Calibrating the Calibrating the Output of a Linear Output of a - PowerPoint PPT Presentation

Calibrating the Calibrating the Output of a Linear Output of a Linear Output of a Linear Output of a Linear Accelerator – TG-5 1 Accelerator – TG-5 1 Updated Updated Updated Updated Malcolm McEwen Malcolm McEwen Ionizing Radiation Standards Ionizing Radiation Standards g g Institute for National Measurement Standards Institute for National Measurement Standards National Research Council, Canada National Research Council, Canada COMP/AAPM Joint Meeting, COMP/AAPM Joint Meeting, Vancouver, 2011 Vancouver, 2011

Background 1. Reference dosimetry for linac beams based on a 60 Co calibration. 2. Simple step-by-step procedure. 3 3. Covers photon and electron Covers photon and electron beams. 4. Widely adopted in NA 2

TG-5 1 rem inder TG-51 is a procedure to give you a measurement of the  absorbed dose to water at a point in a water phantom It’s based on measurements with a calibrated ion chamber:  ,  60 Co Co D D N N k k M M w Q D , w Q ion N D,w is obtained from an ADCL or primary standards N D is obtained from an ADCL or primary standards   laboratory (e.g., NRCC in Canada) k Q is the factor that converts from the calibration beam ( 60 Co) 60  to the uses linac beam, defined by beam quality Q Q can represent a photon or electron beam 

W hy update? Working Group review recommends: Photons : Photons :   i. An updated list of chambers ii. Review of calculated k Q factors iii. Uncertainty analysis i iv. I Implementation l t ti guidance notes (clarification)  Electrons: More widespread revision required required

Part 1 - photon addendum The report will cover the following: A. k Q factors for new chambers B. Recommendations for implementation C. Uncertainty analysis for implementation of TG-51 D D. Comparison of measured and calculated k Q factors Comparison of measured and calculated k Q factors

TG-5 1 photons – w hat stays? Procedure remains unchanged  Continue to follow the procedure in the TG-51 document  TG-51 remains based on a calibration coefficient obtained in Co-60   MV standards and calibration services are already available in certain countries but widespread dissemination in the US is not realistic at the countries but widespread dissemination in the US is not realistic at the present time. Calculated k Q factors Calculated k Q factors   Measured k Q data are available for some chamber types  MV calibration services cannot meet demand in North America  %dd(10) x remains the beam quality specifier  See discussion later 

B. Recom m endations 1. Implementation of TG-51 Addendum 2 2. k Q data sets k data sets 3. Reference-class ionization chamber 4. Choice of polarizing voltage 5. Measurement of polarity correction, P pol 6. Effective point of measurement 7. Use of small volume chambers in relative dosimetry 7 U f ll l h b i l ti d i t 8. Non-water phantoms 9. Application to flattening-filter-free linacs pp g

B.1 I m plem entation of Addendum  The addendum should be implemented!  Minor changes in experimental procedure  New equipment may be required  Development of uncertainty budget may take some time ti

B.2 k Q data sets 1. For chambers listed in both this addendum and the original TG-51 protocol, the k Q factors and the original TG 51 protocol, the k Q factors listed in the addendum should be used. 2. For chambers that are not listed in either the original TG-51 protocol or in this addendum the recommendations of Section XI of TG-51 should be followed.

B.3 Proposed Cham ber spec Specification developed for cylindrical (thimble) chambers p p y ( )  3 sub-types (NOTE: WGTG51 definitions) –  0.6 cm 3 reference chambers (e.g., NE2571, PR-06C) i. f h b ( ) 3 0.125 cm 3 scanning chambers (e.g., PTW31010, IBA CC13) ii. iii. 0.02 cm 3 micro chambers (e.g., Exradin A16, Pinpoint TM ) iii. 0.02 cm micro chambers (e.g., Exradin A16, Pinpoint )

Exam ples 0.125 cm 3 3 PTW31010 Scanning chamber 1010 0.01 cm 3 IBA CC01 Micro chamber CC01 CC01 Exradin A12 Exradin A12 0 6 cm 3 0.6 cm 3 ‘Farmer’ chamber A12 NE2577 0.25 cm 3 ‘Short Farmer’ NE2577

B.3 Proposed Cham ber spec Based on objective assessment of chamber performance Measurand Specification p Chamber settling Must be less than a 0.5 % change in reading from beam-on to stabilization P leak < 0.1 % of chamber reading P pol P pol < 0.4 % correction (0.996 < P pol < 1.004) 0.4 % correction (0.996 P pol 1.004) < 0.5 % maximum variation in P pol with energy (total range) P ion = P init +P gen P gen Correction must be linear with dose per pulse P init Initial recombination must be < 0.002 at 300 V init Correction follows Boag theory for chamber dimensions. Difference in initial recombination correction between opposite polarities < 0.1 % Chamber stability Chamber stability Must exhibit less than a 0.3 % change in calibration coefficient over Must exhibit less than a 0.3 % change in calibration coefficient over the typical recalibration period of 2 years

B.3 Cham ber spec Based on results in the literature we can state that at least the following meet this specification: NE2571 and NE2611 o PTW30010, PTW30012, PTW30013, PTW31013 o Exradin A12, A12S, A19, A18, A1SL o IBA FC65-G, FC65-P, FC23-C, CC25, CC13 o Capintec PR-06C o i) majority are 0.6 cm 3 ‘Farmer-type’ chambers ii) 5 scanning chambers, NO microchambers iii) A-150 chambers explicitly excluded

B.3 a Parallel-plate cham bers?  TG-51 did not recommend any parallel-plate chambers for photon beam dosimetry   Studies had indicated issues with the operation of such Studies had indicated issues with the operation of such chambers in Co-60 beams but little info on MV performance 1.01 1.00 for Co-60) � Larger chamber-to-chamber 0.99 variations than for cylindrical chambers (normalised to 1 0.98 � Polarity correction larger than 0.97 for cylindrical chambers and more NACP #1 0.96 variable variable NACP #1 k Q NACP #2 NACP #3 0.95 Quadratic fit to data 0.94 0.55 0.60 0.65 0.70 0.75 0.80 TPR 20,10 Reference: McEwen, Duane and Thomas, IAEA Symposium, Nov 2002

B.3 a Parallel-plate cham bers  There is nothing inherently wrong with the parallel- plate configuration   More recent measurements indicate better More recent measurements indicate better performance – but still not as good as cylindrical chambers 1.00 1 00 PPC05 0.5 % PPC05-83, 25 MV 0.99 4 MV 0.0 % I monitor 0.98 ive variation of I PPC05 / 8 MV -0.5 % 0.97 k Q 0.96 -1.0 % 0.95 0.95 25 MV 5 V relati -1.5 % 0.94 U = +100 V U = -100 V -2.0 % 0.93 0 5 10 15 20 25 30 35 40 45 50 55 min 83 111 156 157 158 159 160 161 474 475 Time since beginning of measurement Serial number of chamber See also Poster SU-E-T-103 Reference: Kapsch And Gomola, IAEA Symposium, Nov 2010

B.4 Polarizing voltage   M M M M P P P P P P l P P , corr w raw TP ion pol elec l  Recombination correction directly affects measurement of absorbed dose  Recombination correction well established but not always straightforward b t t l t i htf d  2-voltage technique as set out in TG-51 applicable only to chambers exhibiting ideal behaviour behaviour  Many examples in literature of anomalous behaviour

B.4 Polarizing voltage 0.30 References: DeBlois et al (Med. Phys., 2000), McEwen (Med. Phys., 2010) 0.25 0.20 n /D pp ) Slope(P ion 0.15 0.10 "good" chambers Linear fit CC08 0.05 CC13 CC04 0.00 2 4 6 8 10 12 14 16 equivalent electrode separation 2 (mm 2 )

B.4 Polarizing voltage Based on results in the literature we can state the following:  Not all chambers follow standard ‘Boag’ theory  Manufacturers’ statements on voltage limits need g verifying (at least for chamber types, if not individual chambers)   Going to a higher polarizing voltage can lead to a Going to a higher polarizing voltage can lead to a larger uncertainty in the measurement  Recombination can be a function of the sign of the charge collected charge collected  Addendum recommends a maximum value of 300 V (lower values may be required for small-volume chambers) chambers)

B.5 Measurem ent of P pol The polarity effect has many potential sources: 1) guard ring distortion, 2) secondary electron emission produces negative current independent of polarity of the electrode, 3) low energy electron ejection from chamber wall which is not 3) low energy electron ejection from chamber wall which is not compensated by electrode, 4) uneven distribution of the space charge, 5) virtual variation of active volume due to space charge distortion, 6) stopping of the fast electrons in the collecting electrode not balanced by the ejection of the recoil electrons, 7) collection of current outside the chamber volume due to leakage in solid insulator.