IR change and dose rate linearity - Ra

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IR change and
dose rate linearity
Mai-Britt K. Jørgensen, Lone Hoffmann, Lars H. Præstegaard, Maria F.
Jensen
Aarhus University Hospital
Department of Medical Physics
www.regionmidtjylland.dk
Change of IR beam components
 Motivation
 The IR beams on Aarhus accelerators have never
been changed. Therefore the voltage is much
smaller than what Varian recommends. Varian
recommends changing them ~once a year
 A mechanical way to change the DLG?
 Method
 Before and after change of IR components:
 3 PF, 3 DRGS and 3 DRMLC tests with MLC
initialisation in between.
Change of IR beam components
 Results
 No change in deviations for the PF, DRGS and
DRMLC tests
 Discussion
 Would you persue this isssue?
Dose rate linearity
 Motivation
 Two types of dose rate control:
 Pulse drop servo and pulse leght servo 2
 How well does the accelerator perform at
different dose rates
 Method
 Arcs in service mode with different dose rates
and same gantry speed
 Static gantry delivery in service mode
 All measurements performend with Farmer
chamber in a square box of solid water
Dose rate linearity:
Pulse drop servo
Dose rate linearity:
Pulse length servo 2
0.60
0.40
0.20
Output
0.00
0
100
200
300
400
-0.20
-0.40
-0.60
-0.80
-1.00
DR
Acc8
Acc5
500
600
700
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