Use of the IC Profiler detector array for comprehensive machine QA

1
ESTRO QA Dosimetry Satellite Symposium 9.5.11
Use of the IC Profiler detector array for
comprehensive machine QA
Steve Morgan, Medical Physics Dept, Sussex Cancer
Centre
2
Outline

• Device description
• Streamlining beam dosimetry QA
• MLC calibration
• Summing up

3
Device description
4
Device description
Detectors 251 parallel plate ion chambers
IC volume 0.05cm3
IC dimensions 3mm x 7mm
IC spacing 5mm
Field of view 32cm x 32cm
Build-up 0.9g/cm2
Weight 5kg
5
Beam steering

• 4 plot axes captured simultaneously highly
  efficient
• Particularly advantageous for primary steering
  (1R, 1T) investigations in which adjustments
  simultaneously affect both inplane and crossplane
  symmetry

6
Fine adjustments

• 5mm pitch allows subtle optimisations to be made
• eg Bending Fine adjustment following Bending
  Magnet replacement

Profile minimum re-centred
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Integrated exposures

• Excellent signal-to-noise ratio
• Good performance down to small MUs

100MU
10MU
3MU
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Photon energy

• Flatness sensitive indicator of energy

BC 38.0V, d10 67.0
BC 40.5V, d10 67.5
BC 42.5V, d10 68.0
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Streamlining beam dosimetry QA
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Guiding principles

• Reduce the number of different equipment
  deployments
• Reduce the number of excursions into the
  treatment room for set-up adjustments
• Make each exposure multi-purpose where possible
• Improve efficiency and quality

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Electron set-up

• Gantry mount accommodates 25x25cm applicator
• Electron wedge simultaneous output, energy
  symmetry

Electron Wedge
Gantry 90
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Traditional schedule
Annual test frequency Annual test frequency Annual test frequency Annual test frequency
Test Set-up G0 G90 G180 G270 Readings Excursions

Photons (2 energies) Output Farmer/Solid W 12 4 4 4 48 24
Photons (2 energies) Energy (QI) Farmer/Solid W 12 1 1 1 90 45
Photons (2 energies) Wedge Factor Farmer/Solid W 12 4 4 4 72 24
Photons (2 energies) Output factors ICO4/Large SW 4 40 4
Photons (2 energies) Basic flatness Doublecheck 12 24 24
Photons (2 energies) Full flatness Schuster 4 4 4 4 128 4
Photons (2 energies) TPS profiles ICO4/Water tank 1 1 day set-up measurement 1 day set-up measurement

S
Electrons (7 energies) Output Roos/Solid W 12 1 1 98 98
Electrons (7 energies) Energy Roos/Solid W 12 1 1 196 196
Electrons (7 energies) Output factors Roos/Solid W 1 49 49
Electrons (7 energies) Basic flatness Doublecheck 12 84 12
Electrons (7 energies) Full flatness Schuster 4 1 1 42 6
Electrons (7 energies) TPS profiles ICO4/Water tank 1 1 day set-up measurement 1 day set-up measurement

Totals 6 871 486
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Proposed schedule
Annual test frequency Annual test frequency Annual test frequency Annual test frequency
Test Set-up G0 G90 G180 G270 Readings Excursions

Photons (2 energies) Output Farmer(G0)/ ICP 12 4 4 4 24 12
Photons (2 energies) Energy (QI) ICP 12 4 4 4 48 12
Photons (2 energies) Wedge Factor ICP 12 4 4 4 48 -
Photons (2 energies) Output factors ICP 4 40 4
Photons (2 energies) Basic flatness Daily QA3 (260) Covered in daily run-up Covered in daily run-up
Photons (2 energies) Full flatness ICP 12 4 4 4 48
Photons (2 energies) TPS profiles ICP/Large SW 1 1 hour measurement time 1 hour measurement time

S
Electrons (7 energies) Output Roos(G0)/ ICP 12 1 1 84 84
Electrons (7 energies) Energy ICP/Elec Wedge 12 1 1 98 12
Electrons (7 energies) Output factors ICP 1 49 49
Electrons (7 energies) Basic flatness Daily QA3 (260) Covered in daily run-up Covered in daily run-up
Electrons (7 energies) Full flatness ICP 12 1 1 - -
Electrons (7 energies) TPS profiles ICP/Large SW 1 2 hours measurement time 2 hours measurement time

Totals 6 3 871 439 486 173
50 fewer readings
65 fewer trips down the maze
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Possible schedule
Annual test frequency Annual test frequency Annual test frequency Annual test frequency
Test Set-up G0 G90 G180 G270 Readings Excursions

Photons (2 energies) Output Farmer(G0)/ ICP 12 12 12 12 24 12
Photons (2 energies) Energy (QI) ICP 12 12 12 12 96 12
Photons (2 energies) Wedge Factor ICP 12 4 4 4 48 -
Photons (2 energies) Output factors ICP 4 40 4
Photons (2 energies) Basic flatness Daily QA3 (260) Covered in daily run-up Covered in daily run-up
Photons (2 energies) Full flatness ICP 12 12 12 12 96
Photons (2 energies) TPS profiles ICP/Large SW 1 1 hour measurement time 1 hour measurement time

S
Electrons (7 energies) Output Roos(G0)/ ICP 12 12 12 84 84
Electrons (7 energies) Energy ICP/Elec Wedge 12 12 12 252 12
Electrons (7 energies) Output factors ICP 1 49 49
Electrons (7 energies) Basic flatness Daily QA3 260 Covered in daily run-up Covered in daily run-up
Electrons (7 energies) Full flatness ICP 12 12 12 - -
Electrons (7 energies) TPS profiles ICP/Large SW 1 2 hours measurement time 2 hours measurement time

Totals 6 3 871 689 486 173
20 fewer readings
65 fewer trips down the maze
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MLC calibration
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MLC relative position (minor offsets)
MLC leaf
Elekta MLCi
7mm
3mm
Alternate chambers sit under centre of MLC leaf
10mm
Partial volume effect
100

50
20/mm
0
-2mm
0
-1mm
1mm
2mm
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MLC relative position (minor offsets)
MLC leaf
Elekta MLCi
7mm
3mm
Alternate chambers sit under centre of MLC leaf
10mm
Partial volume effect
100

50
20/mm
0
-2mm
0
-1mm
1mm
2mm
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MLC relative position (minor offsets)
MLC leaf
Elekta MLCi
7mm
3mm
Alternate chambers sit under centre of MLC leaf
10mm
Partial volume effect
100
50

20/mm
0
-2mm
0
-1mm
1mm
2mm
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MLC relative position (minor offsets)
MLC leaf
Elekta MLCi
7mm
3mm
Alternate chambers sit under centre of MLC leaf
10mm
Partial volume effect
100
50

20/mm
0
-2mm
0
-1mm
1mm
2mm
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MLC relative position (minor offsets)
MLC leaf
Elekta MLCi
7mm
3mm
Alternate chambers sit under centre of MLC leaf
10mm
Partial volume effect
100
50

20/mm
0
-2mm
0
-1mm
1mm
2mm
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MLC calibration is referenced to backup jaw
Backup Jaw
MLC21
MLC20
MLC19
MLC22
Reference signal
MLC position signal
110
110
100
100
20/mm
90
90
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Perfect alignment not necessary!
Backup Jaw
MLC22
MLC21
MLC20
MLC19
Reference signal
MLC position signal
110
110
100
100
90
90
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Major gains and offsets - penumbra interpolation

• 5mm detector spacing - unlikely to have two
  detectors sitting within the linear (20-80)
  portion of the penumbra
• Square root interpolation model finds 50 edge
• Model and parameters tested by shifting ICP in
  1mm increments (and comparisons with film)

100
75
50
25
0
0
-5mm
-10mm
5mm
10mm
1 (t sc)
s
(t sc)
1 -
Dpenumbra(s)
v(s2 n)
2
where (t sc) Transmission Scatter (outside
beam)
and n (20-80 penumbra width / 1.5)2
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Calibration sequence 25MU exposures
MLC minor offsets (central 30 leaves) Backup jaw
major gains offsets
3
4
1
2
5
15
-5
-15
Y2 Jaw
Y2 MLC
Y1 Jaw
Y1 MLC
Major gains offsets
5
6
Further 4 exposures if calibrating all 40 MLC
leaves
-15
-5
5
15
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PiMLiCo software interface
Major gain/offset calculator
Export minor offsets to linac
Out-of-tolerance results
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Electronic update of minor offsets
White cells indicate updated items
Import
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System performance

• Can detect and correct minor offsets of 0.2mm
• Agreement with film/water tank within
  experimental error

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Summing up
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Advantages and opportunities
Efficient and precise tool for beam optimisation.
Fewer test set-ups. Multi-purpose exposures.
Significant time and consumables savings
associated with MLC calibration and X-ray/light
coincidence tests.
Time-to-competence for new staff expected to
reduce.
Alternative to water tank for TPS data baseline
checks.
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Thank you
steve.morgan_at_bsuh.nhs.uk