Machine-specific QC in
Aarhus using EPID
Mai-Britt K. Jørgensen, Lone Hoffmann, L.P. Muren, Lars H.
Præstegaard, Rune Hansen, Jørgen B.B. Pedersen, Maria F. Jensen
Aarhus University Hospital
Department of Medical Physics
www.regionmidtjylland.dk
Rutine QC
IMRT
Machine specific Sweeping gap
Picket fence (PF)
VMAT
Sweeping gap
Picket fence (PF)
Dose rate versus
gantry speed (DRGS)
Dose rate versus MLC
speed (DRMLC)
Patient specific
PDI
PDI
(Delta4)
(Delta4)
(PTW729 array)
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Picket fence, PF
For every MLC and gab…
 Position:

p ( x MLC , gap ) 
y  I ( x MLC , y )
I ( x MLC , y )  Threshold

y
I ( x MLC , y )  Threshold
 Relative rotation of EPID and collimator
 Intensity (”width”):
I sum ( x MLC , gap ) 
 I (x
MLC
I ( x MLC , y )  Threshold
, y)
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PF: ex. ”Eye test”
PF:
ex. Deviation in position, tolerance ±0.3mm
0.4
Deviation in position (mm)
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
0
10
20
30
40
MLC leaf pair number
50
60
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PF:
ex. Deviation in intensity, tolerance ±300
400
Deviation in intensity
300
200
100
0
-100
-200
-300
0
10
20
30
MLC leaf pair number
40
50
60
PF: ex. ”Eye test version 2”
PF:
In total ~30 test on each accelerator, one every month
Counts
Deviations
8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
Accelerator
Position
Intensity
Relative rotation
9
10
Dose rate vs. gantry speed, DRGS
I corr ( x MLC , y ) 
Deviation(
I DRGS ( x MLC , y )
 100 %
I Open ( x MLC , y )
x MLC ) j 
I corr (x MLC , y)
I corr (x MLC , y)
j
1
j [1 , m ]
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DRGS: ex. typical deviation
1.2
Normalised scale
1
0.8
0.6
0.4
0.2
0
0
100
200
300
400
500
600
700
-0.2
y (pixels)
DRGS
Open field
DRGS: ex. cable issue
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DRGS:
In total ~30 test on each accelerator, one every month
Deviation in DRGS test
14
12
Counts
10
8
6
4
2
0
1
2
3
4
5
6
Accelerator
7
8
9
10
Dose rate vs. MLC speed, DRMLC
I corr ( x MLC , y ) 
Deviation(
I DRMLC ( x MLC , y )
 100 %
I Open ( x MLC , y )
x MLC ) j 
I corr (x MLC , y)
I corr (x MLC , y)
j
1
j [1 , m ]
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DRMLC:
ex.
segment 4 DR120, 0.4cm/s
1.6
1.4
1.2
1
Norm
0.8
0.6
0.4
0.2
0
0
100
200
300
400
500
600
700
-0.2
Pixel
Leaf 30
Leaf 30 baggrund
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DRMLC:
ex.
segment 2 DR600, 2.4cm/s
1.6
1.4
1.2
1
Norm
0.8
0.6
0.4
0.2
0
0
100
200
300
400
-0.2
Pixel
Leaf 30
Leaf 30 baggrund
500
600
700
DRMLC:
ex.
segment 4 DR120, 0.4cm/s
segment 1 DR480, 1.6cm/s
1.4
1.2
1
Norm
0.8
0.6
0.4
0.2
0
0
100
200
300
400
-0.2
Pixel
Leaf 30
Leaf 30 baggrund
500
600
700
DRMLC:
In total ~30 test on each accelerator, one every month
Deviations in DRMLC
35
30
Counts
25
20
15
10
5
0
1
2
3
4
5
6
Accelerator
7
8
9
10
Time consumption
 Measurements:
~10 min
 Analysis (home-made software): ~1 min
 Total:
~11 min
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Effect of chain adjustments on
the DRGS and DRMLC tests
 Mehtod
 3 DRGS and 3 DRMLC test with a loose chain
 3 DRGS and 3 DRMLC test with a thight chain
 Result, comparing loose and thight chain
 DRGS: Standard deviation was unchanged
 DRMLC: Std 0.6 (loose) -> 0.2 (tight)
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Loose chain:
unstable characteristic
1.4
1.2
1
Norm
0.8
0.6
0.4
0.2
0
0
100
200
300
400
-0.2
Pixel
Leaf 30
Leaf 30 baggrund
500
600
700
Tight chain:
Stable characteristic
1.4
1.2
1
Norm
0.8
0.6
0.4
0.2
0
0
100
200
300
400
-0.2
Pixel
Leaf 30
Leaf 30 baggrund
500
600
700
Discussion
 Deviations in PF are covered in next, next
session
 Deviations in DRGS and DRMLC
 Chain adjustments have an effect on DRGS and
DRMLC test, but it does not solve everything
 Cable issues
 Beam adjustments?
 For more information on the calculations and tolerances:
Med. Phys. 38, 1425 (2011);
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