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Klinik und Poliklinik für Strahlentherapie Direktor: Prof. Dr. Dr. M. Flentje Small Field Dosimetry Relative dosimetry: detector choice and corrections Otto A Sauer Otto A Sauer

Klinik und Poliklinik für Strahlentherapie Direktor: Prof. Dr. Dr. M. … · 2010. 7. 20. · Klinik und Poliklinik für Strahlentherapie Direktor: Prof. Dr. Dr. M. Flentje Small

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  • Klinik und Poliklinik für StrahlentherapieDirektor: Prof. Dr. Dr. M. Flentje

    Small Field DosimetryRelative dosimetry: detector choice and corrections

    Otto A SauerOtto A Sauer

  • Relative dosimetry

    Measurement tasks:

    • Rel. profiles (OARs)

    • Rel. depth dose• Tissue phantom ratios (TPR)

    2Otto Sauer: small field dosimetry 20.07.2010

    • Total scatter factor (output measured in phantom, Scp)

    • Head scatter factor (output measured in air Sc)

    • Phantom scatter factor Sp

  • Necessary and desired detector properties

    • High spatial resolution• High signal, low noise• Low energy dependence• Low directional dependence• high dose, resp. dose rate

    linearity

    • Some detectors

    Ionization chamber PTW

    31010, 0.125cc

    Ionization chamber PTW

    31006, PinPoint, 0.015cc

    Shielded Si-diode Sc.-Wellhöfer PFD Photon, 0.0003cc

    3Otto Sauer: small field dosimetry 20.07.2010

    linearity• High stability• Handy, easy to use in a

    clinic, robust• inexpensive

    0.0003cc

    Mini Si-diode Sc.-Wellhöfer SFD stereotactic, 0.000017cc

    Si-MOSFET Thomson &

    Nielsen TN-502 RD

    Diamond detector PTW

    60003, ca. 0.002cc

    microLion Chamber 0.0017cc

  • Detector response

    Reference dosimetry:• Product of correction factors should be known

    (use CoP)

    ( ) ( ) ( )rrr NMkD ⋅= ∏

    4Otto Sauer: small field dosimetry 20.07.2010

    (use CoP)

    Relative dosimetry:• Product of correction factors k should be constant or

    known

  • Detectors

    • Polymer gel – 3D, difficult readout, calibration, preparation

    • Radiochromic film – 2D, difficult calibration, stability of scanning

    • Micro ionization chamber– Volume effect, signal/noise, background

    • TLD, alanine, MOSFET, all non direct reading 1D detectors

    5Otto Sauer: small field dosimetry 20.07.2010

    • TLD, alanine, MOSFET, all non direct reading 1D detectors– Position uncertainty

    • Diamond– Dose rate dependence, volume effect, robustness

    • Si-diodes– High spatial resolution, energy dependent response

  • Polymer gel dosimetry / volume effect

    *: MC(Verhagen et al PMB 43 1998 2755–2768)

    Gel

    Pappas et al MP 32 2005 1513–1520

    6Otto Sauer: small field dosimetry 20.07.2010

  • Volume effect (1)

    • Mean relative dose on the sensitive area of the detector perpendicular to the beam axis

    • (assumption:

    7Otto Sauer: small field dosimetry 20.07.2010

    • (assumption: constant sensitivity

    • → volume effect is overestimated)

    Kawachi et al. Medical Physics, Vol. 35, No. 10, 2008, 4591f

  • Volume effect (2)

    • Deconvolution with the detector response function K

    • Dm: measured dose

    • D: “true” dose

    • F:

    8Otto Sauer: small field dosimetry 20.07.2010

    • F: Fourier transform

    [ ] [ ] [ ])()()(

    )()()(

    xuKFuDFxDF

    duxuKuDxD

    m

    m

    −=

    −= ∫+∞

    ∞−

    Garcia-Vicente et al Med. Phys. 25, 1998, 202f

  • Dose rate dependence

    A diamond detector• may have a huge dose rate

    dependence• is not really small (> 3mm)• has a huge production

    spread• has low energy

    liquid ionization chamber

    Single & double diode

    9Otto Sauer: small field dosimetry 20.07.2010

    • has low energy dependence

    Natural diamondDetector response for different dose rates in a 6 MV bremsstrahlung beam expressedas ratios of detector response and dose rate, determined by a Farmer ionization chamber, andnormalized at the lowest dose rate. The error bars represent the statistical uncertainty (1SD).[Westermark et al. Phys. Med. Biol. 45 (2000) 685–702]

  • Energy dependence, cavity theory

    Silicon detector

    ( ) detdetdetdet

    1 DkkDL

    LkD Ep

    wwpw ′′′=′

    −+′= α

    µµα

    10Otto Sauer: small field dosimetry 20.07.2010

    • strong increase of response below hν = 200 keV

    • Small: width < 1mm

  • Relative dose from photons below 200 keV for 60Co• 5cm x 5cm @ 1cm and @ 20cm depth

    11Otto Sauer: small field dosimetry 20.07.2010

    N Chofor et al 2007 Phys. Med. Biol. 52 N137 doi: 10.1088/0031-9155/52/7/N03and personal communication

    Possible change of response outside the useful beam

  • Response versus field size

    • Decreasing response of Si-detectors with decreasing field size

    • Reason: depletion of low energy photons

    • For small fields volume averaging of the 0.125 cm³ large IC is dominant

    12Otto Sauer: small field dosimetry 20.07.2010

    Sauer, Wilbert MP 34, 2007, 1983-1988

  • perturbation

    Ionization chamber:• wall, central electrode,

    displacement, cavity,• (saturation, humidity, T&p, polarization)

    Diode:• housing, shielding,

    13Otto Sauer: small field dosimetry 20.07.2010

    • housing, shielding, contacting

    • Field size dependence of perturbation factors?

    C. McKerracher, D.I. Thwaites / Radiotherapy and Oncology 79 (2006) 348–351

    Assumption: perturbation for solid state detectors is approx. constant as long as field is larger than detector diameter.

    5mm7mm

  • Profile measurements

    Recommendations• To detect the penumbra correctly use a small diode• Check the detector response outside the geometrical field• Correct for over/under-response or use an appropriate

    detector. [e.g. (shielded) diode or radiochromic film]

    Off axis profiles at the

    14Otto Sauer: small field dosimetry 20.07.2010

    Off axis profiles at the isocentre (FAD D 100 cm), 6 cm deep, measured withdiamond, diode, and film and calculated using MC (EGS4), for (a) 7 mm and

    (b) 23 mm SRScollimators.

    Heydarian et al PMB 41 (1996) 93–110

  • Profile measurements: directional dependence

    15Otto Sauer: small field dosimetry 20.07.2010

    Beddar el al (1994), PMB 37(10)

    Dasu el al (1998), PMB 43(1)

  • Depth dose and TPR measurements

    • Use a small diode• (energy dependence, i.e. depth

    dependence of response is hardly effective for small fields)

    • Estimate the changing volume effect with depth

    • Carefully align CAX with scan

    16Otto Sauer: small field dosimetry 20.07.2010

    • Carefully align CAX with scan direction

    • Measure TPR directly• Examine data

    – Fit to a function– Corroborate by different means

  • Output measurement, total scatter factor Scp

    Field size dependence of

    ( ) ( ) ( ) ( )

    ( )( )zAD

    zADS

    DAkAkDAksD

    refw

    wcp

    Epw

    ,

    ,

    detdet

    =

    ==

    0 2 4 6 8 10 12 14 16 180.0

    0.2

    0.4

    0.6

    0.8

    1.06 MV meas.

    IC PiP DiGre DiYe MOS1 MOS2 DIAre

    lativ

    e D

    ose

    SES / cm

    0.0 0.5 1.0 1.5 2.0 2.50.0

    0.2

    0.4

    0.6

    0.8

    17Otto Sauer: small field dosimetry 20.07.2010

    Field size dependence of• perturbation correction factor

    kp?• energy correction factor kE

    for different detectors ?• volume effect

    0 2 4 6 8 10 12 14 16 180.0

    0.2

    0.4

    0.6

    0.8

    1.06 MV corr.

    IC PiP DiGre DiYe MOS1 MOS2 DIA

    Dos

    e / G

    y/10

    0MU

    SES / cm

    0.0 0.5 1.0 1.5 2.0 2.50.0

    0.2

    0.4

    0.6

    0.8

    ( )brnn

    n

    eSrk

    rPD −∞∞ −++

    = 1

  • Output measurement Scp• measurement with an ionization chamber

    high perturbation?

    ( ) ( )( )( )

    ( ) [ ]A

    Aairwater,refref

    ref

    refrefw

    refwcp

    reff

    z,AM

    zA,M

    z,AD

    zA,DAS ==

    18Otto Sauer: small field dosimetry 20.07.2010

    •R. Capote et al., Med. Phys. 31(9), (2004) 2454–2465 : ∼ 3% inside the irregular (∼2×2cm2) IMRT-field and outside field ∼ 9%

    •H. Bouchard and J. Seuntjens, Med. Phys. 31(9), (2004): 10% in a realistic dynamic IMRT field and large for fields that provoke the lateral electron equilibrium (LEE)

    •A. L. McNiven et. al., Med. Phys. 33(11), (2006) 3997-4004: The conventional approach ofcalculating this dose conversion factor, using a ratio of detector readings only, is not suitable for small-

    field dosimetry unless the conversion factor can be shown to be unity!

  • Output measurement Scp• measurement with a silicon detector

    high energy dependence

    ( ) ( )( )( )

    ( ) [ ] [ ]A

    A

    diodeA

    A

    diode

    refref

    ref

    refref

    refdiodecp

    refErefp

    kkz,AM

    zA,M

    z,AD

    zA,DAS ==

    19Otto Sauer: small field dosimetry 20.07.2010

    kp = 1

    for detector < A

    kE = as+b

  • Output measurement Scp• Cross-calibrate a Si-diode at medium

    field sizes with a small IC• Evaluate energy/field size dependence• Avoid volume effects (use a small

    detector)

    1.04 6 MV IC

    Li et al MP 22, 1995, 1167-1170

    20Otto Sauer: small field dosimetry 20.07.2010

    0 2 4 6 8 10 12 14 16 18

    0.96

    0.98

    1.00

    1.02

    DiYe MOS1 MOS2 DIA

    IC PiP DiGre

    Sig

    nal r

    atio

    s

    SES / cm

    Eklund, Ahnesjö PMB 2010

    LEE

    Sauer, Wilbert MP 34, 2007, 1983-1988

  • Standard uncertainty: Type B (non -statistical)

    0.02

    0.03MOSDiGre

    DiYe

    DIA

    STDs

    raw06 corr06 raw10 corr10

    stan

    dard

    dev

    iatio

    n / G

    y/10

    0MU

    0 2 4 6 8 10

    0.28

    0.30

    0.32

    0.34

    0.36

    10 MV

    6 MV

    corrected dose for SES = 4 mm

    dos

    e /G

    y

    approx. detector area / mm²

    21Otto Sauer: small field dosimetry 20.07.2010

    0.0 0.8 1.6 2.4 4 8 12 160.00

    0.01

    stan

    dard

    dev

    iatio

    n / G

    y/10

    0MU

    SES / cm

    approx. detector area / mm²

    Decrease for s> 8mm, increase for s=4mm, p?

  • Head/Collimator scatter SC and phantom scatter SP• Some planning systems want SP and SC data

    • SC• Use a cap on a diode• Brass or other high Z is acceptable

    • SP• calculate from S and S

    22Otto Sauer: small field dosimetry 20.07.2010

    • calculate from SCP and SP• Use tabulated SP data

    • Alternatively:• Calculate SC from tabulated SP data and measured SCP

  • Conclusions

    • For small photon fields, detector response is not always known with high certainty

    • A small (shielded) diode works, • determine dependence of response on energy spectrum and other

    dependences for each individual detector

    23Otto Sauer: small field dosimetry 20.07.2010

    • Corroborate the measurements by– Using different detectors– Fitting data to analytical functions– Survey the literature

  • Thank you!

    24Otto Sauer: small field dosimetry 20.07.2010

    [email protected]