development of a device to measure constancy of output, … m magness.pdf · 2015-10-27 ·...
TRANSCRIPT
Development of a Device to Measure
Constancy of Output, Symmetry and
Energy of a Clinical Linear Accelerator
Using Low-Cost Photodiodes
Mark Magness, Velindre Cancer Centre
Monitoring the beam
Monitoring the beam
• Output – how much radiation is
being delivered?
Monitoring the beam
• Output – how much radiation is
being delivered?
• Symmetry – the shape of the
beam being delivered.
Monitoring the beam
• Output – how much radiation is
being delivered?
• Symmetry – the shape of the
beam being delivered.
• Energy – How deep is the beam
being delivered?
Monitoring the beam
• Output – how much radiation is
being delivered?
• Symmetry – the shape of the
beam being delivered.
• Energy – How deep is the beam
being delivered? <2%
Current Daily Constancy Device
Varian CTB CN-732-A
Why Photodiodes?
• Using diodes instead of chambers in the
device would eliminate the need for HT.
• Existing radiation diodes are prohibitively
expensive to have one device per machine.
• Normal Photodiodes are designed to detect
photons and have an obvious plane of
sensitivity to position perpendicular to the
beam.
• Small, light package, no assembly.
3 types of Diode
1N4001 VTB8440B BPW34
Radiation Hardening
• 64.5cm from target (Shadow Tray
Mount)
• 10mm Buildup
• 40 x 40 field
• 6MeV electrons (Adjusted to deliver
1200Mu/min)
2.5kGy All Readings are in % relative to the initial reading
99
100
101
102
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Diode 1N4001
97
98
99
100
101
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Diode VTB8440B
90
110
130
150
170
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Exposure No.
Diode BPW34
5.2kGy
106
107
108
109
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Diode 1N4001
46
47
48
49
50
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Diode VTB8440B
2940
2960
2980
3000
3020
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Exposure No.
Diode BPW34
All Readings are in % relative to the initial reading
6.5kGy
136
137
138
139
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Diode 1N4001
46.5
47.5
48.5
49.5
50.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Diode VTB8440B
2600
2620
2640
2660
2680
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Exposure No.
Diode BPW34
All Readings are in % relative to the initial reading
Diode Vs Chamber
• 25 readings Vs chamber
• Raw diode % variance from mean =
±0.23%
• After comparing against thimble chamber
and subtracting beam variance the actual
diode variance = ±0.16%
Symmetry Prototype
CNC PCB Milling
The Block
Full Prototype
Software
Final Device
Device Aspects
• Uses Low-cost photodiodes to measure Output,
Symmetry and Energy
• Internal temperature sensor for temperature
correction of readings, reduces user error
• Micro-switch interlocked block eliminates spurious
readings through incorrect build-up usage
• Software Based Front End
• All readings and calibration factors are recorded
both locally and backed up to a network database,
reducing chance of loss
Acknowledgments
Machine Maintenance, VCC
• Tim Register – Liaising with heads of departments regarding budgets, training etc.
• Stephen Hayes – Original Concept, Analogue Circuit Design
• Andrew Bevan – Temperature Sensor test and selection, Thermocouple circuit design
• Anthony Frost – Interface box design and build
Mechanical Workshop, VCC
• Andrew Edwards – Help with development of CNC PCB Milling
• Robin Hymers – Device Block and Enclosure Design and Build
Medical Physics, VCC
• Richard Jarvis – Developing the correct specification for the device
• All the other physicists - For answering my random questions, taking comparison readings, etc.