Transcript Use of the IC Profiler detector array for comprehensive machine QA

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
Outline
1.
2.
3.
4.
Device description
Streamlining beam dosimetry QA
MLC calibration
Summing up
Device description
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
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
Fine adjustments
 5mm pitch allows subtle optimisations to be made
 eg Bending Fine adjustment following Bending Magnet
replacement:
Profile minimum re-centred
Integrated exposures
 Excellent signal-to-noise ratio
 Good performance down to small MUs
100MU
10MU
3MU
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%
Streamlining beam dosimetry QA
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
 Use chambers only for absolute dose output at gantry 0°
 Use IC Profiler for all other measurements
Electron set-up
 Gantry mount: accommodates 25x25cm applicator
 Electron wedge: simultaneous output, energy & symmetry
Electron Wedge
Gantry 90°
Traditional schedule
Test
Set-up
Output
Annual test frequency
Photons (2 energies)
G0°
G90°
G180°
G270°
Readings
Excursions
Farmer/Solid W
12
4
4
4
48
24
Energy (QI)
Farmer/Solid W
12
1
1
1
90
45
Wedge Factor
Farmer/Solid W
12
4
4
4
72
24
Output factors
ICO4/Large SW
4
40
4
Basic flatness
Doublecheck
12
24
24
Full flatness
Schuster
4
128
4
TPS profiles
ICO4/Water tank
1
4
4
4
1 day set-up & measurement
S
Electrons (7 energies)
Output
Roos/Solid W
12
1
1
98
98
Energy
Roos/Solid W
12
1
1
196
196
Output factors
Roos/Solid W
1
49
49
Basic flatness
Doublecheck
12
84
12
Full flatness
Schuster
4
42
6
TPS profiles
ICO4/Water tank
1
Totals
6
1
1
1 day set-up & measurement
871
486
Proposed schedule
Test
Set-up
Output
Annual test frequency
Photons (2 energies)
G0°
G90°
G180°
G270°
Readings
Excursions
Farmer(G0°)/ ICP
12
4
4
4
24
12
Energy (QI)
ICP
12
4
4
4
48
12
Wedge Factor
ICP
12
4
4
4
48
-
Output factors
ICP
4
40
4
Basic flatness
Daily QA3
Full flatness
ICP
12
TPS profiles
ICP/Large SW
1
(260)
Covered in daily run-up
4
4
4
48*
1 hour measurement time
S
Electrons (7 energies)
Output
Roos(G0°)/ ICP
12
1
1
84
84
Energy
ICP/Elec Wedge
12
1
1
98
12
Output factors
ICP
1
49
49
Basic flatness
Daily QA3
Full flatness
ICP
12
TPS profiles
ICP/Large SW
1
Totals
6 3
(260)
Covered in daily run-up
1
1
-
-
2 hours measurement time
871 439
486 173
Possible schedule
Test
Set-up
Output
Annual test frequency
Photons (2 energies)
G0°
G90°
G180°
G270°
Readings
Excursions
Farmer(G0°)/ ICP
12
12
12
12
24
12
Energy (QI)
ICP
12
12
12
12
96
12
Wedge Factor
ICP
12
4
4
4
48
-
Output factors
ICP
4
40
4
Basic flatness
Daily QA3
Full flatness
ICP
12
TPS profiles
ICP/Large SW
1
Electrons (7 energies)
Thus saving 30 miles of
walking in one year (for a
four linac department)
(260)
Covered in daily run-up
12
12
12
96*
1 hour measurement time
S
Output
Roos(G0°)/ ICP
12
12
12
84
84
Energy
ICP/Elec Wedge
12
12
12
252
12
Output factors
ICP
1
49
49
Basic flatness
Daily QA3
260
Full flatness
ICP
12
TPS profiles
ICP/Large SW
1
Totals
6 3
Covered in daily run-up
12
12
-
-
2 hours measurement time
871 689
486 173
MLC calibration
MLC relative position (‘minor offsets’)
Elekta MLCi
7mm
3mm
10mm
Alternate chambers sit
under centre of MLC leaf
MLC leaf
Partial volume effect
100%
50%
×
20%/mm
0%
-2mm -1mm
0
1mm 2mm
MLC relative position (‘minor offsets’)
Elekta MLCi
7mm
3mm
10mm
Alternate chambers sit
under centre of MLC leaf
MLC leaf
Partial volume effect
100%
50%
×
20%/mm
0%
-2mm -1mm
0
1mm 2mm
MLC relative position (‘minor offsets’)
Elekta MLCi
7mm
3mm
10mm
Alternate chambers sit
under centre of MLC leaf
MLC leaf
Partial volume effect
100%
50%
×
20%/mm
0%
-2mm -1mm
0
1mm 2mm
MLC relative position (‘minor offsets’)
Elekta MLCi
7mm
3mm
10mm
Alternate chambers sit
under centre of MLC leaf
MLC leaf
Partial volume effect
100%
50%
×
0%
-2mm -1mm
0
1mm 2mm
20%/mm
MLC relative position (‘minor offsets’)
Elekta MLCi
7mm
3mm
10mm
Alternate chambers sit
under centre of MLC leaf
MLC leaf
Partial volume effect
100%
50%
×
0%
-2mm -1mm
0
1mm 2mm
20%/mm
MLC calibration is referenced to backup jaw
Backup Jaw
MLC21
MLC19 MLC20
MLC22
Reference signal
MLC position signal
110%
110%
100%
100%
90%
90%
20%/mm
Perfect alignment not necessary!
Reference signal
MLC position signal
110%
110%
100%
100%
90%
90%
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)
Dpenumbra(s)
=
100%
75%
50%
25%
0%
1 – (t + sc)
2
-10mm -5mm 0
× 1-
5mm 10mm
s
√(s2 + n)
+ (t + sc)
where (t + sc) = Transmission + Scatter (outside beam)
and
n = (20-80% penumbra width / 1.5)2
Calibration sequence – 25MU exposures
MLC minor offsets (central 30 leaves); Backup jaw major gains & offsets
3
1
2
4
-5
+5
-15
Y1 Jaw
Y1 MLC
+15
Y2 Jaw
Y2 MLC
Major gains & offsets
5
6
-5
Further 4 exposures if
calibrating all 40 MLC leaves
+5
-15
+15
PiMLiCo software interface
Major gain/offset calculator
Export minor offsets to linac
Out-of-tolerance results
Electronic update of minor offsets
White cells indicate
updated items
Import
System performance
 Can detect and correct minor offsets of ~0.2mm
 Agreement with film/water tank within experimental error
Re-calibration of all MLC and diaphragm settings
20 mins
QA of major gains/offsets + central 30 leaf pairs
5 mins
Summing up
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.
Thank you
[email protected]