be huge (~100%). • Steady state – depends on dissolved species (0 to few %). Remedy: • H2 – saturated, high-purity water in a sealed glass vessel. N.V. Klassen and Carl K. Ross, J. Res. Natl. Inst. Stand. Techol. 107, 171-178 (2002). Heat Transport Causes: • Dose non-uniformities • Changes in heat capacity at material interfaces Principal mechanisms: • Conduction (linear) • Convection (nonlinear) Effects on signals: • Distortion throughout entire waveform. • Errors amplified by extrapolation procedures for extracting T. Remedies: • Restrict experimental runs to a few exposures at a time ~0.2% for conduction. • Operate calorimeter at 3.98 oC 0% error for convection. Heat Transport cont’d Questions regarding existing remedies: • Would the system exhibit a stable, steady-state behavior? Interruptions to reestablish thermal equilibrium might not be necessary. • If so, to what extent would convection contribute? Possibility to operate at room temperature instead of 3.98 oC. 10 y = sqrt(m1^ 2+m2 ^ 2 /m0^4) 9 10 -5 1 8 10 7 10-5 6 10 -5 0.003536 5 2.5438e-07 m2 0.000190 95 Chi sq 0.000108 9 NA R 0.99999 NA 0.1 0.01 5 10-5 0.001 4 10 -5 0 2000 4000 6000 8000 1 10 4 0.001 0.01 ~ time (sec) Error 0.005988 6 -5 FFT amplitude ~ Temperature (K) Val ue m1 0.1 f (Hz) 1 Heat Transport cont’d Variation of “apparent” dose rate with shutter freq/period demonstrates agreement with heat equation (i.e. conduction only) except at higher shutter periods. Measured dose rate (Gy/min) steady-state operation at room temperature appears feasible at shorter exposure periods. 1.20 0.98 0.96 0.94 0.92 0.90 0.88 Oct-09 1.15 1.10 May-Jul-2009 1.05 finite element simulation 1.00 30 0.95 130 0.90 230 Vessel removed 0.85 0.80 10 100 1000 Shutter on time (s) 10000 330 Heat Transport cont’d 2 t tu 2 u D g ( x ) f (t ) uu cv p g zˆ o zˆ even harmonics? Application Note: chamber calibration Bilateral comparison with BIPM: • BIPM: primary standard graphite calorimeter • NIST: A12 reference ionization chamber – “pre-calibrated” in 60Co – Directly calibrated in 6- and 18-MV xrays using room-temperature, sealed water calorimeter Calibration runs in Clinac beams - “run” = 20 to 30 cycles of 1 minute on/off - A12 and calorimeter runs interleaved • 6 MV – 14 runs • 18 MV – 8 runs • Convection test: vary dose-rate and look for nonlinearity in calorimeter response. App Note cont’d: chamber calibration 2.00 calorimeter Comparison of calorimeter and scaled A12 (TG-51) measurements: • Agreement within uncertainties at both beam qualities. • Results shown are provisional, as we are determining revised uncertainties on heat defect and beam perturbation estimates (Monte Carlo). chamber Gy 1.90 18 MV 1.909 +/1.912 +/- 0.0022 0.0022 1.907 +/- 0.0014 6 MV 1.617+/-0.0021 1.615 +/- 0.0021 1.617 +/- 0.0025 1.80 1.70 1.60 0 5 10 15 20 25 Calorimeter runs/chamber runs/chamber runs Calcorimeter runs Test of calorimeter linearity over ~5x change in dose rate for an 18 MV beam. • Clinac allows 5 discrete dose-rate levels (calibration runs were done at middle value). • Slope of fit: (7.999e-3 ± 0.02e-3) Gy/MU Expected: 8 mGy/MU • Suggests that convection is negligible (even at these elevated dose rates). MU “Everything becomes a project”… • • • • • 60Co high energy x-rays electron beams proton beams HDR brachytherapy • • • Each one of these… requires …redetermining each one of these or Heat defect glass Excess heat vessel Heat transport Perturbation Field non-uniformity or … What if we could measure this? After all, water is a sort of 3D imaging medium for dose (if we could just figure out how to extract the information). Ultrasonic Thermometry – 1D Co-60 Co-60 Shutter Frequency Counter PPLL Malyarenko E, Heyman J, Chen-Mayer H and Tosh R, “High Resolution Ultrasonic Thermometer for Radiation Dosimetry,” J. Acoust. Soc. Am., 124 3481-90 (2008) UST 1D cont’d Co-60 First use of ultrasonic thermometry to detect absorbed dose to water… UST 1D – cont’d Co-60 No convection barriers convection… simple behavior suggests that its effects may be readily deconvolved. UST 2D Eugene V. Malyarenko, Joseph S. Heyman, H. Heather Chen-Mayer and Ronald E. Tosh, “Time-resolved Radiation Beam Profiles in Water Obtained by Ultrasonic Tomography,” Metrologia 47 3 (2010) UST 2D cont’d UST 2D cont’d Spring 2008 Spring 2011 Summer 2011 ???????? Water Calorimetry at NIST 1994-present Summer 2011 Spring 2008 Spring 2011 ????????