Dosimetric Uncertainties in Reference and Relative Dosimetry of Small Fields

Transcription

1 Dosimetric Uncertainties in Reerence and Relative Dosimetry o Small Fields Jan Seuntjens, Ph.D., FAAPM, FCCPM McGill University Health Centre Canada

2 Outline o Presentation Uncertainty concepts and requirements Dosimetry standards, calibration chain and calibration uncertainties Recap o physics o small ields & dosimetric uncertainties Small ield reerence dosimetry and sources o uncertainty Output actors and sources o uncertainty

3 Learning Objectives Learn about the sources o uncertainties in reerence dosimetry o conventional and small ields Learn about new upcoming dosimetry recommendations or small ield dosimetry and components o uncertainty

4 Uncertainties - GUM GUM: ISO Guide to the Expression o Uncertainty in Measurement procedure to estimate the total uncertainty in your measurement more than you ever wanted to know about probability distributions, uncertainty budgets, degrees o reedom, coverage actors and how to turn a guess into an estimate More useul: the best way to ensure that you take all uncertainty components into account properly how to build an uncertainty budget! NIST produced an explanatory document to the GUM (- NIST Technical Note 1297)

5 Uncertainty categories An Error is the dierence between the true value o a quantity or variable and its estimate. I we know the Error, we can apply a correction to arrive at the true value o the quantity Uncertainties are not Errors! Categories or types o uncertainties: A and B Type A. those which are evaluated by statistical methods; sometimes wrongly called random uncertainties, more correctly: "component o uncertainty arising rom a random eect" Type B. those which are evaluated by other means; sometimes wrongly called systematic uncertainties, more correctly: component o uncertainty arising rom a systematic eect A type A uncertainty rom one uncertainty budget can become a Type B uncertainty in another uncertainty budget

6 Uncertainty requirements in SRT Gradients are on the order o 20% per mm With this gradient, a 10% dose uncertainty will lead to an uncertainty in a proile width o 10%/20% * 1 mm = 0.5 mm on both sides o the proile, i.e., 1 mm width uncertainty GammaKnie: 10% higher dose in centre o the ield means a 1 mm widening o the 50% isodose line For a 18 mm target this means the treated volume may increase by 25%! the eect o dosimetric uncertainty translates in signiicant changes in treated volume

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8 Radiation Dosimetry Traceability Calibration Chain Veriication, QA and audits 8

9 Calibration chain PSDL (NIST or NRCC) N D,w ADCL, SSDL ADCL beam Q N D,w ic AAPM-CLA Clinical beam Q D w ic IROC

10 Primary Dosimetry Standard Instrument that allows the determination o absorbed dose according to its deinition Preerably 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. the path to base SI units is not always as direct as we would like PSDLs are primary standards dosimetry laboratories

11 Absorbed dose to water Dose to water is determined directly, at a point, by measuring the temperature increase: c w : speciic heat capacity o water (4180 J kg -1 K -1 ) : temperature increase (0.25 mk/gy) k c :heat loss correction actor k p : perturbation o radiation ield correction actor k dd : non-uniormity o lateral dose proile corr. Factor : water density dierence correction actor h: heat deect

12 Practical realization The NRC water calorimeter, Ottawa, Canada (Seuntjens et al 1999 A status report on the NRC sealed water calorimeter. PIRS 0584)

13 Uncertainty: primary standard ~0.5% Seuntjens and Duane (2009) Metrologia 46 S39

14 Uncertainty: reerence dosimetry TG-51 Update: Uncertainty budget broken down into: Measurement Calibration data Inluence quantities Typical values discussed but emphasis on individual users constructing site-speciic uncertainty budgets or their calibration situations McEwen et al Medical Physics 41, (2014)

15 Physics o small ields and impact on uncertainties 18

16 Recap: What constitutes small-ield conditions? Beam-related small-ield conditions the existence o lateral charged particle disequilibrium partial geometrical shielding o the primary photon source as seen rom the point o measurement Detector-related small-ield condition detector size compared to ield size

17 Lateral charged particle loss broad photon ield narrow photon ield volume volume A small ield can be deined as a ield with size smaller than the lateral range o charged particles is a measure o the degree o charged particle equilibrium or transient equilibrium

18 Lateral charged particle loss Concept o r LCPE MC calculations, Seuntjens (2013)

19 Detector size relative to ield size Small ield conditions exist when one o the edges o the sensitive volume o a detector is less then a lateral charged particle equilibrium range (r LCPE ) away rom the edge o the ield r LCPE (in cm) = TPR 20, (Li et al Med Phys 22, ) Slide courtesy: H. Palmans

20 Source occlusion Large ield conditions Small ield conditions (Figure courtesy M.M. Aspradakis et al, IPEM Report 103)

21 Overlapping o beam penumbras deinition o ield size is not unique Das et al Med Phys 35:

22 Detector-related small ield condition Meltsner et al., Med Phys 36:339 (2009) Exradin A16 outer diameter Exradin A16 inner diameter Based on criterion 1, one could claim that the GammaKnie 18 or 14 mm diameter ields are not small (quasi point source + electron equilibrium length about 6 mm).

23 Detector dependence o output actor From Sanchez-Doblado et al Phys Med 23:58-66

24 Measurements with small-ield detectors Sauer & Wilbert Med Phys 34, (2007) IC = PTW (0.125 cm 3 ) PiP = PTW (Pinpoint) SES = size o equivalent square

25 Detector issues in small ield dosimetry Energy dependence o the response Perturbation eects Central electrode Wall eects Fact that cavity is dierent rom water, luence perturbation Volume averaging These eects depend somewhat on the beam spot size

26 Detector issues in small ield dosimetry Dosimetry protocol values (e.g., TG-51) o these actors are applicable usually only in TCPE and only or the conditions: 10 x 10 cm 2 ; z re = 10 cm; SSD or SAD 100 cm 29

27 Stopping power ratio water to air 0.5% eect Very small eects! Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)

28 Role o dierent perturbation actors PP31006 and PP31016 chambers Crop et al., Phys Med Biol 54:2951 (2009)

29 Magnitude o correction actors 8 mm x 8 mm ield, 10 cm depth (0.6 mm, 2 mm spot sizes) Very large eects! Crop et al., Phys Med Biol 54:2951 (2009)

30 A14P chamber Collecting electrode diameter: 1.5 mm Separation: 1 mm Narrow 1.5 mm ield D w /D air 60% Ratio o dose-to-water to doseto-air averaged over cavity volume Stopping power ratio w/air O-axis distance (mm) Paskalev, Seuntjens, Podgorsak (2002) AAPM Proc. Series 13, Med. Phys. Publishing, Madison, Wi,

31 Summary o issues leading to dosimetric uncertainties in small ields Beam dependent issues Beam ocal spot size Lateral disequilibrium How do we measure beam quality in practice? Detector eects There is no ideal detector Volume averaging and luence perturbation eects Corrections depend on beam spot size

32 Small ields: upcoming guidelines, data and uncertainties 35

33 2 components: 1. Clinical reerence beam 2. Clinical small ield Med. Phys. 35, 5179 (2008) 36

34 REFERENCE DOSIMETRY, wq, Q DwQ,, QQ, Q, Q D M N k k Broad beam reerence ield re 0 0 Machine speciic reerence ield re Radiosurgica l collimators d = 1.8 cm RELATIVE DOSIMETRY D w D, w Q Q Q, Clinical ield,, Q N D k, w, Q Q 0 Q, k Q Hypothetical reerence ield re 0 k Q Ionization chamber,, Q,, Q re re micro MLC 10 cm x 10 cm CyberKnie 6 cm GammaKnie d = 1.6/1.8 cm Tomotherapy 5 cm x 20 cm Q M, Q,, Q k Q, M 37 Q Q

35 Components o small ield reerence dosimetry

36 How to speciy beam quality in small ields? Data rom BJR Suppl 25 Sauer (2009) Med. Phys. 36: 4168

37 Beam quality in small ields Palmans 2012 Med Phys 39 (9), 5514 e.g. or PDD 10X (10) = %dd(10) X PDD 10 ( 10) PDD 10 1 c ( s) c 2 1 e 10s t e s t PDD 10(s) MV 21 MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV 5 MV 4 MV s / cm PDD10 x ( 10) PDD PDD ( 10), ( 10) 20. 0, PDD PDD ( 10) ( 10) (TG-51)

38 Beam quality speciier or Tomotherapy AAPM TG-148 (Langen et al Med Phys 37: ): dd(10) x[ht-re]

39 Correction actor data correction actors are small or the larger-ield

40 Correction actor data (cont d) Francescon et al: Phys. Med. Biol. 57 (2012)

41 Correction actor data (cont d) correction actors are small or the larger ield

42 Volume averaging in FFF beams A chamber o cavity length o 24 mm underestimates dose by 1.5 % in the 6 cm ield on Cyberknie! Slide courtesy: H Palmans

43 Volume averaging in FFF beams Pantelis et al Med Phys 37:2369

44 Volume averaging in FFF beams Slide courtesy: H Palmans

45 Components o small ield output actors

46 Output actors (1) (2) (2) (2) (1) (1) (2) (1),,,,,, Q rel Q rel Q Q Q Q Q Q Q Q M M M M M M k k Q Q Q Q Q w Q Q w Q Q Q w Q w Q Q Q k M M M D M D M M D D,,,,,,,, Q Q w Q w Q Q Q M M D D k,,,, Where:

47 Output actors example CyberKnie M / M60 (2),,,, Q Q Q kq (1) k Pantelis et al Med Phys 37: 2369 Slide courtesy: H. Palmans A16 PinPoint Diode Diode EDGE Alanine TLD EBT ilm Polymer gel diameter / mm M / Mre A16 PinPoint Diode Diode EDGE Alanine TLD EBT ilm Polymer gel Diode Diode EDGE (M/M 60 ) 2 /(M/M 60 ) 1 ratio o correction actors (MC or vol) PinPoint Diode Diode EDGE Alanine TLD EBT ilm Polymer gel diameter / mm TLD EBT ilm ExrA16 PinPoint SHD USD EDGE alanine TLD EBT GEL Polymer gel detector PinPoint Diode Diode EDGE Alanine diameter / mm ( M / M re ) * k,

48 Experimental and MC studies E.g. PTW unshielded diode in MLC collimated square ields Experiment: Monte Carlo: correction actor with re int Griesbach et al 2005 Med Phys 32:3750 (rel. diamond) Krauss www (rel. LIC) Ralston et al 2012 PMB 57:2587 (rel. PS / z = 5 cm, diode 1) Ralston et al 2012 PMB 57:2587 (rel. PS / z = 5 cm, diode 2) it Experimental Data or Table square ield size / cm correction actor with re int it Monte Carlo Data or Table Monte Carlo < Francescon et al Med Phys 38: square ield size / cm

49 Eect o dierent parameters on the correction actors P. Francescon, et al, Med. Phys. 38, (2011). Parameters varied: 1. Linac model 2. Spot size FWHM 3. Energy o the electron source 4. Distance between exit window and target

50 Uncertainty in correction actor introduced due to ield size deinition Cranmer Sargison et al Med. Phys. 38, (2011) Benmakhlou et al Med. Phys. 41, (2014)

51 Summary Small ield dosimetry is complex There are hety perturbation eects that can have signiicant impact on reerence dosimetry procedures and output actors Current good practice or reerence and relative dosimetry in static small MV photon ields can be expressed as Choice o suitably small detector which is known to minimally perturb luence Careul experimental setup Correct or volume averaging and energy dependence o detectors Corroboration o data with peers and use o detectors o dierent design New protocol is upcoming Machine-speciic reerence ields deined, corrections are small Data on correction actors is being collected Uncertainty analyses ongoing

52 The 1-sigma uncertainty on the realization o absorbed dose to water under reerence conditions in broad photon beams, in the primary standards laboratory, is typically: 2% 1. 4% 2. 2% 17% 12% 68% 1% 3. 1% % %

53 Correct answer: (4) 0.5% Discussion: Uncertainties in absorbed dose to water standards vary slightly rom primary laboratory to primary laboratory but all PSDLS arrive at typical values o around 0.5%. The uncertainty on calibrations in a ical context vary anywhere rom 0.9% to 2.1% in ideal versus more routine occasions, respectively. Re: or example, Seuntjens and Duane 2009 Photon Absorbed Dose Standards. Metrologia 46 S39. McEwen et al 2014 Med. Phys. 41 (4),

54 A condition or radiation ields to be small or the purpose o reerence dosimetry can generally be ormulated as 6% 76% 9% 3% 6% 1. Radiation ields with diameter o less than 3 cm 2. Radiation ields or which lateral charged particle equilibrium is lost whether the detector is absent or not; 3. Radiation ields or which the stopping power ratio, water-to-air, is drastically (> 3%) dierent rom the value in TG-51 reerence (10 x 10 cm 2 ) ields; 4. Radiation ields or which the PSDL-traceable ionization chamber calibration coeicient is valid; 5. Radiation ields in which the absorbed dose to water cannot be measured accurately.

55 Correct answer: (2) Radiation ields or which lateral charged particle equilibrium is lost in absence or presence o the detector. Discussion: Stopping power ratios do not vary signiicantly in small ields. PSDL traceable calibration coeicients in small ields do not (yet) exist. Absorbed dose can be measured accurately in small ields. Re: or example, Aspradakis et al, IPEM Report 103, Small ield MV photon dosimetry

56 Indicate the single set o two largest contributors to correction actors and their uncertainties or commercial air-illed ionization chambers in small photon ields 2% 3% 77% 15% 3% 1. The stopping power ratio water to air and the central electrode eect 2. The stopping power ration water to air, and the chamber wall eect 3. The luence perturbation eect and the volume averaging eect 4. The stopping power ratio, water to air, and the volume averaging eect 5. The ionization chamber wall eect and the stem eect

57 Correct answer: (3) The luence perturbation eect and the volume averaging eect Discussion: stopping power ratios are not very sensitive to changes in radiation beam size, nor are wall correction, central electrode corrections or stem eects. The large eects observed in small ields lie in volume averaging and luence perturbation eects. Re: example: Crop et al 2009 Phys Med Biol 54(9). p

58 Reviews on small ield dosimetry R. Alonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjäll, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S. Vatnitsky, A new ormalism or reerence dosimetry o small and nonstandard ields, Med. Phys. 35, (2008). I. J. Das, G. X. Ding, and A. Ahnesjö, Small ields: Nonequilibrium radiation dosimetry, Med. Phys. 35, (2008) M. Aspradakis, J. Byrne, H. Palmans, J. Conway, K. Rosser, J. Warrington, and S. Duane, Small ield MV photon dosimetry, IPEM Report No. 103 (Institute o Physics and Engineering in Medicine, York, 2010). H Palmans (2011) CN-182-INV006, Small and composite ield dosimetry: the problems and recent progress. IDOS Conerence, Vienna. Note: There is a literature explosion (since 2008) on the subject o small ield dosimetry and correction actors. The reviews / reports above are not that recent! `

59 Acknowledgments IAEA committee Palmans (Chair) Andreo Huq Mackie Ulrich Kilby Izewska Capote Alonso Seuntjens AAPM Committees TG-178 (Goetsch et al) TG-155 (Das et al) WGDPCB (Seuntjens et al) ICRU Report Committee: Prescription, Recording and Reporting o Stereotactic Radiosurgery using small ields Seuntjens (Chair) Lartigau (Co-chair) Ding Goetsch Cora Roberge Nuyttens Grégoire, Jones Main commission

60 Dosimetric Uncertainties in Reerence and Relative Dosimetry o Small Fields Jan Seuntjens, Ph.D., FAAPM, FCCPM McGill University Health Centre Canada