Dosimetry and QA of proton and heavier ion beams

Transcription

1 Dosimetry and QA of proton and heavier ion beams Stanislav Vatnitskiy EBG MedAustron GmbH Wiener Neustadt, Austria

2 Content Introduction Reference dosimetry Methods, detectors, protocols Dosimetry in non-reference conditions and QA measurements Methods, detectors Recommendations

3 about 25 treatment facilities are established another 20 proton and light-ion beam centers are planned to be open in next 5 years

4 Dosimetry tasks in proton and heavier ion beam radiotherapy Acceptance testing and commissioning of delivery beam lines Reference calibration of clinical beams Commissioning of treatment planning systems Periodic QA checks Verification of dose delivery

5 Ensure exact delivery of prescribed dose Consistent and harmonized dosimetry guidelines Accurate beam calibration Perform planning of high-precision conformal therapy Provide interchange of clinical experience and treatment protocols between facilities Provide standardization of dosimetry in radiobiology experiments

6 Reference dosimetry Basic output calibration of a clinical beam, is a direct determination of absorbed dose per MU in water under specific reference conditions Reference dosimetry techniques Faraday Cup Calorimetry Ionization chamber

7 Absorbed dose determination in reference conditions for proton and heavier ion beams Faraday Cup Calorimeter Lack of national and international dosimetry standards Thimble air-filled ionization chamber

8 FC-based calibration of the ionization chamber D w = (N/A) (S/ρ) w * x N : number of protons per MU collected in the FC A : effective area of the beam (S/ρ) w : stopping power for water at the incident proton energy

9 N Dw( Q, FC) = ( S/ ) A FC-based calibration of the ionization chamber 10 D ρ ( w QFC, ) w NDwFC,, = corr M FC IC

10 1995 PTCOG Dosimetry intercomparison Institution s statement of proton dose ratio to the mean Institution FC calibration

11 Calorimetry-based calibration of the ionization chamber Main characteristics of calorimetry dosimetry: Energy imparted to matter by radiation causes an increase in temperature T. Dose absorbed in the sensitive volume is proportional to T. T is measured with thermocouples or thermistors. Calorimetric dosimetry is the most precise of all absolute dosimetry techniques.

12 Calorimetry-based calibration of the ionization chamber Two types of absorbed dose calorimeter are used in charged particle beams: In graphite calorimeters the average temperature rise is measured in a graphite body that is thermally insulated from surrounding bodies (jackets) by evacuated vacuum gaps.

13 Calorimetry-based calibration of the ionization chamber In sealed water calorimeters low thermal diffusivity of water enables the temperature rise to be measured directly at a point in continuous water.

14 Calorimetry-based calibration of the ionization chamber D ( Q, cal) = c Θ V (1 + D ) w T

15 Calorimetry-based calibration of the ionization chamber Dw( Qcal, ) = c Θ V (1 + DT),, N Dwc = D (, ) w Qcal corr M

16 Water calorimetery in a 250 MeV proton beam at LLUMC Water Calorimeter

17 Ion chamber measurements (dummy calorimeter) in a 250 MeV proton beam Dummy calorimeter with IC

18 Graphite calorimetry protons at CCO carbon ions at NIRS Sakama et al 2008 Palmans et al 2004

19 Absorbed dose determination in reference conditions for light ion beams Faraday Cup Calorimeter Lack of national and international dosimetry standards Thimble air-filled ionization chamber

20 Protocols/Code of Practice 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

21 N D,w - based formalism - IAEA TRS-398 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 factor

22 Ionization chambers Both cylindrical and parallel plate chambers are recommended for reference dosimetry Parallel plate chambers yield higher u c in absolute D w, though are better suited for relative dosimetry cylindrical chambers recommended for SOBP width>2 cm ICRU/IAEA Report 78: dosimetry equipment For SOBP widths < 2 cm parallel plate chambers must be used Versatile electrometer with cable and connectors fitting to the electrometer and all chambers thermometer, barometer

23 Water phantoms for reference dosimetry Water is the only standard and most universal phantom material for proton and heavier ions dosimetry measurements

24 Reference calibration of proton and heavier ion beams: reference conditions Passive Scattering protons, carbon ions Calibration at SOBP Goitein, Lomax and Pedroni, 2002 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

25 Reference geometry passive beam delivery relative dose relative dose off-axis distance, cm depth, cm phantom protons ion chamber

26 Proton spot scanning calibration at PSI dose model predicts the number of protons/gy 10x10x10 cm 3 box is filled in with homogeneous dose distribution to a dose of 1Gy IC 10x10x10 ( Pedroni et al) ion chamber is placed at a residual range 5g/cm 2

27 A solid phantom rather than a water phantom is used in scanned ion beam A field of 5 cm x 5 cm rather than 10 cm x 10 cm is used The reference depth is in the plateau region of a monoenergetic Bragg peak (depth of 7 mm) instead of the center of a SOBP The calibration is dependent on the initial particles energy and several energy points are selected

28 ( ) ( s ) Lack of standards => Q o = 60 Co This approach would ultimately lead to a dosimetry system, where the dose applied to a patient is traceable to the dosimetry standards of the national PSDL. k Q = s ( ) wair, W p wair, 60 Q air Q Q Co W p = p p p p ( ) air Q dis wall cav cel 1 for protons 1 for heavier ions 1 for 60 Co Co p Co

29 Stopping powers for proton beams Basic proton stopping powers from ICRU 49 Calculation using MC code PETRA following Spencer- Attix cavity theory Transport included secondary electrons and nuclear inelastic process

30 1.140 water/air stopping-power ratio alpha particles carbon ions protons C (Salamon) Ne (Salamon) Ar (Salamon) He (Salamon) p (ICRU-49) He (ICRU-49) C (H and B) residual range (g cm -2 ) Ratio of stopping powers water/air for heavy ions calculated using the computer codes developed by Salamon (for C, Ne, Ar and He) and by Hiraoka and Bichsel (for C). Data for protons and He from ICRU 49. A constant value of s w,air = 1.13 adopted in TRS 398 (ignores fragments)

31 Geitner et al 2006

32 Values of w/e for protons and carbon ions deduced from comparison of ionization chamber 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 2008

33 Proton perturbation factors Palmans et al. 2000

34 intra-track one single track inter-tracks multiple tracks dose or dose-rate independent dose or dose-rate dependent Courtesy of P. Andreo The user should verify recombination corrections against independent method

35 Initial recombination in pp chamber in different LET beams Kanai t. et al., Phys. Med. Biol. (1998) 43,

36 Issues to be resolved in upcoming ICRU report on light-ion beams The proposed approach in upcoming ICRU Report on light-ion beams would ultimately lead to a dosimetry system, where the dose applied to a patient is traceable to the dosimetry standards of the national PSDL. TRS 398 may be adopted for light-ion beam dosimetry with beamline specific adjustments The currently recommended values of s w,air (and W air ) for absolute dosimetry should be re-considered Uncertainties in stopping powers, including those of the I-values for different tissues (5-10%), must be taken into account to reestimate what precision is really achievable in clinical practice

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

38 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

39 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

40 Detectors for measurements in non-reference conditions Passive detectors Active detectors

41 Diodes for characterization of small beams Because of their small size silicon diodes are convenient for profile measurements and characterization of small proton beams. Ionization chamber Diode The response of diodes must always be checked against ionometric measurements before use.

42 Diodes for characterization of small proton beams Newhauser et al, 2003

43 1.1 Characterization of small proton beams 126 MeV protons, collimator 30 mm, no modulation Markus chamber Diode DEB 50 Diamond detector PinPoint chamber Relative dose Depth in water, mm

44 Characterization of scanned proton pencil beams Depth dose distribution Beam monitor calibration Gillin et al, 2010

45 Multi-detector systems for characterization and QA of proton and carbon beams courtesy by PTW Cirio et al, 2004 Nichiporov et al courtesy by IBA

46 Alanine ESR proton dosimetry Advantages: Signal is preserved after readout Exhibits long term stability Near tissue equivalent Linear dose response Disadvantages: Low sensitivity Reduction of Bragg Peak ESR technique is quite elaborous Palmans, 2003

47 2-D dosimetry and QA with fluorescent screen and CCD camera Boon et al, 2000

48 2-D dosimetry and QA with fluorescent screen and CCD camera Courtesy of J. Heese E. Pedroni et al 2005

49 2-D dosimetry and QA with fluorescent screen and CCD camera Rosenthal et al, 2004

50 TLD film for 2-D characterization of clinical proton beams Olko et al, 2004

51 Radiochromic film for characterization of clinical proton beams 1.0 MD- 55 film Kodak XV-2 film 0.8 A. Entrance B. SOBP region relative dose off-axis data, mm

52 Radiochromic film 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 water equivalent depth, cm

53 3-D Gel dosimetry for characterization of clinical proton beams Magic Gel Heufelder et al, 2003

54 3-D Gel dosimetry for characterization of clinical proton beams Heufelder et al, 2003

55 Current status of proton and heavier ion beam dosimetry 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

56 Current status of proton and light-ion beam dosimetry: PROTONS Passive beam delivery Active beam delivery ICRU Report 78 IAEA TRS 398 (center of SOBP) CARBON IONS Active beam delivery IAEA TRS 398 (Plateau or center SOPB) PROTONS Stereotactic radiosurgery MC

57 Acknowledgements Members of ICRU/IAEA Committee Report 78 O. Jäkel and colleagues at DKFZ E. Pedroni and colleagues at PSI H. Palmans