Measurement of Ionizing Radiation

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????1. The Physics of Radiation Therapy
Faiz M. Khan2. Introduction to Radiological
Physics and Radiation Dosimetry Frank H.
Attix,
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• Daily Sun Nuclear Daily QA 2

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• Beam uniformity

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• Output calibration

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Measurement of Absorbed Dose
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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

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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.

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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 )

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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)
  

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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

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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

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Relationship Between Kerma, Exposure, and
Absorbed Dose

• Absorbed Dose and Kerma

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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

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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)

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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

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Calculation of Absorbed Dose from Exposure

• Absorbed Dose to Any Medium

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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.

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Farmer Chamber
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Parallel-Plate Chamber
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Electrometer
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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

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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

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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

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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

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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

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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

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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)

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CALIBRATION OF MEGAVOLTAGE BEAMS TG-21 PROTOCOL
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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

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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

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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

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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

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Cavity-Gas Calibration Factor (Ngas)
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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

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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

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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

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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 )

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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

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Chamber as a Bragg-Gray Cavity

• Electron Beams
• For an electron beam of mean energy Ez , at depth
  Z of measurement

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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

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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

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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
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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

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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

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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

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• 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.

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• 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.

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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.
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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
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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.

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RESULTS
60Co
60Co
pp
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RESULTS
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RESULTS
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RESULTS
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RESULTS
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RESULTS
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RESULTS
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RESULTS
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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