The TG-51 protocol (Med Phys 26, 1847-70, 1999)

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The TG-51 protocol (Med Phys 26, 1847-70, 1999)
The TG-51 protocol is based on absorbed dose to
water calibration (also in a Co-60 beam). The
chamber calibration factor is denoted
. The calibrated chamber can be used in any beam
modality (photon or electron beams) and any
energy, in water. The formalism is simpler than
the TG-21, but it is applicable in water only.
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Equipment Needed

• Ion chamber and electrometer
• calibration traceable to standard laboratory
• waterproofing for ion chamber ( if needed) lt1mm
  PMMA
• water phantom (at least 30x30x30 cm3)
• lead foil for photons 10MV and above
• 1 mm ? 20
• system to measure air pressure and water
  temperature

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Obtain an Absorbed-dose to Water Calibration
Factor
60Co source
dmax
?
D
M (corrected)
Dose to water per unit charge (reading)
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Quality Conversion Factor
Ideally, for a given chamber individual
calibration factor should be
obtained for each beam quality Q used in the
clinic. So that This is impractical, as the
standard laboratory may not have the particular
beam quality Q available, thus a quality
conversion factor kQ is introduced to convert the
calibration factor for Co-60 to that for the beam
quality Q.
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General Formalism
In a Co-60 beam
In any other photon beam Q (only cylindrical
chamber allowed at present)
Converts beam quality (energy) from Co-60 to Q.
In any electron beam R50 (both cylindrical and
parallel-plate chambers allowed)
Converts modality from photon to electron.
Gradient correction for cylindrical chambers
Converts electron energy from ecal to R50.
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Charge measurement
Polarity correction
Press/temperature
Pelec
Electrometer correction
Ion recombination correction
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kQ values for cylindrical chambers in photon beams
NRC-CNRC
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Determination of Photon Beam Quality Q
TG-21 photon beam quality (energy) determined by
the ionization ratio (d20cm to d10cm)
TG-51 photon beam quality (energy) determined by
dd(10)x, the percent depth dose at d10cm due
to photons only.
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Point of Measurement and Effective Point of
Measurement
Effective point of measurement
?
r
point of measurement
rcav
parallel plate
cylindrical
Photon r 0.6 rcav electron r 0.5 rcav
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Percent Depth-Dose (ionization) for photon beams
Percent depth dose to be measured at SSD 100 cm
for a 10?10 cm2 field size.
Parallel-plate chamber measured curve
II. Cylindrical chamber measured curve I, needs
to be shifted by 0.6 rcav to get curve II. Curve
II is the percent dose (percent ionization)
curve, including contaminated electrons.
100
II
I
80
depth-dose (ionization)
60
dd(10)
40
20
5
10
15
20
Depth in water (cm)
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Beam Quality Specification (photons)
For this protocol, the photon beam quality is
specified by dd(10)x, the percent depth-dose at
10 cm depth in water due to the photon component
only, that is, excluding contaminated
electrons. For low energy photons (lt10 MV with
dd(10) lt 75) dd(10)x dd(10) (contaminated
electron is negligible) For high energy photons
(gt10 MV with 75ltdd(10)lt89) dd(10)x ?
1.267dd(10) 20.0 A more accurate method
requires the use of a 1-mm thick lead foil placed
about 50 cm from the surface. dd(10)x
0.89050.00150dd(10)pb dd(10)pb foil at
50 cm, dd(10)pbgt73
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Percent depth dose measured at SSD 100 cm for a
10?10 cm2 field size with a 1mm Pb filter placed
at 50 cm from the surface.
1mm Pb filter
100
II
I (cylindrical chamber)
80
60
( dd)Pb
dd(10)Pb
40
20
50 cm
5
10
15
20
Depth in water (cm)
Curve II is the (dd)Pb curve, with the
contaminated electrons in the original beam
removed, but generates its own contaminated
electrons.
dd(10)x 0.89050.00150dd(10)pb dd(10)pb
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Reference conditions for Photon Beam Calibration
photon source
100 cm
10 cm
10?10 cm2
10?10 cm2
10 cm
water
water
SSD setup
SAD setup
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Photon Beam Dosimetry
where M is the fully corrected (temperature,
pressure, polarity, recombination) chamber
reading, kQ is the quality conversion factor,
is the absorbed dose to water chamber
calibration factor
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Reference conditions for Electron Beams
Electron source

• Depth dref 0.6 R50 - 0.1 cm
• where R50 is the depth in water at which the dose
  is 50 of the maximum dose. dref is approx. at
  dmax
• Field size (why different field sizes for
  different energies?)
• gt 10x10 cm2 on surface R50lt8.5 cm
• gt 20x20 cm2 on surface R50gt8.5 cm
• SSD as used in clinic between 90 cm and 110 cm
  (typically 100 cm)

SSD
dref
water
SSD setup
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Percent Depth-ionization for electron beams
Percent depth ionization to be measured at SSD
100 cm for field size ? 10?10 cm2 (or ?20?20 cm2
for Egt20 MeV).
Parallel-plate chamber measured curve
II. Cylindrical chamber measured curve I, needs
to be shifted by 0.5 rcav to get curve II. Curve
II is the percent ionization curve.
100
II
I
80
depth-ionization
60
40
I50
20
R50 1.029I50 0.06 (cm) for 2?I50 ?
10 cm R50 1.059I50 0.37 (cm) for I50
gt 10 cm
2
4
6
8
Depth in water (cm)
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Electron Beam Dosimetry ( )
depends on users beam must be measured in clinic.
parallel plate chamber
100
80
dref0.5rcav
depth-dose
60
cylindrical chamber
dref
40
20
Same as shifting the point of measurement
downstream by 0.5rcav.
2
4
6
8
Depth in water (cm)
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Electron Beam Dosimetry (Kecal )
Kecal is the photon-to-electron conversion
factor, for an arbitrary electron beam
quality Qecal, taken as R50 7.5 cm. The values
of Kecal are available in the TG-51 protocol.
Parallel-plate chambers
chamber Attix Capintec PTB Exradin Holt Markus NACP
kecal 0.883 0.921 0.901 0.888 0.900 0.905 0.888
cylindrical chambers
chamber Exradin A12 NE2571 NE2581 PR-06 N23331 N30004
kecal 0.906 0.903 0.885 0.900 0.896 0.905
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Electron Beam Dosimetry ( )
is the electron quality conversion factor
converting from Qecal to Q.
for a number of cylindrical and parallel-plate
chambers are available in Figs. 5-8 in the TG-51
protocol. It can also be calculated from the
following expressions
Cylindrical
Parallel-plate
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kR50 for Cylindrical Chambers
NRC-CNRC
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kR50 for Parallel Plate Chambers
NRC-CNRC
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Electron Beam Dosimetry
where M is the fully corrected chamber reading,
is the correction factor that accounts for
the ionization gradient at the point of
measurement (for cylindrical chamber only)
is the electron quality conversion factor. kecal
is the photon to electron conversion factor,
fixed for a given chamber model is the
absorbed dose to water chamber calibration factor
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Summary - photons

• get a traceable
• measure dd(10)Pb with lead foil (shift depth if
  necessary)
• deduce dd(10)x for open beam from dd(10)Pb
• measure Mraw at 10 cm depth in water (no depth
  shift !!!)
• M PionPTPPelecPpol Mraw
• lookup kQ for your chamber

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Summary - electrons

• get a traceable
• measure I50 to give R50 (shift depth if
  necessary)
• deduce dref 0.6 R50 -0.1 cm (approx. at dmax)
• measure Mraw at dref (no depth shift !!!)
• M PionPTPPelecPpol Mraw
• lookup kecal for your chamber
• determine (fig, formula)
• establish (Mraw 2 depths)

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Depth-ionization vs. Depth-dose
For photon beams, depth-ionization is considered
to be equivalent to depth-dose. For electron
beams, depth-ionization must be converted to
depth-dose by multiplying the stopping power
ratio Why?