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Measurement of Ionizing Radiation

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Title: Measurement of Ionizing Radiation Author: Joseph Shiau Last modified by: user Created Date: 9/11/2002 4:33:50 PM Document presentation format – PowerPoint PPT presentation

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Title: Measurement of Ionizing Radiation


1
?????????? ????? ???
???? 1. The Physics of Radiation Therapy
Faiz M. Khan 2. Introduction to Radiological
Physics and Radiation Dosimetry Frank H.
Attix,
2
???????????????
3
  • ????????
  • Daily Sun Nuclear Daily QA 2

4
  • ????????
  • Beam uniformity

5
  • ????????
  • Output calibration

6
Measurement of Absorbed Dose
7
The Roentgen
  • The roentgen is an unit of exposure ( X ). The
    ICRU defines X as the quotient of dQ by dm where
    dQ is the absolute value of the total charge of
    the ions of one sign produced in air when all the
    electrons ( or - ) liberated by photons in air
    of mass dm are completely stopped in air.
  • X dQ / dm
  • The SI unit is C/kg but the special unit is
    roentgen ( R )
  • 1R 2.58 10-4 C/kg

8
The Roentgen
  • Charged Particle Equilibrium (CPE ) Electron
    produced outside the collection region, which
    enter the ion-collecting region, is equal to the
    electron produced inside the collection region ,
    which deposit their energy outside the region.

9
Radiation Absorbed Dose
  • Exposure photon beam, in air, Elt3MeV
  • Absorbed dose for all types of ionizing
    radiation
  • Absorbed dose is a measure of the biologically
    significant effects produced by ionizing
    radiation
  • Absorbed dose dE/dm
  • dE is the mean energy imparted by ionizing
    radiation to material of dm
  • The SI unit for absorbed dose is the gray (Gy)
  • 1Gy 1 J/kg
  • ( 1 rad100ergs/g10-2J/kg, 1cGy1rad )

10
Relationship Between Kerma, Exposure, and
Absorbed Dose
  • Kerma ( K ) Kinetic energy released in the
    medium.
  • K dEtr / dm
  • dEtr is the sum of the initial kinetic energies
    of all the charged particles liberated by
    uncharged particles ( photons) in a material of
    mass dm
  • The unit for kerma is the same as for dose, that
    is, J/kg. The name of its SI unit is gray (Gy)

11
Relationship Between Kerma, Exposure, and
Absorbed Dose
  • Kerma ( K ) Kcol and Krad are the collision and
    the radiation parts of kerma
  • K Kcol Krad
  • the photon energy fluence, ?
  • averaged mass energy absorption coefficient, men
    / r

12
Relationship Between Kerma, Exposure, and
Absorbed Dose
  • Exposure and Kerma
  • Exposure is the ionization equivalent of the
    collision kerma in air.
  • X (Kcol)air ( e/w )
  • w/e 33.97 J/C

13
Relationship Between Kerma, Exposure, and
Absorbed Dose
  • Absorbed Dose and Kerma

14
Relationship Between Kerma, Exposure, and
Absorbed Dose
  • Absorbed Dose and Kerma
  • Suppose D1 is the dose at a point in some
    material in a photon beam and another material is
    substituted of a thickness of at least one
    maximum electron range in all directions from the
    point, then D2 , the dose in the second material,
    is related to D1 by

15
Calculation of Absorbed Dose from Exposure
  • Absorbed Dose to Air
  • In the presence of charged particle equilibrium
    (CPE), dose at a point in any medium is equal to
    the collision part of kerma.
  • Dair ( Kcol )air X ( w/e )
  • Dair(rad) 0.876 ( rad/R) X (R)

16
Calculation of Absorbed Dose from Exposure
  • Absorbed Dose to Any Medium
  • Under CPE
  • Dmed / Dair (men/r)med / (men/r )air A
  • A ?med / ?air
  • Dmed(rad) fmed X (R) A
  • fmed roentgen-to-rad conversion factor

17
Calculation of Absorbed Dose from Exposure
  • Absorbed Dose to Any Medium

18
Calculation of Absorbed Dose from Exposure
  • Dose calculation with Ion Chamber In Air
  • For low-energy radiations, chamber wall are thick
    enough to provide CPE.
  • For high-energy radiation, Co-60, build-up cap
    chamber wall to provide CPE.

19
Farmer Chamber
20
Parallel-Plate Chamber
21
Electrometer
22
Calculation of Absorbed Dose from Exposure
  • Dose calculation with Ion Chamber In Air
  • X M Nx D f.s. ftissue X Aeq
  • Nx is the exposure calibration factor for the
    given chamber

23
Calculation of Absorbed Dose from Exposure
  • Dose Measurement from Exposure with Ion Chamber
    in a Medium
  • Dmed M Nx W/e (men/r)med / (men/r)air Am

24
The Bragg-Gray Cavity Theory
  • Limitations when calculate absorbed dose from
    exposure
  • Photon only
  • In air only
  • Photon energy lt3MeV
  • The Bragg-Gray cavity theory, on the other hand,
    may be used without such restrictions to
    calculate dose directly from ion chamber
    measurements in a medium

25
The Bragg-Gray Cavity Theory
  • Bragg-Gray theory
  • The ionization produced in a gas-filled cavity
    placed in a medium is related to the energy
    absorbed in the surrounding medium.
  • When the cavity is sufficiently small, electron
    fluence does not change.
  • Dmed / Dgas ( S / r )med / ( S / r )gas
  • (S / r)med / (S / r)gas mass stopping power
    ratio for the electron crossing the cavity

26
The Bragg-Gray Cavity Theory
  • Bragg-Gray theory
  • Dmed / Dgas ( S / r )med / ( S / r )gas
  • Jgas the ionization charge of one sign produced
    per unit mass of the cavity gas

27
The Bragg-Gray Cavity Theory
  • The Spencer-Attix formulation of the Bragg-Gray
    cavity theory
  • F(E) is the distribution of electron fluence in
    energy
  • L/r is the restricted mass collision stopping
    power with ? as the cutoff energy

28
Effective Point of Measurement
  • Plane Parallel Chambers
  • at the inner surface of the proximal collecting
    plate
  • Cylindrical Chambers
  • Shift proximal to the chamber axis by
  • 0.75r for an electron beam (TG-21)
  • 0.5r for an electron beam (TG-25)
  • 0.6r for photon beams, 0.5r for electron
    beams(TG-51)

29
CALIBRATION OF MEGAVOLTAGE BEAMS TG-21 PROTOCOL
30
Cavity-Gas Calibration Factor (Ngas)
  • The AAPM TG-21 protocol for absorbed dose
    calibration introduced a factor (Ngas ) to
    represent calibration of the cavity gas in terms
    of absorbed dose to the gas in the chamber per
    unit charge or electrometer reading.
  • For an ionization chamber containing air in the
    cavity and exposed to a Go-60 g ray

31
Cavity-Gas Calibration Factor (Ngas)
  • Ngas is derived from Nx and
  • other chamber-related parameters, all
    determined for the calibration energy, e.g.,
    Co-60

32
Cavity-Gas Calibration Factor (Ngas)
  • Once Ngas, is determined, the chamber can be
    used as a calibrated Bragg-Gray cavity to
    determine absorbed dose from photon and electron
    beams of any energy and in phantoms of any
    composition
  • Ngas, is unique to each ionization chamber,
    because it is related to the volume of the chamber

33
Cavity-Gas Calibration Factor (Ngas)
  • Nx XM-1
  • Dgas Jgas ( W/e )
  • Ngas D gas Aion M-1
  • Assume Aion 1
  • Ngas D gas M-1
  • D gas M ( W/e ) / (rair Vc )
  • Ngas ( W/e ) / (rair Vc )
  • if the volume of the chamber is 0.6 cm3, its Ngas
    will be 4.73 107 Gy/C

34
Cavity-Gas Calibration Factor (Ngas)
35
Chamber as a Bragg-Gray Cavity
  • Photon Beams
  • Suppose the chamber, with its build-up cap
    removed (it is recommended not to use buildup cap
    for in-phantom dosimetry), is placed in a medium
    and irradiated by a photon beam of given energy

36
Chamber as a Bragg-Gray Cavity
  • Photon Beams
  • Dose to medium at point P corresponding to the
    center of the chamber will then be
  • P corresponding to the chamber's effective point
    of measurement

37
Chamber as a Bragg-Gray Cavity
  • Photon Beams
  • Pion
  • correction factor for ion recombination losses
  • Prepl
  • corrects for perturbation in the electron and
    photon fluences at point P as a result of
    insertion of the cavity in the medium
  • Pwall
  • accounts for perturbation caused by the wall
    being different from the medium

38
Chamber as a Bragg-Gray Cavity
  • Photon Beams
  • The AAPM values for Prepl and Pwall have been
    derived with the chamber irradiated under the
    conditions of transient electronic equilibrium
    (on the descending exponential part of the depth
    dose curve )

39
Chamber as a Bragg-Gray Cavity
  • Electron Beams
  • When a chamber, with its build-up cap removed, is
    placed in a medium and irradiated by an electron
    beam
  • usually assumed that the chamber wall does not
    introduce any perturbation of the electron
    fluence
  • thin-walled (?0.5 mm) chambers composed of low
    atomic number materials (e.g., graphite, acrylic)
  • Pwall 1

40
Chamber as a Bragg-Gray Cavity
  • Electron Beams
  • For an electron beam of mean energy Ez , at depth
    Z of measurement

41
Chamber as a Bragg-Gray Cavity
  • Electron Beams
  • Prepl
  • fluence correction
  • increases the fluence in the cavity since
    electron scattering out of the cavity is less
    than that expected in the intact medium
  • Gradient correction
  • Displacement in the effective point of
    measurement, which gives rise to a correction if
    the point of measurement is on the sloping part
    of the depth dose curve

42
Chamber as a Bragg-Gray Cavity
  • Electron Beams
  • Recommends that the electron beam calibration be
    made at the point of depth dose maximum
  • Because there is no dose gradient at that depth,
    the gradient correction is ignored
  • Prepl , then, constitutes only a fluence
    correction
  • for cylindrical chambers as a function of mean
    electron energy at the depth of measurement and
    the inner diameter of ion chamber

43
Chamber as a Bragg-Gray Cavity
  • Electron Beams
  • a depth ionization curve can be converted into a
    depth dose curve using

A
A
A
B
B
B
44
Chamber as a Bragg-Gray Cavity
  • Electron Beams
  • The gradient correction, however, is best handled
    by shifting the point of measurement toward the
    surface through a distance of 0.5r
  • For well designed plane-parallel chambers with
    adequate guard rings, both fluence and gradient
    corrections are ignored, i.e., Prep, 1 the
    point of measurement is at the front surface of
    the cavity

45
Calibration Phantom
  • The TG-21 protocol recommends that calibrations
    be expressed in terms of dose to water
  • polystyrene, or acrylic phantoms may be used,
    but, requires that the dose calibration be
    reference to water
  • Scaling factors
  • SF d plastic / d water m water / m plastic

46
Calibration Phantom
  • A calibration phantom must provide at least 5 cm
    margin laterally beyond field borders
  • and at least 10 cm margin in depth beyond the
    point of measurement
  • Calibration depths for a megavoltage photon beams
    are recommended to be between 5- and 10-cm depth,
    depending on energy
  • For electron beams, the calibration depth
    recommended by TG-21 is the depth of dose maximum
    for the reference cone

47
  • ????????
  • Monthly
  • Keithley 35-040 electrometer NE 2571 Farmer
    chamber
  • Victoreen 530 electrometer PTW N30001 Farmer
    chamber
  • Solid phantom Acrylic, Polystyrene, and solid
    water
  • Sun Nuclear Daily QA 2
  • et al.

48
  • ????????
  • Annual
  • Keithley 35-040 electrometer NE 2571 Farmer
    chamber
  • Victoreen 530 electrometer PTW N30001 Farmer
    chamber
  • Solid phantom Acrylic, Polystyrene, and solid
    water
  • Sun Nuclear Daily QA 2
  • WellHoffer water phantom IC 10 chamber
  • et al.

49
  • ??????
  • ???????
  • ???????????
  • ?????

50
INTRODUCTION
AAPM TG-51 has recently developed a new protocol
for the calibration of high-energy photon and
electron beams used in radiation therapy. The
formalism and the dosimetry procedures
recommended in this protocol are based on the
used of an ionization chamber calibrated in terms
of absorbed dose-to-water in a standards
laboratorys Co-60 gamma ray beam. This is
different from the recommendations given in the
AAPM TG-21 protocol, which are based on an
exposure calibration factor of an ionization
chamber in a Co-60 beam. The purpose of this work
is to compare the determination of absorbed
dose-to-water in reference conditions in
high-energy photon and electron beams following
the recommendations given in the two protocols.
51
METHODS AND MATERIALS
  • Calibrations of photon beams ( nominal energy of
    6 and 10 MV ) and electron beams ( nominal energy
    of 6, 8, 10, 12, 15 and 18 MeV ), generated by a
    Siemens KDS-2 linac, are performed.
  • Farmer-type ( NE 2571 ) ionization chamber was
    used for photon beam dosimetry and plane parallel
    ( PTW Markus ) chambers was used for electron
    beam dosimetry.
  • Absorbed-dose-to-water calibration factor, ND,w ,
    and exposure calibration factor, Nx , for
    Farmer-type chamber were provided by NATIONAL
    RADIATION STANDARD LABORATORY INER.
  • Plane-parallel chamber was calibrated against
    calibrated cylindrical chamber in a 18 MeV
    electron beam, as recommended in TG-21, TG-39 and
    TG-51.

60Co
52
METHODS AND MATERIALS
60Co
  • ND,w 4.5394 cGy/nC , expanded uncertainty 1
    ( k2 ). Date of report 2001/12/24, report No.
    NRSL 90084.
  • Nx 4.7179 R/nC , expanded uncertainty 1 (
    k2 ).
  • Date of report 2001/10/17, report No.
    NRSL 90073.
  • Keithley electrometer and Nucleartron water
    phantom were used in this study.
  • The depth of clinical dosimetry for electron
    beams was performed at dref , dref 0.6R50 0.1
    ( cm ), as recommended in TG-51.
  • Depth ionization measurements along the central
    axis were made by using the Markus chamber and
    referenced to that of a 0.12 cm3 RK chamber
    mounted on the head of the machine.

53
RESULTS
60Co
60Co
pp
54
RESULTS
55
RESULTS
56
RESULTS
57
RESULTS
58
RESULTS
59
RESULTS
60
RESULTS
61
Discussion and Conclusion
  • The doses at 10 cm in water for 6 MV and 10 MV
    photon beams and the doses at dref in water for 6
    to 18 MeV electron beams determined with TG-51
    and TG-21 are within 0.3 and 1.4 .
  • According to TG-51, P-P chambers must be used
    for reference dosimetry in electron beams of
    energies 6 MeV or less. In the meantime, NRSL
    provided ND,W factor for Farmer-type chamber
    only. So, the ND,W factor of a P-P chamber should
    be determined by using the cross calibrating
    method.
  • Measurements at the IAEA Dosimetry Lab. have
    shown that at Co-60, the absorbed dose to water
    determined by using the ND,W is about 1 higher
    than that by using the Nx. But, it is different
    in this study. Detailed analysis should be done
    including the data given in the two protocols and
    the calibration factors provided from air-kerma
    and absorbed dose to water.

Co-60
Co-60
Co-60
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