Chamber with Various Radiotherapy Beams Makan Farrokhkish, B.Sc . - PowerPoint PPT Presentation

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

𝑻 𝑵 ∝ Dose ∙ Area Large Ion Chamber Collimating elements Photon Source

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

Varian Millennium Collimator Elekta MLCi2 Elekta Agility

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

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

Objectives  IQM Intrinsic and effective spatial sensitivity

-500 V 0 V

1.6 Central Gradient Profile ( Normalized To Centre) 1.6 Normalized Signal 1.4 Normalized Signal 1.4 1.2 1.2 1.0 1.0 Loss of lateral 0.8 0.8 Electron 0.6 Equilibrium 0.6 0.4 0.4 0.2 0.2 0.0 0.0 -15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15 Off Axis Distance (cm) Off Axis Distance (cm)

1.6 1.2 Canteral None-Gradient Profile ( Normalized To Centre) Normalized Signal 1.4 1.0 1.2 0.8 1.0 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 -15 -15 -10 -10 -5 -5 0 0 5 5 10 10 15 15 Off Axis Distance (cm) Off Axis Distance (cm)

IQM Intrinsic Response (Gradient Direction) TB 6X TB 6FFF 10 FFF 6X Agility 1.60 1.40 Normalized IQM Signal 1.20 1.00 0.80 0.60 0.40 0.20 0.00 -15 -10 -5 0 5 10 15 Off Axis Distance In Transverse Plane (cm)

IQM Intrinsic Response (None Gradient Direction) TB 6X TB 6 FFF 10 FFF 6X Agility 1.05 1.00 Normalized IQM Signal 0.95 0.90 0.85 0.80 0.75 -15 -10 -5 0 5 10 15 Off Axis Distance In Radial Plane (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field True Beam 6 MV True Beam 10 MV Int Gradient 1.20 Slope R Value IQM Signal Normalized to Centre Int Gradient -0.050 1.000 1.15 True Beam 6 MV -0.030 0.986 True Beam 10 MV -0.031 0.931 1.10 1.05 1.00 0.95 0.90 0.85 0.80 -3 -2 -1 0 1 2 3 Off axis field shift (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field MLCi2 6 MV MLCi2 18 MV Int Gradient 1.20 Slope R Value IQM Signal Normalized to Centre Int Gradient -0.050 1.000 1.15 MLCi2 6 MV -0.026 0.997 MLCi2 18 MV -0.024 0.974 1.10 1.05 1.00 0.95 0.90 0.85 0.80 -3 -2 -1 0 1 2 3 Off axis field shift (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field Agility 6 MV Agility 18 MV Int Gradient 1.20 Slope R Value IQM Signal Normalized to Centre Int Gradient -0.050 1.000 1.15 Agility 6 MV -0.230 0.954 Agility 18 MV -0.012 0.771 1.10 1.05 1.00 0.95 0.90 0.85 0.80 -3 -2 -1 0 1 2 3 Off axis field shift (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field True Beam 6FFF True Beam 10FFF Int Gradient 1.20 IQM Signal Normalized to Centre 1.15 1.10 1.05 1.00 0.95 0.90 0.85 0.80 -3 -2 -1 0 1 2 3 Off axis field shift (cm)

IQM Response Characteristics  Response as a function of field size

4.50 REF 𝑱𝑹𝑵 𝑺𝑭𝑮 𝑻𝒋𝒉𝒐𝒃𝒎 = C ∙ 𝒚 𝟑 4.00 𝟐𝟏𝟏 Normalized IQM REF Signal 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 0 2 4 6 8 10 12 14 16 18 20 Side of square (cm)

REF 4.5 Normalized IQM REF Signal 4  Deviations from REF caused by: 3.5 3 • Difference in LINAC output with 2.5 field size 2 • Difference in beam flatness and 1.5 symmetry 1 0.5 • Difference in beam transmission 0 through collimating elements 0 2 4 6 8 10 12 14 16 18 20 Side of square (cm)

True Beam 6 MV iX 6 MV REF 1.0 Signal % Diff With respect to REF 0.9 Side of True Beam iX 6 MV IQM Signal Normalized to 10X10 Square (cm) 6 MV 0.8 1 -22.2 -10.7 0.7 3 -15.9 -12.6 0.6 5 -9.7 -7.6 8 -3.3 -2.6 0.5 10 0.0 0.0 0.4 0.3 0.2 0.1 0.0 1 3 5 7 9 Side of square (cm)

True Beam 6 MV iX 6 MV REF 4.5 Signal % Diff With respect to REF 4.0 Side of True Beam iX 6 MV IQM Signal Normalized to 10X10 Square (cm) 6 MV 3.5 10 0.0 0.0 15 5.4 4.0 3.0 20 8.8 6.0 2.5 2.0 1.5 1.0 0.5 0.0 10 12 14 16 18 20 Side of square (cm)

IQM Response Characteristics  Response as a function of beam energy

True Beam 10 MV Agility 18 MV MLCi2 18 MV iX 18MV 1.4 IQM signal normalized to 6 MV 1.2 1.0 0.8 0.6 0 5 10 15 20 Side of square (cm)

True Beam 10FFF 6 FFF 1.2 10 FFF IQM signal normalized to 6 MV FFF 1.0 0.8 0.6 0 5 10 15 20 Side of square (cm)

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

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 20 1.000 0.999 - - 40 1.001 0.999 - - 100 1.001 0.999 - - 400 1.000 1.000 1.000 - 600 1.000 1.000 1.000 - 800 - - 0.999 1.000 1000 - - 1.000 - 1200 - - 1.000 0.999 1400 - - 1.000 - 1600 - - - 0.999 2000 - - - 0.999 2400 - - - 0.999

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

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