Instrumentation for Verification of Dose

Transcription

1 Instrumentation for Verification of Dose S. Vatnitsky MedAustron GmbH, Wiener Neustadt, Austria Presented to: Educational Workshop PTCOG 53 Shanghai, China, June 9-11, 2014

2 Consistent and harmonized dosimetry guidelines Accurate beam calibration Goals Ensure exact delivery of prescribed dose Perform planning of high-precision conformal therapy Provide interchange of clinical experience between facilities Dosimetry tasks Acceptance testing & commissioning BDS Reference calibration of clinical beams Acceptance testing & commissioning TPS Periodic QA checks Provide standardization of dosimetry in radiobiology experiments Verification of dose delivery 2

3 Absorbed dose determination in reference conditions for light ion beams Faraday Cup Calorimeter A tool for beam monitor calibration Lack of national and international dosimetry standards Thimble air-filled ionization chamber 3

4 Protocols/COP for proton and heavier ion beam dosimetry Only a Protocol based on standards of absorbed dose to water is being recommended by ICRU/IAEA Reports for protons and heavier ions 4

5 Absorbed dose determination in reference conditions for light ion beams: Step one At the standards laboratory ionization chamber is provided with a calibration factor in terms of the radiation quantity S in a beam of quality Q o : N S,Qo Q o (Co-60) S No national and international dosimetry standards Q o (Co-60) M Qo Q o (proton beam) calorimeter N S,Q M S o Q o reference conditions: N D, w, p D ( ) wp, cal M corr N D,w,Qo P, T, h, S, etc 5

6 Absorbed dose determination in reference conditions for light ion beams: Step two The ionization chamber is then subsequently placed at a reference depth in water in the user s beam, of quality Q. Q D D S w Q M Q N S Q f,,, Q, Q o o M Q f D, S Q, Q o is the instrument reading in the user s beam, suitably corrected to the reference conditions for which N S,Q is valid o is any overall factor necessary to convert both from the calibration quantity S to dose D and from the calibration quality Q o to the user s quality Q 6

7 N D,w - based formalism: IAEA TRS-398/ICRU 78 D w (z ref ) at any user quality Q (photons, electrons, protons, heavier ions) D w,q = M Q N D,w,Q o k Q,Qo corrected instrument reading at Q calibration coefficient at Q o beam quality correction factor Q o (proton beam) calorimeter D M N k w, Q Q D, w, p Q, p 7

8 N D,w - based formalism: IAEA TRS-398/ICRU 78 k Q => Q o = 60 Co s s w, air W p w, air 60 Q air Q Q Co W air Co p => Q o = proton beam Co k Qp, s s w, air w, air Q p 8

9 Stopping powers Proton beams water/air stopping-power ratio Heavier ion beams alpha particles carbon ions protons C (Salamon) Ne (Salamon) Ar (Salamon) He (Salamon) p (ICRU-49) He (ICRU-49) C (H and B) 1.13 Basic proton stopping powers from ICRU residual range (g cm -2 ) A constant value of s w,air = 1.13 adopted in TRS 398 (ignores fragments) Calculation using MC code PETRA following Spencer- Attix cavity theory Transport included secondary electrons and nuclear inelastic process Geitner et al

10 Calorimetry-based determination of W-values D ( Q, cal) cv (1 D ) w T D w ( Q, cal ) M cor N D, w, 60 Co ( S ( S w, air w, air ) ) 60 Q Co ( W ( W air air 60 Co ) ) Q ( p) ( p) Q Co

11 Values of W/e for protons and carbon ions deduced from comparison of IC and calorimeter measurements (w air/e )p / (J C -1 ) Proton beam 34.2 J C -1 Delacroix et al., 1997 Palmans et al., 2004 Palmans et al., 1996 Hashemian et al., 2003 Siebers et al., 1995 Schulz et al., 1992 Brede et al., 2006 Medin et al., J C J C -1 TRS Proton energy / MeV ICRU 78 Sakama et al

12 Transportable graphite calorimeters NPL protons at CCO NIRS - carbon ions Sakama et al 2008 Palmans et al

13 Transportable water calorimeters McGill PTB Protons Calorimetry Gy/MU Ionometry Gy/MU Difference % Calorimetry Gy/MU Ionometry Gy/MU Difference % Scattering 9.087* *10 3 Scanning 1.198* *10 3 Sarfehnia et al., Protons 182 MeV C MeV/u 2.95± ± ± ± 0.08 Brede et al.,

14 k Q s 60 air W 60 p60 sw, air W Q Q pq w, air Co air Co Nylon66-Al Co PMMA-Al &PTW30001 ExrT2 D w,ne2571 / D w,ch C-C &PTW30002 A150-Al &NE2581 IC Chamber # Data from Palmans et al 2001, and Palmans and Verhaegen, Montreal workshop for protons and ions 1 for 60 Co 14

15 Compilation of published data I water I water = 78 ev 400 I water (ev) mean excitation energy of liquid water ICRU 73 (2005) superseded ICRU (2009) tentative ICRU 37 (1984) ICRU 49 (1993) I-water DRF I-water EXP I-water EST Energy deposition (MeV/cm) C 400 MeV/u on water SHIELD-HIT variation with I water is ~1 mm for 2 ev 5 mm value # Depth (cm) 15

16 Influence of a change in the I water and I graphite values on basic dosimetry data and k Q values (results presented by P. Andreo at ESTRO 2013) Decrease of 0.6% in S w,air for Co-60 Decrease of 0.4% in S w,air for protons and heavier ions Net change in W air,protons - increase of 0.6% i.e. W air,protons = ev (current value ev) CONCLUSION: the net effect of all the changes leaves current calculated kq values unaltered 16

17 Recombination corrections for protons and heavier ions Protons and heavier Ion beams: Pulsed (passive) or pulsed scanned (active) beams, no continuous beams! k s a o M a a M M 2 M 2 Not all beams are pulsed for determination of recombination 2 Two-voltage method k s a o M a a M M 2 M 2 2 1/ M 1/ M b / V General recombination 17

18 Recombination corrections for proton beams ICRU 78 Synchrotrons (Repetition < 0.5 Hz, Acceleration 0.5 1s) Cyclotrons (small pulses, high repetition, high dose per pulse) dose per pulse (0.2 Gy) pulse length 400μs maximum transit time for the ionization chamber 152 μs (300 V) and 76 μs (600 V) Lorin et al, 2008 Effective pulse duration is long compared to ion collection time of ion chamber continuous beam k s 2 V N / VL 1 2 V / V M / M N L 18 N L Ion collection time of ion chamber shorter compared to pulse duration Scanned continuous beam The user should verify recombination corrections against independent method

19 Reference calibration: reference conditions Passive Scattering protons, carbon ions Calibration at SOBP ( Pedroni et al) Reference conditions are facility specific Protons spot scanning Carbon ions spot scanning Calibration at SOBP Calibration at plateau Plateau versus SOBP: superposition of beams with different intensities not continuous and reproducible mix of particles with high and low LET fluence corrections are small 19

20 Proton spot scanning calibration PSI (Pedroni et al, 2005) D w = (N/A) (S/) w * x Number of protons/mu IC 10x10x10 MD ACC (Gillin et al, 2010) Dose/MU 20

21 Standard uncertainties in determination of D w (TRS 398, ICRU 78) u(n D,w SSDL ) = 0.6 k Q calc k Q,p Co-60 gamma-rays 0.9 High-energy photons 1.5 High-energy electrons Proton beams Heavier ions

22 Dosimetry in non-reference conditions Relative dose measurements require no detector calibration other than verification of linearity of response within assumed dynamic range of measurement conditions Dosimetry tasks Routine daily clinical physics activity Beam line commissioning Collecting data for TPS Periodic QA Beam characteristics Depth dose Lateral profiles Output factors 22

23 Detectors for measurements in non-reference conditions Active detectors: Ion chambers, diodes, diamond detector, scintillators (single and multiple) Direct display of the current dose rate or the accumulating dose Passive detectors: Destructive TLD Non-destructive Films, alanine Probe accumulates the dose during irradiation. The value of dose is obtained after irradiation with read-out device 23

24 Characterization of small proton beams MeV: no modulation, aperture 30 mm 126 MeV protons, collimator 30 mm, no modulation Markus chamber Diode DEB 50 Diamond detector PinPoint chamber Relative dose Depth in water, mm The user should carefully select detectors depending on beam size 24

25 Characterization of scanned proton pencil beams Depth dose distribution Dose profiles Gillin et al, 2010 Kimstrand et al

26 Multi-detector systems for characterization and QA of proton and carbon beams courtesy by PTW courtesy by IBA Nichiporov et al courtesy by IBA 26

27 2-D dosimetry: fluorescent screen and CCD camera Boon et al 2000 Courtesy CMS Lin et al, 2013 E. Pedroni et al Rosenthal et al

28 2-D dosimetry: TLD Radiochromic film 1.0 MD- 55 film Kodak XV-2 film 0.8 A. Entrance B. SOBP region relative dose Olko et al off-axis data, mm 28

29 Radiochromic films and gels for characterization of clinical proton beams 155 MeV, modulation 3 cm, normalized at 11 cm parallel plate ionization chamber MD-55 film, 70 Gy relative dose Heufelder et al, water equivalent depth, cm 29

30 Additional reading 30

31 Conclusion Implementation of ICRU Report 78 IAEA TRS 398 harmonize clinical dosimetry at proton and heavier ion beam facilities provide a level of accuracy comparable to that in calibration of photon and electron beams 31

32 Acknowledgements Members of ICRU/IAEA Committee Report 78 O. Jäkel and colleagues at HIT T. Lomax and colleagues at PSI Thank you for your attention! 32