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Kilovoltage X-ray dosimetry in the clinic Philip Mayles The Clatterbridge Cancer Centre
Kilovoltage X-ray dosimetry
in the clinic
Philip Mayles
The Clatterbridge Cancer Centre
Kilovoltage Codes of Practice
Germany DIN 1988 and 1996
UK IPEMB (IPEM) 1996 and 2005
IAEA TRS 277 2nd edn 1997
Holland NCS 1997
USA AAPM TG61 2001
IAEA TRS 398 2004
Formula for Medium Energy
• Measure at 2 cm deep
IPEM COP Klevenhagen et al Phys Med Biol 41 2605-2625 1996
In phantom
Formula for Low Energy
• Measure in air
In air
IPEM COP Klevenhagen et al Phys Med Biol 41 2605-2625 1996
Formula for Low Energy
• Measure in air
AAPM COP Ma et al Med Phys 28 868-893 2001
In air
Formula for Low Energy
• Measure in air
In air
IPEMB recommends use of the in air protocol if
the prescription is to the max dose for medium
energy
IPEMB Addendum Aukett et al Phys Med Biol 50 2739-2748 2005
IPEM code for Very Low Energy
• Measure at surface of phantom
• Note that kch is based on measurement
comparing to in air measurement
IPEMB COP Klevenhagen et al Phys Med Biol 41 2605-2625 1996
IPEMB Addendum Aukett et al Phys Med Biol 50 2739-2748 2005
In phantom
Grenz ray chamber
PTW23342
In phantom
Grenz ray chamber
PTW23342
In phantom
How are Low and Medium Energy
defined ?
• IPEMB based on HVL
– Medium energy 0.5 - 4 mm Cu (>160 kV)
– Low energy 1 – 8 mm Al (50–160 kV)
– Very low energy 0.035-1 mm Al (8–50 kV)
How is Energy defined ?
• Penetrative power
– Depth dose
– First Half Value Layer
• Energy spectrum
– Generating potential
– Filtration
Measurement of HVL
In air
AAPM COP Ma et al Med Phys 28 868-893 2001
Specification of Beam Quality
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 50 100 150 200 250 300
kV
HV
L m
m A
l
Standards Lab
TECDOC 1455
Clatterbridge
0
0.5
1
1.5
2
2.5
3
0 20 40 60 80 100 120
kV
HV
L m
m A
l
Standards Lab
TECDOC 1455
Clatterbridge
Low Energy Medium Energy
Adapted from TRS398
Does Beam Quality Matter ?
HVL mm Al
Nk
Factors in Calculation: M
• Electrometer reading corrected for
temperature, pressure, ion recombination,
polarity effect and electrometer accuracy
In air In phantom
AAPM COP Ma et al Med Phys 28 868-893 2001
Factors in Calculation: M
• Electrometer reading corrected for
temperature, pressure, ion recombination,
polarity effect and electrometer accuracy
In air In phantom
IPEM COP Klevenhagen et al Phys Med Biol 41 2605-2625 1996
Factors in Calculation: M
• Recombination correction
VH and VL are the high and low voltages
and MH and ML are the meter readings
Important for short SSDs
In air In phantom
AAPM COP Ma et al Med Phys 28 868-893 2001
Factors in Calculation: μen/ρ air
• AAPM and IPEMB agree to better than
0.3% but the sources of data were not
independent
In air
Dose is tissue dependent μen/ρ air
AAPM COP Ma et al Med Phys 28 868-893 2001
In air
Factors in Calculation: μen/ρ 2cm
In phantom
AAPM COP Ma et al Med Phys 28 868-893 2001
1 10 100
Factors in Calculation: Bw
• Since the backscatter factor is
fundamentally a water-kerma ratio, reliable
measurements are non-trivial. Therefore
for the application of this protocol
backscatter factors should not be
measured in the clinic
In air
AAPM COP Ma et al Med Phys 28 868-893 2001
Factors in Calculation: Bw
• Since the backscatter factor is
fundamentally a water-kerma ratio, reliable
measurements are non-trivial. Therefore
for the application of this protocol
backscatter factors should not be
measured in the clinic
• Largest difference AAPM:IPEMB is 0.4%
In air
AAPM COP Ma et al Med Phys 28 868-893 2001
Factors in Calculation: Pstem
• 1.0 if field size for calibration is same as
measurement
• Likely to be important only for plane
parallel chambers
• Need to know the value for the reference
chamber
In air
Factors in Calculation: kch
• AAPM: PQchamPsheath
• IPEM: kch
In phantom
Factors in Calculation: kch
• AAPM: PQchamPsheath
• IPEM: kch
• TRS277: kupu
In phantom
Phantom Materials
• Compared Solid Water (RMI Gammex) and
Plastic Water (Computerized Imaging Reference Systems) to H2O
• Percentage Depth Dose
– Solid water: up to 2.4% difference (75kVp beam 40mm deep)
– Plastic water: up to 23.2% difference (75kVp beam 2mm deep)
In phantom
Hill et al Phys Med Biol 50 N331-N344 2005
Phantom Materials
• Compared Solid Water (RMI Gammex) and
Plastic Water (Computerized Imaging Reference Systems) to H2O
• Percentage Depth Dose
– Solid water: up to 2.4% difference (75kVp beam 40mm deep)
– Plastic water: up to 23.2% difference (75kVp beam 2mm deep)
• Absolute dose measurement:
In phantom
Hill et al Phys Med Biol 50 N331-N344 2005
Are In Air and In Water measurements
equivalent ?
In air In phantom
Ma et al Med Phys 25 2376-2384 1998
300kV 3.67mm Cu
Are In Air and In Water measurements
equivalent ?
In air In phantom
Ma et al Med Phys 25 2376-2384 1998
300kV 3.67mm Cu
1.22
0.964
Issues with Backscatter Method
Thanks to Rhydian Caines and Richard Clements
Depth Dose measurement
• Measurement of doses close to the
surface is difficult
• Ma et al recommend using an NACP
chamber but found differences of 2.4%
• With a diode the differences were up to
10%
In phantom
New Devices
New uses are being proposed
Zeiss Intrabeam Ariane Papillon 50
Methodology for New Devices
• Ebert et al have carried out a thorough
evaluation of the dosimetry
• However, the uncertainties in dosimetry
probably place the priority on consistency
between centres
• An intercomparison jig has been proposed
by Armoogum and Watson Ebert et al Med Phys 30 2424-2431 2003
Armoogum and Watson Z Med Phys 18 120-127 2008
Uncertainty Estimates
Summary
• Protocols give reliable results at the point
of measurement
• Accurate measurement of percentage
depth doses is problematic
• A protocol should be chosen which
measures the dose close to the
prescription point
Kilovoltage Codes of Practice
Germany DIN 1988 and 1996
UK IPEMB (IPEM) 1996 and 2005
IAEA TRS 277 2nd edn 1997
Holland NCS 1997
USA AAPM TG61 2001
IAEA TRS 398 2004
TRS 398
• Calibrate chamber in absorbed dose to
water
• is a chamber specific factor
which corrects for differences between the
reference beam and the actual beam
In phantom
Is TRS 398 the answer ?
• Only PTB offers an absorbed dose to water
calibration
• Other standards labs may offer a calibration
based on one of the protocols
• Jurado et al have used it with calibrations from
PTB
• IAEA TECDOC 1455 shows agreement within
3% at low energies
In phantom
Jurado et al BJR 78 1-12 2005
