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Machine-specific QC in Aarhus using EPIDMai-Britt K. Jørgensen, Lone Hoffmann, L.P. Muren, Lars H. Præstegaard, Rune Hansen, Jørgen B.B. Pedersen, Maria F. JensenAarhus University HospitalDepartment of Medical Physics

Rutine QC

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IMRT VMAT

Machine specific Sweeping gap

Picket fence (PF)

Sweeping gap

Picket fence (PF)

Dose rate versus gantry speed (DRGS)

Dose rate versus MLC speed (DRMLC)

Patient specific PDI

(Delta4)

(PTW729 array)

PDI

(Delta4)

Picket fence, PF

For every MLC and gab…Position:

Relative rotation of EPID and collimator

Intensity (”width”):

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ThresholdyxI

ThresholdyxIMLC

MLC

MLC

MLC

y

yxIy

gapxp

),(

),(

),(

),(

ThresholdyxI

MLCMLCsum

MLC

yxIgapxI),(

),(),(

PF: ex. ”Eye test”

PF: ex. Deviation in position, tolerance ±0.3mm

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-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 10 20 30 40 50 60

MLC leaf pair number

De

viat

ion

in p

osi

tion

(m

m)

PF: ex. Deviation in intensity, tolerance ±300

-300

-200

-100

0

100

200

300

400

0 10 20 30 40 50 60

MLC leaf pair number

Dev

iatio

n in

inte

nsity

PF: ex. ”Eye test version 2”

PF: In total ~30 test on each accelerator, one every month

Deviations

012345678

1 2 3 4 5 6 7 8 9 10

Accelerator

Cou

nts

Position Intensity Relative rotation

Dose rate vs. gantry speed, DRGS

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%100),(),(

),( yxIyxI

yxIMLCOpen

MLCDRGSMLCcorr

1y),(xI

y),(xI )xDeviation(

],1[MLCcorr

MLCcorr

jMLC mj

j

DRGS: ex. typical deviation

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700

y (pixels)

Nor

mal

ised

sca

le

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

0

2

4

6

8

10

12

14

1 2 3 4 5 6 7 8 9 10

Accelerator

Cou

nts

Dose rate vs. MLC speed, DRMLC

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%100),(),(

),( yxIyxI

yxIMLCOpen

MLCDRMLCMLCcorr

1y),(xI

y),(xI )xDeviation(

],1[MLCcorr

MLCcorr

jMLC mj

j

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600 700

Pixel

No

rm

Leaf 30 Leaf 30 baggrund

DRMLC: ex. segment 4 DR120, 0.4cm/s

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-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500 600 700

Pixel

No

rm

Leaf 30 Leaf 30 baggrund

DRMLC: ex. segment 2 DR600, 2.4cm/s

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200 300 400 500 600 700

Pixel

No

rm

Leaf 30 Leaf 30 baggrund

DRMLC: ex. segment 4 DR120, 0.4cm/s segment 1 DR480, 1.6cm/s

DRMLC: In total ~30 test on each accelerator, one every month

Deviations in DRMLC

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10

Accelerator

Co

un

ts

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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-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200 300 400 500 600 700

Pixel

No

rm

Leaf 30 Leaf 30 baggrund

Loose chain: unstable characteristic

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 100 200 300 400 500 600 700

Pixel

No

rm

Leaf 30 Leaf 30 baggrund

Tight chain: Stable characteristic

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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