1. 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

2. Overview of talk Why we do dosimetry What we use Method of calibration
Need to know dose delivered
What we use
Thimble ion chambers
Hierarchy traceable to NPL
Method of calibration
Following IPSM COP
Chamber factor in practice
How linked to routine QA

3. Why do we carry out dosimetry?
Radiation causes biological damage
Need to quantify dose delivered
Delivery of prescription

4. Bragg-Gray Principle Tissue Ξ water
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

5. What do we use to carry out MV photon dosimetry?
Thimble ionisation chambers
Small volume of air (typically <1ml)
Potential difference of a few hundred volts
Used with a calibrated electrometer
Outer electrode
(graphite wall)
Central Al
electrode
Basic design of FARMER thimble chamber
Insulator

6. What do we use to carry out MV photon dosimetry?
Hierarchy of chambers
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

7. Summary of calibration method
Described in IPSM COP
Intercompare chambers – using same radiation qualities as will be measured by the field chamber
Correct for Ion recombination losses
Measure Q.I. to find NPL correction factor
Calculate chamber factor to convert electrometer reading to absorbed dose to water

8. Calibration Factor
Ms/Mf - ratio of secondary standard reading to field instrument reading
Is/If - ion recombination factor ratio
ND - NPL secondary standard calibration factor

9. Method for MV calibration
Chambers placed in intercomparison jig in water tank
Depth of 5cm in 10x10cm field
95cm TSD
100mu delivered
Interchanged
Ratio calculated
Lasers
Secondary standard
Field chamber
Thin window in plotting tank

10. Measuring Quality Index
Measure of radiation quality
Ratio of ion recombination corrected instrument readings –
20cm and 10cm depths
10x10cm field
Constant TCD of 100cm
Used to find NPL calibration factor from certificate

11. Results Nominal beam energy 6MV 10MV Is 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.

12. Calibration factors in practice
LINAC calibrated to give 1Gy
5cm depth
10x10cm field
100mu
TSD of 95cm
Calibration in Perspex rather than water
Requires PBF

13. Calibration factors in practice
CFlumped = CF x PBF x If
Nominal beam energy
6MV
10MV
PBF
1.014
CFlumped (Gy/nC)
0.937
0.931
Previous CFlumped (Gy/nC)
0.943
0.930
Error
0.6%
0.1%
Found to be consistent with previous factors

14. Summary 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

15. Thank you for listening