Detectors for small field dosimetry

Transcription

1 Detectors or small ield dosimetry Hugo Palmans MedAustron, Wiener Neustadt, Austria and National Physical Laboratory, Teddington, UK 1

2 Overview Ideal detector Water calorimeter Ionization chamber Alanine Diodes, diamond, ilm, TLD, 2

3 Which detector to use? Read literature: IPEM report 103 Manuacturers manuals, e.g. PTW 3

4 IPEM report 103 4

5 Which detector to use? Read literature: IPEM report 103 Manuacturers manuals, e.g. PTW 5

6 Which detector to use? 6

7 Ideal detector Detector properties Stability Dose linearity Dose rate linearity Dose per pulse linearity Energy response Spatial resolution Size o detector Orientation Recommendation Short-term detector response should be better than 0.1% or a total combined dose o many hundreds o kgy rom multiple exposures. linearity better than 0.1% should be possible over a dose range o at least three orders o magnitude (e.g Gy) Linear accelerators are typically operated at dose rates o 100 Gy/s (instantaneous) and 0.01 Gy/s (averaged) and thereore detectors should be linear to better than 0.1% over this range. A detector s response with changing dose per pulse should remain stable to better than 0.1% ater correction or ion recombination. Essentially water equivalent The choice o suitable detector in terms o spatial resolution is usually based on a trade-o between adequately high signal-to-noise ratio and dosimeter size. The detector size should be such that the volume averaging correction is not larger than 5%. The response o a detector should ideally be independent o the orientation o the detector with 7

8 Calorimetry Radiation energy turns into heat heat is tiny, but measurable our primary standards or absorbed dose are calorimeters 8

9 0.5 mm Water calorimeter - thermistor probes (LSDG) 0.6 mm thermistor

10 Water calorimeter chemical heat deect D w = c w 1. D T. 1 - h 10

11 Water calorimetry why does it not work or small ields Heath low: broad ield small ield 11

12 Problem: dose determination with ion chambers D D s p w = air w, air M W D = air m e air = M r V air W e OF = D D ield re = M ield M re ( s ( s w, air w, air ) ield ) re p p ield re correction actor 12

13 Which one is right? M /M re PTW (0.125cc) PTW (PinPoint) PTW (0.3cc) square ield width / mm Krauss, Vienna, Feb. 2009, 13

14 Ion chambers or small ield dosimetry - Water to air stopping power ratios Eklund&Ahnesjö, PMB 53:4231 (2008) Andreo&Brahme PMB 8:839 (1986) 14

15 Calibration in a 60 Co beam N D, w = D M w, 60 corr, Co 60 Co P 0 D w, 60 Co 15

16 Dosimetry in a high-energy x-ray beam M corr, D w, (P 0 )=M corr, N D,w k Only B! 16

17 OF Dosimetry in a high-energy x-ray beam square ield width / mm M corr, D w, (P 0 )=M corr, N D,w k Only B! 17

18 OF Dosimetry in a high-energy x-ray beam square ield width / mm M corr, D w, (P 0 )=M corr, N D,w k Only B! 18

19 OF Dosimetry in a high-energy x-ray beam square ield width / mm M corr, D w, (P 0 )=M corr, N D,w k Only B! 19

20 OF Dosimetry in a high-energy x-ray beam square ield width / mm M corr, D w, (P 0 )=M corr, N D,w k Only B! 20

21 OF Plane-parallel chamber square ield width / mm 21

22 OF Plane-parallel chamber square ield width / mm 22

23 OF Plane-parallel chamber square ield width / mm 23

24 Ion chambers: recombination 24 LeRoy et al., PMB 56: (2011)

25 Ion chambers: polarity Agostinelli et al., Med Phys 35: (2008) 25

26 Which one is right? M /M re PTW (0.125cc) PTW (PinPoint) PTW (0.3cc) Krauss, Vienna, Feb. 2009, square ield width / mm 26

27 Which one is right? M /M re PTW (electron diode) PTW (photon diode) PTW (MicroLion) PTW (0.125cc) PTW (PinPoint) PTW (0.3cc) Krauss, Vienna, Feb. 2009, square ield width / mm 27

28 Which one is right? M /M re PTW (electron diode) PTW (photon diode) PTW (MicroLion) PTW (0.125cc) PTW (PinPoint) PTW (0.3cc) square ield width / mm Krauss, Vienna, Feb. 2009, 28

29 Diodes or small ield dosimetry Sauer and Wilbert 2007 Med Phys 34:

30 Diamond detectors 1.20 (m en /r) carbon,water volume averaging correction actor k vol Volume averaging correction actor (p vol ) E E E E E photon energy / MeV Side o square ield [cm] Diamond (diameter 2 mm) Diamond (diameter 3.2 mm) Diamond (diameter 4 mm) Diamond (4.3 x 4.3 mm 2 ) Diamond (5 x 3 mm 2 ) side o square ield / cm Lechner 2013 Radiother Oncol 109:356 30

31 Diodes and diamonds 31

32 Organic scintillators Cr. Louis Archambault 32

33 Liquid ionization chambers 33

34 Radiochromic ilm Ralston 2012 Phys Med Biol 57:

35 Alanine Amino acid / readout by ESR Pellets 2.5 mm thick and Ø 5 mm or 2.5 mm Water-equivalent Density 1.2 g cm -3 Bergstrand 2003 Phys Med Biol 48:

36 Real time detectors: diodes, diamond, Al2O3:C and scintillators Passive detectors Some others Table I. Summary o properties o detectors used in this study solid state detectors Detector type Dimensions o sensitive volume [mm] Material Z e Material density [g/cm 3 ] Electron density [e - /g] relative to water TLD chips TLD-100 LiF:Mg,Ti TLD µ-cubes RPLD (GD-302M), Japan Length 6, diameter 1.5 TLD-100 LiF:Mg,Ti Silver activated phosphate glass Alanine, NPL Thickness 2.3, diameter 5 Alanine/Parain Alanine, DTU Thickness 2.8, diameter 4.8 Alanine/Parain IBA SFD diode Thickness 0.06, diameter 0.6 Silicone IBA PFD diode Thickness 0.06, diameter 2 Silicone IBA EFD diode Thickness 0.06, diameter 2 Silicone PTW diamond Thickness Sensitive area Diamond 6 approximately Al 2 O 3 :C, DTU Al 2 O 3 :C Saint Gobain BCF-60 scintillator 1 Length 1, diameter 1 Polystyrene Saint Gobain BCF-60 scintillator 2 Length 2, diameter 0.5 Polystyrene the density varies depending on the purity o the diamond used

37 Field output actors correction actors / components Scott et al Phys. Med. Biol. 57:

38 Other proposals to actorize SF correction actors Bouchard et al Med. Phys. 36:4654 Crop et al Phys. Med. Biol. 54:2951 Czarnecki and Zink 2013 Phys. Med. Biol. 58:2431 Fenwick et al. 2013: Phys. Med. Biol. 58:

39 Correction actors or unshielded diodes Francescon et al 2011, Med Phys 38:6513 Benmakhlou et al 2014, Med Phys 41:

40 Ch 5 Practical implementation dosimetry / availability, re data k, re 40

41 Ch 6 Practical implementation relative dosimetry / correction actors or OF e.g. CyberKnie / iris collimator E 0 =7.0 MeV, FWHM spot 2.1 mm Field size / mm PTW PTW PTW PTW SN Edge Exradin A PTW PTW microlion Francescon et al Med Phys 57:3741 k,, re re 41

42 , Field output actors Monte Carlo calculated correction actors CyberKnie Dierent FWHM primary source k, k k,,,, = = a M M,, b,, = 60 mm collimator,, Francescon et al 2008 Med Phys 35:

43 43 Field output actors: (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 =,,,,

44 Field output actors - CyberKnie:,, ( 1 ) ratio o correction actors (MC or vol),, ( 6 Point ode ode GE nine D T ilm lymer gel k 2 ) Pantelis et al Med Phys 37:2369 M / M60 k A16 PinPoint Diode Diode EDGE Alanine TLD EBT ilm Polymer gel M / Mre diameter / mm Diode Diode EDGE (M/M 60 ) 2 /(M/M 60 ) diameter / mm TLD ExrA16 PinPoint EBT ilm SHD USD EDGE alanine TLD EBT GEL Polymer gel detector PinPoint Diode Diode EDGE Alanine TLD EBT ilm Polymer gel diameter / mm ( M / M re ) * k, PinPoint Diode Diode EDGE Alanine 44

45 Field output actors correction actors PTW IBA SFD 45

46 Field output actors correction actors PTW PTW PTW

47 47