IAEA/AAPM Code of Practice for the Dosimetry of Static Small Photon Fields. Jan Seuntjens McGill University Montréal, Canada

Transcription

1 IAEA/AAPM Code o Practice or the Dosimetry o Static Small Photon Fields Jan Seuntjens McGill University Montréal, Canada

2 Acknowledgements IAEA/AAPM small and composite ield working group: Hugo Palmans (Chair), Rodolo Alonso, Pedro Andreo, Roberto Capote, Saiul Huq, Joanna Izewska, Jonas Johansson, Warren Kilby, T Rock Mackie, Ahmed Meghziene, Karen Rosser, Jan Seuntjens, Wolgang Ullrich Edmond Sterpin, Mania Aspradakis, Simon Duane, Hugo Palmans, Pedro Andreo or discussions on a variety o aspects related to this eort.

3 Disclosures My work is supported in part by the Canadian Institutes o Health Research, the Natural Sciences and Engineering Research Council, Canada through operating grants and training grants. Sun Nuclear Corporation provided untied unding to support the graphite probe calorimeter project. Some brand names o commercial products are mentioned in this presentation. This does not represent any endorsement o one product or manuacturer over another 3

4 Learning Objectives Review the problems o small ield dosimetry and the solutions that have been identiied Learn about the IAEA-AAPM recommendations and data or small ield dosimetry

5 Overview The problems in small-ield dosimetry The IAEA dosimetry ormalism Conclusions

6 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

7 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

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

9 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

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

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

12 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).

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

14 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

15 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 15

16 Stopping power ratio water to air 0.5% eect Very small eects! Andreo&Brahme PMB 8:839 (1986) Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)

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

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

19 Benmahklou and Andreo (2013) Correction actors or ionization chambers

20 Diodes or small ield dosimetry Sauer and Wilbert 2007 Med Phys 34:1983-8

21 Shielded and unshielded diodes Benmahklou and Andreo (2013)

22 22 Benmahklou and Andreo (2013)

23 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

24 What are the single set o two largest contributors to correction actors and their uncertainties or commercial air-illed ionization chambers in small photon ields? 1% 1. The stopping power ratio and the central electrode eect 5% 75% 17% 3% 2. The stopping power ratio and the chamber wall eect 3. The luence perturbation eect and the volume averaging eect 4. The stopping power ratio and the volume averaging eect 5. The ionization chamber wall eect and the stem eect

25 Correct answer: 3 The luence perturbation eect and the volume averaging eect Discussion: The ield size dependence o stopping power ratios is 0.5% or less. For most ionization chambers the ield size dependence o wall corrections is limited to a ew percent. The volume averaging and luence perturbation corrections are potentially very large (on the order o 10-30% or more depending on the situation) Reerence: Crop et al (2009) Phys Med Biol Bouchard et al (2009) Med Phys 36 (10),

26 Which two competing eects lead to ield size dependent correction actors o unshielded diode detectors? 1. Intrinsic energy dependence o Si in photon beams and volume averaging 2. Intrinsic energy dependence o Si in photon beams and perturbation eects 3. Polarity eect and recombination 4. Polarity eect and electrometer calibration 5. Recombination eect and diode doping 78% 14% 3% 5% 0%

27 Correct answer: 2 Intrinsic energy dependence o Si in photon beams and electron luence perturbation eects Discussion: Volume averaging is usually small in diodes because o the small size o the sensitive volume. Diodes are not polarized by an external bias, so there is no polarity eect. Recombination eects and diode doping are not relevant in this context. Reerences: Francescon et al 2011, Med Phys 38: 6513 Benmakhlou et al 2014, Med Phys 41:

28 IAEA TECDOC small ield dosimetry Code o Practice / working document Physics relevant to reerence and relative dosimetry Formalism Instrumentation Practical implementation Machine-speciic reerence dosimetry Relative dosimetry Data

29 ratio o dose to kerma Ch. 2 - Physics o small ields e.g. Small ield conditions LCPE source occlusion detector size Co-60 6 MV 10 MV 15 MV beam radius / cm Seuntjens MC Aspradakis et al 2010 IPEM Report 103 Meltsner et al Med Phys 36:339-50

30 Reerence Fields Small Fields REFERENCE DOSIMETRY, w, D, w,,, 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, Clinical ield,, N D k, w, 0, Hypothetical reerence ield re 0 k k Ionization chamber,,,, 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 M,,, k, M 30

31 Ch3. Formalism (Alonso et al) / D w in machine speciic reerence () ields Chamber calibrated speciically or the ield D w, M N,, D w Chamber calibrated or the conventional reerence ield and generic correction actors are available D re re M N,, D, w, k 0, 0 w re= =10 x 10 cm 2 0 = 60 Co Chamber calibrated or the conventional reerence ield and generic correction actors not available D re re, M N D, w, k, 0 0 w k, re,

32 Equivalent square ields - WFF beams: BJR 25 - equivalent ield size is energy independent FFF beams: equivalent ield size is energy dependent; Tables are provided or 6 MV and 10 MV

33 Ch 3. Formalism / equations or beam quality in non-standard reerence ields or TPR 20,10 (10) = TPR 20, TPR 20, 10 ( 10) TPR 20, 10 ( s) d ( 10 1 d ( 10 s) s) TPR 20,10(s) MV 21 MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV (Palmans 2012 Med Phys 39:5513) MV 4 MV (b) s / cm

34 PDD 10(s) Ch 3. Formalism / equations or beam quality in non-standard reerence ields or PDD 10X (10) = %dd(10) X PDD 10 ( 10) PDD 10 1 c ( s) c 2 1 e 10s t e s t MV 21 MV 18 MV 15 MV 12 MV 10 MV 8 MV 6 MV (Palmans 2012 Med Phys 39:5513-9) MV 4 MV (d) s / cm PDD10 x ( 10) PDD PDD ( 10), ( 10) 20. 0, PDD PDD ( 10) ( 10) (TG-51)

35 Note about volume averaging in FFF beams Pantelis et al Med Phys 37:2369

36 Volume averaging in FFF beams

37 Ch3. Formalism / determination o ield output actors Field output actor relative to reerence ield (re stands here or a conventional reerence or ield) M, re, re, k re re, re M re Field output actor relative to reerence ield using intermediate ield or daisy chaining method,, re re int M (det) M ( IC) int, M int int (det) M re re ( IC) K, re re where K,, re re k,, int det (det) k int int,, re re ( IC)

38 Ch 4 Instrumentation Required equipment, detectors, phantoms or dosimetry Required equipment, detectors, phantoms or relative dosimetry

39 Ch 5 Practical implementation dosimetry Reerence conditions or beam quality and dosimetry Overall correction actors or ionization chambers Correction or inluence quantities Measurement in plastic phantoms and crosscalibration

40 Ionization chambers, recombination, polarity LeRoy et al., PMB 56: (2011) Agostinelli et al., Med Phys 35: (2008)

41 Note on the use o plastic phantoms (Seuntjens et al 2005, Med. Phys. 32: 2945)

42 Ch 5 Practical implementation dosimetry / availability k, re, re data

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

44 Ch 6 Practical implementation relative dosimetry Required equipment, detectors, phantoms Measurements o proiles and ield output actors Correction actors or determination o output actors

45 Ch 6 Practical implementation relative dosimetry / correction actors or OF Examples o dierent sources o correction actors will be urther discussed in the next presentation (I. Das) IAEA-AAPM code o practice data tables is based on a vetted set o correction actor rom the literature Uncertainty analysis has been perormed

46 correction actor with re int Field output actors correction actors - example PTW unshielded diode Cranmer Sargison 2011 / Elekta 0.97 Cranmer Sargison 2011 / Varian Krauss Schwedas Griesbach 2005 Ralston et al cones 0.94 Ralston et al micromlc 0.93 Fit (exp) Fit (exp + MC) square ield size / cm

47 correction actor with re int Field output actors correction actors - diode IBA SFD unshielded diode Azangwe 2014 Lechner 2013 WFF Lechner 2013 FFF Bassinet 2013 /Novalis+muMLC Cranmer Sargison 2011 / Elekta Cranmer Sargison 2011 / Varian Sauer 2007 Ralston et al cones Ralston et al micromlc Fit (exp) Fit (exp + MC) square ield size / cm

48 Field output actors correction actors PTW Pinpoint

49 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)

50 Output actors validation methodology (1) (2) (2) (2) (1) (1) (2) (1),,,,,, rel rel M M M M M M k k w w w w k M M M D M D M M D D,,,,,,,, w w M M D D k,,,, Where:

51 Output actors example CyberKnie A16 PinPoint Diode Diode EDGE Alanine TLD EBT ilm Polymer gel k,, ( k 1 ) ratio o correction actors (MC or vol) M / M60,, ( 2 ) 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 Diode Diode EDGE (M/M 60 ) 2 /(M/M 60 ) diameter / mm TLD EBT ilm ExrA16 PinPoint SHD USD EDGE alanine TLD EBT GEL Polymer gel detector ( M / M re ) * k, PinPoint Diode Diode EDGE Alanine TLD EBT ilm Polymer gel PinPoint Diode Diode EDGE Alanine diameter / mm

52 For what purpose is the measurement o the beam quality speciier required? 1. To speciy the correction actors to be applied to the output ratios measured in small ields 2. To speciy small ield output actors 3. To speciy the beam quality correction actor in the ield 4. To ensure the beam is o adequate quality 5. To speciy the absorbed dose calibration coeicient or a small ield 58% 22% 13% 6% 1%

53 Correct answer: 3 To speciy the k,re beam quality correction actor in the ield Discussion: In general, no beam quality measurement is perormed in small ields, only in ields. Reerences: Palmans 2012 Med Phys 39: 5513

54 Conclusions Solutions to most small-ield dosimetry problems have been described and translated in ormalised procedures The IAEA CoP will be coming out in the very near uture timeline < 6 months