Transcript Calibration of an Ionisation Chamber for use in

Calibration of an Ionisation
Chamber for use in
Megavoltage Dosimetry
An insight into the Radiotherapy Placement
undertaken by a Part 1 Trainee Clinical
Scientist
Will Mairs
Princess Royal Hospital - Hull
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Overview of talk
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Why we do dosimetry
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What we use
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Thimble ion chambers
Hierarchy traceable to NPL
Method of calibration
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Need to know dose delivered
Following IPSM COP
Chamber factor in practice
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How linked to routine QA
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Why do we carry out dosimetry?
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Radiation causes biological damage
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Need to quantify dose delivered
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Delivery of prescription
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Bragg-Gray Principle
Absorbed dose in a given
material can be deduced from
the ionisation produced in a
small gas-filled cavity within
that material
 Tissue
Ξ water
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What do we use to carry out MV photon
dosimetry?
Thimble ionisation
chambers
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Small volume of air
(typically <1ml)
Central Al
electrode
Potential difference of a
few hundred volts
Used with a calibrated
electrometer
Basic design of FARMER thimble
chamber
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Outer electrode
(graphite wall)
Insulator
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What do we use to carry out MV
photon dosimetry?
Hierarchy of chambers
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Secondary standards
calibrated against primary
every two years at NPL
NPL provides calibration
factor (ND) in terms of
absorbed dose to water
based on graphite
calorimeter
Field chambers calibrated
against secondary standard
(in house) at least once a
year
Primary Standard
Secondary Standard
Field Chamber
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Summary of calibration method
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Described in IPSM COP
Intercompare chambers – using same radiation
qualities as will be measured by the field chamber
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Correct for Ion recombination losses
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Measure Q.I. to find NPL correction factor
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Calculate chamber factor to convert electrometer
reading to absorbed dose to water
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Calibration Factor
M s Is
CF 
x xN D
Mf If
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Ms/Mf - ratio of secondary standard reading
to field instrument reading
Is/If - ion recombination factor ratio
ND - NPL secondary standard calibration
factor
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Method for MV calibration
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Chambers placed in intercomparison jig in
water tank
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Depth of 5cm in 10x10cm field
95cm TSD
100mu delivered
Interchanged
Ratio calculated
Lasers
Secondary
standard
Field
chamber
Thin
window in
plotting
tank
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Measuring Quality Index
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Measure of radiation quality
Ratio of ion recombination
corrected instrument readings –
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20cm and 10cm depths
10x10cm field
Constant TCD of 100cm
Used to find NPL calibration factor
from certificate
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Results
Nominal beam energy
6MV
10MV
Is
1.008
1.008
If
1.020
1.022
Q.I.
0.667
0.734
ND (cGy/nC)
10.190
10.082
Ms/Mf (nC/div)
8.997
9.034
CF (Gy/nC)
0.906
0.898
Summary of the data required to calculate the calibration
factor for field instrument NE 2571 s/n 3234.
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Calibration factors in practice
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LINAC calibrated to give 1Gy
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5cm depth
10x10cm field
100mu
TSD of 95cm
Calibration in Perspex rather than
water
Requires PBF
readingsin water
PBF 
readingsin perspex
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Calibration factors in practice
CFlumped = CF x PBF x If
Nominal beam energy
6MV
10MV
PBF
1.014
1.014
CFlumped (Gy/nC)
0.937
0.931
Previous CFlumped
(Gy/nC)
0.943
0.930
Error
0.6%
0.1%
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Found to be consistent with previous
factors
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Summary
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It is necessary to carry out dosimetry
Field chambers are traceable to national
standards
Calibration carried out in house following
COP
Chamber used in the field for QA
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Thank you for
listening
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