NPL/IPEM Reference Dosimetry Audits

Russell Thomas

Radiotherapy Standards Users’ Meeting5th June 2007

Background

NPL, at the invitation of IPEM, started conducting reference dosimetry audits in 1995 since then we have conducted:

• 40 MV photon audits• 6 electron audits (1996 code)• 8 electron audits (2003 code)• 6 kV photon audits (first one in 2003)

Audit Format

Basic structure for all reference dosimetry audits:

• Beam quality• Machine output• Field instrument calibration

+ specific areas such as ion recombination+ individual areas of concern

MV photon audit set up

MV photon beam quality indexMV photon beam quality index

Beam quality index (TPR)

0.9800

0.9850

0.9900

0.9950

1.0000

1.0050

1.0100

1.0150

0 10 20 30 40 50 60 70

Audit number

Rat

io N

PL/H

ost NPL/Host

0.998std dev 0.5%

MV photon field instrument calibration

Field Instrument calibration

0.9800

0.9850

0.9900

0.9950

1.0000

1.0050

1.0100

1.0150

1.0200

0 10 20 30 40 50 60 70

Audit number

Rat

io N

PL/

Hos

t

NPL/Host

1.001

std dev 0.7%

MV photon machine output

Machine output (cGy/MU)

0.9850

0.9900

0.9950

1.0000

1.0050

1.0100

1.0150

1.0200

1.0250

0 10 20 30 40 50 60 70Audit number

Ratio

NPL

/Hos

t

NPL/Host 1.003 std dev 0.7%

Nisbet/Host 1.003 std dev 1.0%

Thwaites/Host 1.003 std dev 1.5%

Depth dose measurements in water

Electron measurements in solid water

Electrons 2003 C of P

Machine output

0.994

0.996

0.998

1

1.002

1.004

1.006

1.008

1.01

0 1 2 3 4 5 6A ud it numb e r

Field instrument

0.998

1

1.002

1.004

1.006

1.008

1.01

1.012

1.014

0 1 2 3 4 5 6

Audit number

Ratio

NPL

/Hos

t

NPL/Host 1.003 std dev 0.3%

Nisbet/Host 0.994 std dev 1.8%

NPL/Host 1.002 std dev 0.3%

kV set up for HVL measurements

KV audit results

Beam Quality

0.9

0.95

1

1.05

1.1

0.5 1 1.5 2 2.5 3 3.5 4 4.5

Audit number

Rati

o N

PL

/Ho

st NPL/Host

1.019

std dev 4.8%

KV audit results

Machine output

0.965

0.97

0.975

0.98

0.985

0.99

0.995

1

1.005

1.01

1.015

0 1 2 3 4 5

Audit number

Rat

io N

PL

/Ho

st

Machine output

expected

Field instrument

0.99

0.995

1

1.005

1.01

1.015

0 1 2 3 4 5

Audit number

Ratio

NPL

/Hos

t

NPL/Host 0.999 std dev 0.7% NPL/Host 1.000 std dev 0.5%

What was our goal for 2004-07?So far…..

Region KV photon MV photon Electron 2003

A

B

C

D

E

F

G

H N/A

Helical Tomotherapy

• Clinicians like the dose distributions that tomotherapy can give …

Helical Tomotherapy is …

• … just an “optimised” integrated system to deliver IMRT

• alanine/EPR could be as useful in “ordinary” IMRT

Tomotherapy beam profiles

Open field at isocentre:

length: 40 cm

width: 5 cm, 2.5 cm or 1 cm

Jeraj et al., Med Phys 31 (2004) 396-404

Individual pellet doses (Cromwell)

Measured dose / TPS dose

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

1.031 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103

109

115

121

127

133

139

5 cm beam

2.5 cm beam

1 cm beam

off-axis beam

static beam1.0 ± 2%

Current and Future work

• NPL offers alanine measurements in the NPL-tomo phantom as a routine service to any TomoTherapy user, with– on-site measurements by NPL for UK users– Postal service (phantom and dosimeters) for non-UK users

• NPL would like to apply alanine dosimetry to IMRT delivered by conventional linacs– Volunteers, please, to identify and help investigate issues– Will co-ordinate work with the RTSIG working group on IMRT

Where now for NPL Audits?

• DTI funding from October 2007?• Hospitals buying additional audits • Continue with kV, MV and Electron audits• HDR Brachytherapy• Smallfield and IMRT• Improve tie in with individual audit region• National electron audit of all radiotherapy departments• Measuring impact

Staff changes in NPL Radiation Dosimetry

Departures:Martin PalmerRebecca Nutbrown

Secondment:Russell Thomas (2 years)

Maternity Leave:Clare Lee (9 months)

Staff changes in NPL Radiation Dosimetry

Protection Level Calibration Service –MSP: Martin Kelly ext 8502

Deputy: Graham Bass

Therapy Level Photon Calibration Service –MSP: Graham Bass ext 6503

Deputy: Julia Pearce / James Manning

Therapy Level Electron Calibration Service –MSP: Graham Bass ext 6503

Deputy: Julia Pearce

NPL Reference Dosimetry Audits –Graham Bass ext 6503

Dosimetry@npl.co.uk

Graham Bass is Captain Dosimetry!

Thank You!