dosimetry and qa of proton and heavier ion beams

Dosimetry and QA of proton and heavier ion beams

Stanislav VatnitskiyEBG MedAustron GmbH

Wiener Neustadt, Austria

PTCOG 49 Educational WH 17-19 May 2010, NIRS Japan S. Vatnitskiy

Content

• Introduction• Reference dosimetry

Methods, detectors, protocols• Dosimetry in non-reference conditions and QA

measurements Methods, detectors

• Recommendations

about 25 treatment facilities are established

another 20 proton and light-ion beam centers

are planned to be open in next 5 years

• 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

Dosimetry tasks in proton and heavier ion beam radiotherapy

Consistent andharmonized

dosimetry guidelines

Accurate beam calibration

Perform planning of high-precision conformal therapy

Provide interchangeof clinical experience

and treatment protocols between facilities

Ensure exact delivery of prescribed dose

Provide standardizationof dosimetry in radiobiology

experiments

• 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

Reference dosimetry

Absorbed dosedeterminationin reference

conditions for proton and heavier

ion beamsCalorimeter

Thimble air-filled

ionizationchamber

Faraday Cup

Lack of national andinternational

dosimetry standards

Dw = (N/A) (S/ρ)w * 1.602 x 10 -10

N : number of protons per MU collected in the FCA : effective area of the beam (S/ρ)w : stopping power for water at the incident proton energy

FC-based calibration of the ionization chamber

10( , ) ( / ) 1.602 10w wND Q FC SA

ρ −= × × ×, ,

( , )wD w FC corr

D Q FCNM

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Institution

1995 PTCOG Dosimetry intercomparison

Institution’s statement of proton dose

FC calibration

Main characteristics of calorimetry dosimetry:

• Energy imparted to matter by radiation causes an increase in temperature

• Dose absorbed in the sensitive volume is proportional to

• is measured with thermocouples or thermistors.

• Calorimetric dosimetry is the most precise of all absolute dosimetry techniques.

Calorimetry-based calibration of the ionization chamber

• Two types of absorbed dose calorimeter are used in charged particle beams:

In graphite calorimetersthe average temperaturerise is measured in a graphite body that is thermally insulated from surrounding bodies (jackets) by evacuated vacuum gaps.

Calorimetry-based calibrationof 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.

( , ) (1 )w TD Q cal c V D= ×Θ×∆ × +

( , ) (1 )w TD Q cal c V D= ×Θ×∆ × + , ,( , )w

D w c corr

D Q calNM

Water calorimeteryin a 250 MeV proton beam at LLUMC

Water Calorimeter

Ion chamber measurements (dummy calorimeter) in a 250 MeV proton beam

Dummy calorimeter with IC

Graphite calorimetry protons at CCO carbon ions at NIRS

Palmans et al 2004

Sakama et al 2008

Absorbed dosedeterminationin reference

conditions for light ionbeams

Calorimeter

Thimble air-filled

ionizationchamber

Faraday Cup

Lack of national andinternational

dosimetry standards

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

ND,w - based formalism - IAEA TRS-398

Dw(zref) at any user quality Q(photons, electrons, protons, heavier ions)

Dw,Q= MQ ND,w,Qo kQ,Qo

beamqualityfactor

calibrationcoefficientat Qo

corrected instrument reading at Q

• Both cylindrical and parallel platechambers are recommended for reference dosimetry

• Parallel plate chambers yield higher uc in absolute Dw, though are better suited for relative dosimetry

• cylindrical chambers recommended for SOBP width>2 cm

• For SOBP widths < 2 cm parallel plate chambers must be used

Ionization chambers

ICRU/IAEA Report 78:dosimetry equipment

• Versatile electrometer with cable and connectors fitting to the electrometer and all chambers

• thermometer, barometer

Water phantoms for reference dosimetry

Water is the only standard and most universal phantom

material for proton and heavier ions dosimetry measurements

Reference calibration of protonand heavier ion beams: reference conditions

Passive Scatteringprotons,

carbon ions

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

Goitein, Lomax and Pedroni, 2002

Calibration at SOBP

Protonsspot scanning

phantomprotons

ion chamber

depth, cm0 10 20 30 40

off-axis distance, cm-12 -8 -4 0 4 8 12

Reference geometrypassive beam delivery

Proton spot scanning calibration at PSI

( Pedroni et al)

• dose model predictsthe number of protons/Gy

• 10x10x10 cm3 box is filled inwith homogeneous dose distribution to a dose of 1Gy

10x10x10

• ion chamber is placedat a residual range 5g/cm2

• 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 monoenergeticBragg 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

Lack of standards => Qo = 60Co

( )( )

( )( ) 606060

w air airQ Q Q

Qairw air CoCoCo

s W pk

Q dis wall cav celp p p p p=≈ 1 for protons

≈ 1 for heavier ions

≠ 1 for 60Co

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.

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

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.

0 5 10 15 20 25 301.120

protonsalpha particles

carbon ions

C (Salamon) Ne (Salamon) Ar (Salamon) He (Salamon) p (ICRU-49) He (ICRU-49) C (H and B)

residual range (g cm-2)

A constant value of sw,air = 1.13 adopted in TRS 398 (ignores fragments)

Geitner et al 2006

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

chamber and calorimeter measurements

0 50 100 150 200

Proton energy / MeV

r/e) p

Delacroix et al., 1997Palmans et al., 2004Palmans et al., 1996Hashemian et al., 2003Siebers et al., 1995Schulz et al., 1992Brede et al., 2006Medin et al., 2006

Sakama et al 2008ICRU 78

Proton beam

34.2 J C-1

35.7 J C-1

34.5 J C-1

TRS 398

Proton perturbation factors

Palmans et al. 2000

recombination

initial(columnar)

general(volume)

“intra-track”one single track

dose or dose-rate independent

“inter-tracks”multiple tracks

dose or dose-rate dependent

Courtesy of P. Andreo

The user should verify recombination corrections against independent method

Kanai t. et al., Phys. Med. Biol. (1998) 43, 3549-3558

Initial recombination in pp chamber in different LET beams

Issues to be resolved in upcoming ICRU report on light-ion beams

• TRS 398 may be adopted for light-ion beam dosimetry with beam-line specific adjustments

• The currently recommended values of sw,air (and Wair) 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 re-estimate what “precision” is really achievable in clinical practice

The proposed approach in upcoming ICRU Report on light-ion beamswould ultimately lead to a dosimetry system, where the dose applied to a patient is traceable to the dosimetry standards of the national PSDL.

Standard uncertainties in Dw (TRS 398, ICRU 78)

u(ND,wSSDL) = 0.6 kQ calc

Co-60 gamma-rays 0.9High-energy photons 1.5High-energy electrons 1.4-2.1

Proton beams 2.0-2.3Heavier ions 3.0-3.4

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 characteristicsDepth doseLateral profilesOutput factors

Active detectors:Ion chambers, diodes, diamond detector, scintillators(single and multiple)

Passive detectors:Destructive – TLDNon-destructive –Films, alanine

Direct display of the current dose rate or the accumulating dose

Probe accumulates the dose during irradiation. The value of dose is obtained after irradiation with read-out device

Detectors for measurements in non-reference conditions

Active detectors

Passive detectors

Because of their small size silicon diodes are convenient for profile measurements and characterization of small proton beams.

The response of diodes must always be checkedagainst ionometric measurements before use.

Ionization chamber Diode

Diodes for characterization of small beams

Diodes for characterization of small proton beams

Newhauser et al, 2003

Depth in water, mm

0 20 40 60 80 100 120

Markus chamberDiode DEB 50Diamond detectorPinPoint chamber

126 MeV protons, collimator 30 mm, no modulation

Characterization of small proton beams

Characterization of scanned proton pencil beams

Gillin et al, 2010

Depth dose distribution Beam monitor calibration

Multi-detector systemsfor characterization and QA of proton and carbon beams

Cirio et al, 2004

courtesy by PTW

courtesy by IBANichiporov et al. 2007

Palmans, 2003

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

Alanine ESR proton dosimetry

2-D dosimetry and QA with fluorescent screen and CCD camera

Boon et al, 2000

Courtesy of J. Heese

E. Pedroni et al 2005

Rosenthal et al, 2004

TLD film for 2-D characterization of clinical proton beams

Olko et al, 2004

off-axis data, mm

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

1.0 MD- 55 filmKodak XV-2 film

A. Entrance B. SOBP region

Radiochromic film for characterization of clinical proton beams

155 MeV, modulation 3 cm, normalized at 11 cm

water equivalent depth, cm

0 2 4 6 8 10 12 14 16

1.1parallel plate ionization chamber

MD-55 film, 70 Gy

Radiochromic film for characterization of clinical proton beams

3-D Gel dosimetry for characterization of clinical proton beams

Heufelder et al, 2003

Magic Gel

Heufelder et al, 2003

3-D Gel dosimetry for characterization of clinical proton beams

Implementation of ICRU Report 78IAEA TRS 398

provide a level of accuracycomparable to that

in calibration of photon and electron beams

harmonize clinical dosimetryat proton and heavier

ion beam facilities

Current status of proton and heavier ion beam dosimetry

PROTONSStereotactic radiosurgery

CARBON IONSActive beam delivery

PROTONS Passive beam deliveryActive beam delivery

Current status of proton and light-ion beam dosimetry:

ICRU Report 78IAEA TRS 398(center of SOBP)

IAEA TRS 398(Plateau or center

Acknowledgements

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

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