innovations conference 2014 dr lois holloway does a helical diode array detect clinically...

Liverpool & Macarthur Cancer Therapy Centres


Does a helical diode array detect clinically significant radiotherapy

VMAT delivery errors?


Sankar Arumugam1, Tony Young1, Aitang Xing1, David

Thwaites2, Lois Holloway1-,4

1Liverpool and Macarthur cancer therapy centres and Ingham Institute, NSW, Sydney,

Australia. 2Institute of Medical Physics, University of Sydney, NSW, Australia. 3Centre for Medical Radiation Physics, University of Wollongong, NSW, Australia.4South Western Sydney Clinical School, University of New South Wales, Sydney,

NSW, Australia.

http://www.uow.edu.au/index.html

http://www.uow.edu.au/index.html


Introduction

• VMAT is a highly complex technique.

• Conventional detector systems used

for IMRT verification are limited for

VMAT verification.

• Limited evidence is available on the

sensitivity of detectors.


Aim

To investigate the sensitivity of the ArcCHECK and other

detectors

-in detecting dose errors that might occur during VMAT

delivery

-in detecting errors likely to have clinical impact


VMAT plans considered

• Elekta Synergy linear accelerator and Pinnacle treatment

planning system

• Simple open aperture arc with 5x10 cm2 field size

• 2 Prostate VMAT plans

• Arc length 2500

• CP gantry spacing 40

• 2 Head and Neck VMAT plans

• Arc length 3600

• CP gantry spacing 30


Detector systems studied

ArcCHECK MatriXX Evolution Octavius 4D

Detector arrangement

1386 diode

helical array

1020 ion chambers

2D array within

phantom

729 ion chambers

Detector Size 0.64 mm2 5 mm(ϴ),0.8 mm (h) 2D array rotated within phantom

Detector Spacing 10 mm 7.6 mm 5 x 5 x 5 mm3


Assessment of measurement

ArcCHECK MatriXX Evolution Octavius 4D

Measured

Calculated

Gamma analysis 2%/2mm

and 3%/3mm


Errors considered

• Three types of MLC and dose errors were considered in the study

– All gantry angles

– Gantry angle dependent

– Discrete gantry angles

• Errors were introduced in the treatment delivery file


Errors –All gantry angles

All gantry angles

MLC error

Dose error

-7

-5

-3

-1

1

3

5

7

0 90 180 270 360

MLC

err

or

(mm

)

Gantry Angle (Degs)

-10

-5

0

5

10

0 90 180 270 360

CP

we

igh

t Er

ror

(%)

Gantry Angle (Degs)

-5mm 0 5mm


Errors – Gantry angle dependent

Gantry angle dependent

MLC error

Dose error

0

5

10

15

0 90 180 270 360

CP

Wt

err

or

(%

)

Gantry Angle (Degs)

1%2%3%4%5%7%10%15%

-7

-5

-3

-1

1

3

5

7

0 90 180 270 360

MLC

err

or

(mm

)

Gantry Angle (Degs)

1mm2mm3mm4mm5mm7mm


Errors-Discrete gantry angles

Error CP1Error CP2

Discrete gantry angles

MLC error

Dose error


Sensitivity of the detector system

• Arbitrary gradient = ∆𝛾 𝑝𝑎𝑠𝑠 𝑟𝑎𝑡𝑒

∆𝑚𝑚𝑀𝐿𝐶 𝑜𝑟 ∆%𝑑𝑜𝑠𝑒

g pass rate

Delivery error (MLC shift (mm) or Dose (%)

Low sensitivity (gradient >-2)

high sensitivity (gradient<-6)

medium sensitivity (-6<gradient <-2)


Error to all gantry angles

MLC errors Dose errors

40.0

50.0

60.0

70.0

80.0

90.0

100.0

3%/3mm

γp

ass

ra

te

ArcCHECK

MatriXX Evolution

Octavius 4D

40.0

50.0

60.0

70.0

80.0

90.0

100.0

3%/3mm

γp

ass

ra

te

ArcCHECK

MatriXX Evolution

Octavius 4D

All medium gradientmedium gradient

high gradient

high gradient


Gantry Angle Dependent

MLC errors

40.0

50.0

60.0

70.0

80.0

90.0

100.0

3%/3mm

γp

ass

ra

te

ArcCHECK

MatriXX Evolution

Octavius 4D

Dose errors

All medium gradient

medium gradient

medium gradient

low gradient


40.0

50.0

60.0

70.0

80.0

90.0

100.0

3%/3mm

γp

ass

ra

te

ArcCHECK

MatriXX Evolution

Octavius 4D

40.0

50.0

60.0

70.0

80.0

90.0

100.0

3%/3mm

γp

ass

ra

te

ArcCHECK

MatriXX Evolution

Octavius 4D

Discrete gantry angles

Dose errorsMLC errors

All low gradient

medium gradient

low gradient

low gradient


Detector sensitivity (3%/3mm γ)

Error Type ArcCHECK

Matrixx

Evolution Octavius 4D

Gantry

independent

MLC medium medium medium

Dose medium high high

Gantry

Dependent

MLC medium medium medium

Dose medium low medium

Discrete

control points

MLC low low low

Dose medium low low


Did this matter clinically?

For the ArcCHECK detector and the prostate plans

• For minimum detectable gantry angle independent errors (3 mm MLC; 5% dose).

• For minimum detectable gantry dependent errors (4 mm MLC ;4% dose).

– All minimum detectable errors resulted in

• >5% reduction in mean PTV dose

• >5% increase in rectum EUD

• Errors introduced to the CPs did not have a clinically significant impact.


Conclusions

• Varying levels of sensitivity to introduced errors were seen

• No detector system detected all errors to a high level of

sensitivity

• Different detector systems performed well for different

situations

• It is important that limitations of detectors are understood for

the particular clinical situation which is being considered


Acknowledgements

– The authors are grateful to Mr Peter Douglas, Nucletron Ptv Ltd,

Australia for lending the Octavius 4D system

– Cancer Council NSW for funding our on-going delivery uncertainties

work, project grant number: 1067566

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