The AAPM TG51 Protocol

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The AAPM TG-51 Protocol
D.W.O. Rogers National Research Council of
Canada Ottawa http//www.irs.inms.nrc.ca/inms/irs
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AAPMs TG-51 Protocol
Clinical reference dosimetry of high- energy
photon and electron beams
60Co to 50 MV photons 5 to 50 MeV
electrons
Peter Almond (chair)
Peter Biggs
Bert Coursey
Saiful Huq
Will Hanson
Ravinder Nath
Dave Rogers
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Current Clinical Practice

• use ion chambers
• use NK , air-kerma calibration factor to
  determine Ngas
• basically just mgas
• use cavity theory lots of data to determine the
  absorbed dose to water

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Problems with current practice

• air-kerma based protocols are very complex
• absorbed dose from air-kerma calibration
• calibration in 60Co but want dose in accelerator
  beams
• uncertainty is too large
• air-kerma standards have problems

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Absorbed-dose standards

• standard is for quantity of interest
• different standardsgt robust system
• only for photons (at present)
• very simple protocols possible

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Absorbed-dose standards
3 different approaches - different systematic
uncertainties

• graphite calorimeter (NPL, BIPM,NIST, NRC)
• water calorimeter (NRC, NIST, Gent)
• total energy absorption (PTB)

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Clinical photon dosimetry

• calibrate chambers in each appropriate beam
  quality
• very expensive/impossible in North America
  (ADCLs dont have accelerators)

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Clinical photon dosimetry

• create a new protocol based on absorbed-dose
  calibration in 60Co beams
• much simpler for users
• much easier for calibration labs

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Starts with
absorbed dose to water calibration factor
absorbed dose to water under reference
conditions in a beam of quality Q, at point of
measurement of chamber in its absence (Gy)
fully corrected chamber reading (C)
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Traceability
ND,w must be traceable to the national primary
standard for absorbed dose
In practice, calibrate at an ADCL, NIST or
NRCC Need two independent checks of calibration
in the clinic
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In-water ? waterproof
Ideal waterproof chamber
Waterproofing sheath lt1mm of PMMA or thin latex
rubber
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General formalism

• kQ is the quality conversion factor
• it accounts for ND,w variation with Q

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Photon beams
TG-51 contains kQ values for all known reference
level ion chambers kQ varies by 5
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Electron beams
kQ has 3 components measured in clinic
presented in protocol vs R50
kecal converts to
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Formalism summary

• photons
• electrons

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Measurement phantoms
Reference dosimetry is done in a water phantom
ONLY, at least 30x30x30 cm3
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Charge measurement
Polarity correction Press/temp Electrometer
correction Pelec
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Pion recombination correction
Lower voltages preferred Pion lt1.05
Linear approx. to 2-voltage technique for pulsed
beams
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Beam quality specification
To allow selection of kQ factors Photons dd(10)x
instead of . Electrons R50
directly instead of Eo from R50
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Photon beam quality
dd(10)x depth dose at 10cm from photon
component (i.e. excluding e- contamination) for
10x10cm2 beam at surface for SSD100cm
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Measuring dd(10)
Pgr is 1 at dmax up to 1 less at 10 cm
depth. For measuring depth-dose only, use
effective point of measurement. shift depth-dose
upstream by 0.6rcav
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Photon 0.6rcav depth-shift
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Electron contamination
At 10 MV above, electron contamination affects
Dmax and hence dd(10) dd(10)x 1.2667 dd(10)
- 20.0 where dd(10) is measured value
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blank
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Electron contamination
accelerator head
1mm lead removes variable e- adds known e-
Variable e-
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Lead foil correction

• e- contamination is machine specific
• remove head e- using a 1 mm lead foil at 50cm
  from phantom
• calculate e- effects from lead
• dd(10)x
• 0.89050.00150dd(10)Pbdd(10)Pb

Open beam
Lead in place
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Problems with TPR
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Why use dd(10)x
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Reference Conditions Photons

• depth 10 g/cm2
• field size 10 x 10 cm2
• SSD or SAD setups are allowed
• 60Co calibration factors at 5 g/cm2 in an
  SSD100cm setup are acceptable

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Reference Conditions Photons
SAD
SSD
10 cm
10x10
10x10
10 cm
SSD setup SAD setup
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Clinical Photon Dosimetry
ideally measure kQ using primary standards
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Measured kQ values
Seuntjens et al
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kQ - cylindrical chambers
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kQ comparison to previous
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e- beam quality
R501.029 I50 - 0.063 cm or use good
diode
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R501.029 I50 - 0.063 cm
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Reference conditions e-
Depth dref 0.6 R50 - 0.1 cm field size gt
10x10 cm2 on surface R50lt8.5 cm gt 20x20 cm2 on
surface R50gt8.5 cm SSD as used in clinic
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Dref 0.6 R50 - 0.1 cm
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Electron beam dosimetry
depends on users beam must be measured in clinic
cylindrical
plane-parallel
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kQ for electron beams
converts to
contains chamber variation
contains all R50 variation
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Why so complex for e-?

• converts to

• can be measured
• used with plane-parallel cross-calibration

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Why so complex for e-?
1 for plane-parallel
same for all Farmer chambers same for all
well-guarded p-p
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Plane-parallel chambers
Preferred - cross-calibrate against calibrated
cylindrical chamber
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Plane-parallel chambers
from cyl
Alternatively, use kecal, and proceed
as usual.
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Dose change 60Co (Can)
chamber TG-21 best calc NE2571
1.004 1.001 NE2581
1.009 1.007 NE2561 1.004
1.001 PR-06 1.009
1.005 Exradin A12 1.003
0.998 PTW30001 1.003 0.999
Seuntjens et al, 1999
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Dose change 60Co (US)
chamber TG-21 best calc NE2571
1.015 1.012 NE2581
1.020 1.018 NE2561 1.015
1.012 PR-06 1.020
1.016 Exradin A12 1.014
1.009 PTW30001 1.014 1.010
Seuntjens et al Shortt et al
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Dose change photons (Can)
Dose TG-51/Dose TG-21 at 10 cm
PR-06 NE2571 SL20
6 MV 1.002 0.996 18 MV
1.000 1.002 KD2 6 MV
1.001 0.995 18 MV 0.997
0.997
Ding et al
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Dose change photons (US)
Dose TG-51/Dose TG-21 at 10 cm
PR-06 NE2571 SL20
6 MV 1.012 1.007 18 MV
1.011 1.013 KD2 6 MV
1.012 1.006 18 MV 1.008
1.008
Ding et al Shortt et al
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Dose change electrons (Can)
Dose TG-51/Dose TG-21 at dmax
Energy kecal cross-calibration
6 1.006 0.991 9
1.013 0.998 11 1.014
0.998 13 1.016
1.001 17 1.031 1.015 20
1.026 1.010
Cygler et al
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Dose change electrons (US)
Dose TG-51/Dose TG-21 at dmax
Energy kecal cross-calibration
6 1.017 1.002 9
1.024 1.003 11 1.025
1.003 13 1.027
1.012 17 1.042 1.026 20
1.037 1.021
Cygler et al Shortt et al
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What have we gained?
photons

• uses consistent stopping-power ratios
• more accurate beam quality specification
• accounts for aluminium electrodes
• avoids in-air to in-water conversion

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What have we gained?
electrons

• avoids Eo at surface
• avoids extensive tables of stopping-power ratios
• uses realistic e- beams in sprs

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What have we gained?
Simplicity
For photons, ignore everything answer is still
correct within 5 (slightly worse for e-)
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What have we gained?
Accuracy
Standards of air-kerma absorbed-dose have same
uncertainty. Protocol avoids step from air-kerma
to absorbed dose. kQ can be directly measured
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Summary - photons

• get a traceable
• measure dd(10) with lead foil
• deduce dd(10)x for open beam
• measure Mraw at 10 g/cm2
• M PionPTPPelecPpol Mraw

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Summary -photons (cont)

• lookup kQ for your chamber

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Summary - electrons

• get a traceable
• measure I50 to give R50
• deduce dref 0.6 R50 -0.1 cm
• measure Mraw at dref
• M PionPTPPelecPpol Mraw

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Summary -electrons(cont)

• lookup kecal for your chamber
• determine (fig, formula)
• establish (Mraw 2 depths)

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Equipment Needed

• Ion chamber and electrometer
• calibration traceable to national std
• equipment for 2 independent checks
• voltage supply (2 voltages, both signs)
• waterproofing for ion chamber ( if needed)
  lt1mm PMMA

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Equipment Needed

• 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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Status of TG-51

• TG-51 is approved by AAPM
• in press, Med. Phys. (Sept issue)
• ADCLs ready to provide absorbed dose calibration
  factors
• round robins tighter than for air kerma

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Conclusions

• TG-51 simplifies clinical reference dosimetry
• TG-51 improves the accuracy of clinical reference
  dosimetry
• presentation on-line at
• www.irs.inms.nrc.ca/inms/irs