Dr Lois holloway - Does a helical Diode Array Detect Clinically Significant Radiotherapy VMAT Delivery Errors?

1. Liverpool & Macarthur
Cancer Therapy Centres

2. Does a helical diode array detect
clinically significant radiotherapy
Liverpool & Macarthur
Cancer Therapy Centres
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.

3. 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.

4. Liverpool & Macarthur
Cancer Therapy Centres
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

5. 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
Liverpool & Macarthur
Cancer Therapy Centres

6. Liverpool & Macarthur
Cancer Therapy Centres
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

7. Assessment of measurement
ArcCHECK MatriXX Evolution Octavius 4D
Measured
Calculated
Gamma analysis 2%/2mm
and 3%/3mm
Liverpool & Macarthur
Cancer Therapy Centres

8. Liverpool & Macarthur
Cancer Therapy Centres
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

9. Errors –All gantry angles
All gantry
angles
Liverpool & Macarthur
Cancer Therapy Centres
MLC error
Dose error
7
5
3
1
-1
-3
-5
-7
0 90 180 270 360
MLC error (mm)
Gantry Angle (Degs)
-5mm 0 5mm
10
5
0
-5
-10
0 90 180 270 360
CP weight Error (%)
Gantry Angle (Degs)

10. Errors – Gantry angle dependent
Gantry angle
dependent
Liverpool & Macarthur
Cancer Therapy Centres
MLC error
Dose error
7
5
3
1
-1
-3
-5
15
10
5
0
0 90 180 270 360
CP Wt error (%)
Gantry Angle (Degs)
1%
2%
3%
4%
5%
7%
10%
15%
-7
0 90 180 270 360
MLC error (mm)
Gantry Angle (Degs)
1mm
2mm
3mm
4mm
5mm
7mm

11. Errors-Discrete gantry angles
Liverpool & Macarthur
Cancer Therapy Centres
Error CP1
Error CP2
Discrete gantry
angles
MLC error
Dose error

12. Sensitivity of the detector system
• Arbitrary gradient =
Liverpool & Macarthur
Cancer Therapy Centres
Δ훾 푝푎푠푠 푟푎푡푒
Δ푚푚 푀퐿퐶 표푟 Δ%푑표푠푒
g pass rate
Low sensitivity (gradient >-2)
medium sensitivity (-6<gradient <-2)
high sensitivity (gradient<-6)
Delivery error (MLC shift (mm) or Dose (%)

13. Error to all gantry angles
MLC errors Dose errors
100.0
90.0
80.0
70.0
60.0
50.0
40.0
Liverpool & Macarthur
Cancer Therapy Centres
3%/3mm
γ pass rate
ArcCHECK
MatriXX Evolution
Octavius 4D
100.0
90.0
80.0
70.0
60.0
50.0
40.0
high gradient
3%/3mm
γ pass rate
ArcCHECK
MatriXX Evolution
Octavius 4D
All medium gradient
medium gradient
high gradient

14. Gantry Angle Dependent
MLC errors
100.0
90.0
80.0
70.0
60.0
50.0
40.0
Liverpool & Macarthur
Cancer Therapy Centres
3%/3mm
γ pass rate
ArcCHECK
MatriXX Evolution
Octavius 4D
Dose errors
All medium gradient
medium gradient
low gradient
medium gradient

15. Discrete gantry angles
MLC errors Dose errors
100.0
90.0
80.0
70.0
60.0
50.0
40.0
Liverpool & Macarthur
Cancer Therapy Centres
3%/3mm
γ pass rate
ArcCHECK
MatriXX Evolution
Octavius 4D
100.0
90.0
80.0
70.0
60.0
50.0
40.0
low gradient
3%/3mm
γ pass rate
ArcCHECK
MatriXX Evolution
Octavius 4D
All low gradient
medium gradient
low gradient

16. Detector sensitivity (3%/3mm γ)
Error Type ArcCHECK
Liverpool & Macarthur
Cancer Therapy Centres
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

17. 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.
Liverpool & Macarthur
Cancer Therapy Centres

18. 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
Liverpool & Macarthur
Cancer Therapy Centres

19. 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
Liverpool & Macarthur
Cancer Therapy Centres