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

Makan Farrokhkish, B.Sc.

Department of Radiation Physics, Radiation Medicine Program Princess Margaret Cancer Centre , Toronto

Response Characteristics of a Large-Area Ion Chamber with Various Radiotherapy Beams

Photon Source Collimating elements Large Ion Chamber

𝑻𝑵∝ Dose ∙ Area

• Linac Head Structure
• Beam Energy
• Beam Calibration

Factors that influence IQM signals

Varian Millennium Collimator Elekta MLCi2 Elekta Agility

• 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

IQM Response Characteristics With Different Radiotherapy Beam

• 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

Studied Linac Models

• IQM Intrinsic and effective spatial sensitivity

Objectives

• 500 V

0 V

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

• 15
• 10
• 5

5 10 15

Normalized Signal Off Axis Distance (cm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

• 15
• 10
• 5

5 10 15

Normalized Signal Off Axis Distance (cm)

Central Gradient Profile ( Normalized To Centre)

Loss of lateral Electron Equilibrium

0.0 0.2 0.4 0.6 0.8 1.0 1.2

• 15
• 10
• 5

5 10 15

Off Axis Distance (cm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

• 15
• 10
• 5

5 10 15

Normalized Signal Off Axis Distance (cm)

Canteral None-Gradient Profile ( Normalized To Centre)

IQM Intrinsic Response (Gradient Direction)

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

• 15
• 10
• 5

5 10 15

Normalized IQM Signal Off Axis Distance In Transverse Plane (cm) TB 6X TB 6FFF 10 FFF 6X Agility

IQM Intrinsic Response (None Gradient Direction)

0.75 0.80 0.85 0.90 0.95 1.00 1.05

• 15
• 10
• 5

5 10 15

Normalized IQM Signal Off Axis Distance In Radial Plane (cm) TB 6X TB 6 FFF 10 FFF 6X Agility

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

• 3
• 2
• 1

1 2 3 IQM Signal Normalized to Centre Off axis field shift (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field

True Beam 6 MV True Beam 10 MV Int Gradient

Slope R Value Int Gradient

• 0.050

1.000 True Beam 6 MV

• 0.030

0.986 True Beam 10 MV

• 0.031

0.931

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

• 3
• 2
• 1

1 2 3 IQM Signal Normalized to Centre Off axis field shift (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field

MLCi2 6 MV MLCi2 18 MV Int Gradient

Slope R Value Int Gradient

• 0.050

1.000 MLCi2 6 MV

• 0.026

0.997 MLCi2 18 MV

• 0.024

0.974

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

• 3
• 2
• 1

1 2 3 IQM Signal Normalized to Centre Off axis field shift (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field

Agility 6 MV Agility 18 MV Int Gradient

Slope R Value Int Gradient

• 0.050

1.000 Agility 6 MV

• 0.230

0.954 Agility 18 MV

• 0.012

0.771

0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

• 3
• 2
• 1

1 2 3 IQM Signal Normalized to Centre Off axis field shift (cm)

Effective IQM Spatial Sensitivity for a 3X3 cm2 field

True Beam 6FFF True Beam 10FFF Int Gradient

• Response as a function of field size

IQM Response Characteristics

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 2 4 6 8 10 12 14 16 18 20 Normalized IQM REF Signal Side of square (cm) REF 𝑱𝑹𝑵 𝑺𝑭𝑮 𝑻𝒋𝒉𝒐𝒃𝒎 = C ∙ 𝒚𝟑 𝟐𝟏𝟏

• 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 0.5 1 1.5 2 2.5 3 3.5 4 4.5 2 4 6 8 10 12 14 16 18 20 Normalized IQM REF Signal Side of square (cm) REF

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1 3 5 7 9 IQM Signal Normalized to 10X10 Side of square (cm) True Beam 6 MV iX 6 MV REF

Signal % Diff With respect to REF Side of Square (cm) True Beam 6 MV iX 6 MV 1

• 22.2
• 10.7

3

• 15.9
• 12.6

5

• 9.7
• 7.6

8

• 3.3
• 2.6

10 0.0 0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 10 12 14 16 18 20 IQM Signal Normalized to 10X10 Side of square (cm) True Beam 6 MV iX 6 MV REF

Signal % Diff With respect to REF Side of Square (cm) True Beam 6 MV iX 6 MV 10 0.0 0.0 15 5.4 4.0 20 8.8 6.0

• Response as a function of beam energy

IQM Response Characteristics

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

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

• IQM response as a function of dose rates

IQM Response Characteristics

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

IQM Dose Rate Dependency (Varian TB)

• Only minor deviations in IQM intrinsic spatial sensitivity was
• bserved 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

Summary

• 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

Acknowledgments