Response Characteristics of a Large-Area Ion Chamber with Various Radiotherapy Beams Makan Farrokhkish, B.Sc.

Transcription

1 Response Characteristics of a Large-Area Ion Chamber with Various Radiotherapy Beams Makan Farrokhkish, B.Sc. Department of Radiation Physics, Radiation Medicine Program Princess Margaret Cancer Centre, Toronto

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3 S M Dose Area Photon Source Collimating elements Large Ion Chamber

4 Factors that influence IQM signals Linac Head Structure Beam Energy Beam Calibration

5 Varian Millennium Collimator Elekta MLCi2 Elekta Agility

6 IQM Response Characteristics With Different Radiotherapy Beam IQM Intrinsic and effective spatial sensitivity Response as a function of field size Response as a function of beam energy IQM response as a function of dose rates

7 Studied Linac Models TrueBeam (Varian Medical Systems, Palo Alto, CA). Millennium MLC. 6 MV, 10 MV, 6 FFF, 10 FFF Clinac ix (Varian Medical Systems, Palo Alto, CA). Millennium MLC. 6 MV, 18 MV Infinity ( Elekta AB, Stockholm, Sweden). Agility Collimator. 6MV, 18 MV Infinity ( Elekta AB, Stockholm, Sweden). MLCi2 Collimator. 6 MV, 18 MV

8 Objectives IQM Intrinsic and effective spatial sensitivity

9 -500 V 0 V

10 Normalized Signal Central Gradient Profile ( Normalized To Centre) Loss of lateral 0.8 Electron 0.6 Equilibrium Off Axis Distance (cm)

11 Normalized Signal Canteral None-Gradient Profile ( Normalized To Centre) Off Axis Distance (cm)

12 Normalized IQM Signal IQM Intrinsic Response (Gradient Direction) 1.60 TB 6X TB 6FFF 10 FFF 6X Agility Off Axis Distance In Transverse Plane (cm)

13 Normalized IQM Signal IQM Intrinsic Response (None Gradient Direction) 1.05 TB 6X TB 6 FFF 10 FFF 6X Agility Off Axis Distance In Radial Plane (cm)

14 IQM Signal Normalized to Centre Effective IQM Spatial Sensitivity for a 3X3 cm2 field True Beam 6 MV True Beam 10 MV Int Gradient Slope R Value Int Gradient True Beam 6 MV True Beam 10 MV Off axis field shift (cm)

15 IQM Signal Normalized to Centre Effective IQM Spatial Sensitivity for a 3X3 cm2 field MLCi2 6 MV MLCi2 18 MV Int Gradient Slope R Value Int Gradient MLCi2 6 MV MLCi2 18 MV Off axis field shift (cm)

16 IQM Signal Normalized to Centre Effective IQM Spatial Sensitivity for a 3X3 cm2 field Agility 6 MV Agility 18 MV Int Gradient Slope R Value Int Gradient Agility 6 MV Agility 18 MV Off axis field shift (cm)

17 IQM Signal Normalized to Centre 1.20 Effective IQM Spatial Sensitivity for a 3X3 cm2 field True Beam 6FFF True Beam 10FFF Int Gradient Off axis field shift (cm)

18 IQM Response Characteristics Response as a function of field size

19 Normalized IQM REF Signal REF IQM REF Signal = C x Side of square (cm)

20 Normalized IQM REF Signal REF Side of square (cm) Deviations from REF caused by: Difference in LINAC output with field size Difference in beam flatness and symmetry Difference in beam transmission through collimating elements

21 IQM Signal Normalized to 10X Signal % Diff With respect to REF Side of Square (cm) True Beam 6 MV True Beam 6 MV ix 6 MV REF ix 6 MV Side of square (cm)

22 IQM Signal Normalized to 10X Signal % Diff With respect to REF Side of Square (cm) True Beam 6 MV True Beam 6 MV ix 6 MV REF ix 6 MV Side of square (cm)

23 IQM Response Characteristics Response as a function of beam energy

24 IQM signal normalized to 6 MV 1.4 True Beam 10 MV Agility 18 MV MLCi2 18 MV ix 18MV Side of square (cm)

25 IQM signal normalized to 6 MV FFF 1.2 True Beam 10FFF 6 FFF 10 FFF Side of square (cm)

26 IQM Response Characteristics IQM response as a function of dose rates

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28 IQM Dose Rate Dependency (Varian TB) Signal Normalized To 600 MU / Min For 6 MV,10 MV, 6 FFF And Normalized to 800 MU / Min For 10 FFF Dose Rate 6 MV IQM 10 MV IQM 6 FFF IQM 10 FFF IQM

29 Summary Only minor deviations in IQM intrinsic spatial sensitivity was observed across different energies and platforms The effective spatial sensitivity is affected by beam profile parameters and beam attenuation The signal for square field size depends on the design of the LINAC head components The IQM energy discrimination varies with field size, design of the LINAC head components, and beam profile IQM system exhibits negligible dose rate dependency

30 Acknowledgments Dr. Mohammad Islam Dr. Robert Heaton Bern Norrlinger Dr. David Jaffray Yinkun Wang Andrew Jung Graham Wilson Dr. Ivan Yeung Marlyn McIntosh Renata Chmielewski Thank You