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Dr Lois holloway - Does a helical Diode Array Detect Clinically Significant Radiotherapy VMAT Delivery Errors?TRANSCRIPT
Liverpool & Macarthur Cancer Therapy Centres
Liverpool & Macarthur Cancer Therapy Centres
Does a helical diode array detect clinically significant radiotherapy
VMAT delivery errors?
Liverpool & Macarthur Cancer Therapy Centres
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.
Liverpool & Macarthur Cancer Therapy Centres
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.
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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
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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
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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
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Assessment of measurement
ArcCHECK MatriXX Evolution Octavius 4D
Measured
Calculated
Gamma analysis 2%/2mm
and 3%/3mm
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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
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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
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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
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Errors-Discrete gantry angles
Error CP1Error CP2
Discrete gantry angles
MLC error
Dose error
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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)
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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
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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
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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
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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
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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.
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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
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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