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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