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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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