An investigation of TLD response in a strong magnetic fieldchapter.aapm.org/nccaapm/z_meetings/2018-10-12/04_Agenda...2018/10/12  · An investigation of TLD response in a strong magnetic

An investigation of TLD response in a strong magnetic field

Eric SimieleUniversity of Wisconsin-Madison, Madison, WI

North Central Chapter of the American Association of Physicists in Medicine fall meeting

Oct. 12, 2018

Introduction

Rising interest in performing concurrent imaging during radiotherapy treatment

• Real-time motion and target tracking• Magnetic resonance imaging (MRI)

Dosimetry challenges in a strong magnetic field

• Differences in density between the detector and surrounding medium

Solid-state detectors appear to be a better alternative for dosimetry measurements in magnetic fields1-3

2

e-Water

Air

1.5 T

3

Various solid-state detectors have been investigated previously1-4

Increases in detector response of up to 20% have been observed2

Change in response depends on:• Detector geometry • Orientation of detector, photon beam,

and magnetic field direction

Detector response characterization is necessary

Introduction

PTW 60003 diamond detector

4

Goal

The goal of this work is to investigate the response of Thermo-luminescent dosimeters (TLDs) with and without a magnetic field present

Methods and Materials

5

TLD-100• Microcubes (1x1x1) mm3

• Chips (3x3x1) mm3

Elekta Precise® medical linac• Photon beam energy of 8 MV

With and without 1.29 T field• Bruker EPR magnet• Set using a Group3 ® Hall probe

Various dose to water (Dw) levels:• Microcubes – 2 Gy• Chips – 2 Gy and 5 Gy

Dw with and without a magnetic field determined by the PTB

1

PresenterPresentation NotesMathis et al 2014 (Poster - 2014 CSM) Found TLDs to be “not significantly affected by the B field within the experimental uncertainty (6%)”Mathis et al 2014 – AAPM Annual Meeting Abstract – “No significant differences (less than 2% difference) in the performance of TLD, OSLD, or PRESAGE dosimeters due to exposure to a strong magnetic field were observed”Wen et al 2016 - Annual Meeting Abstract – “For TLDs, the ratio of signals with the B field to signals without the B field averaged over three dose levels was 1.003 +/- 0.016…” “uncertainty of 2%”Wang et al. 2016 – AAPM Annual Mtg Abstract - 1.002 for TLDs, statistical uncertainty 3%

Methods and Materials

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All measurements performed in a PMMA water phantom

• Designed to fit between pole shoes of the magnet

• Exterior dimensions of (7x21x21) cm3

• Wall thicknesses of 0.5 cm

Measurements performed at:

• Effective depth of 10 cm• SSD of 110 cm

Field size of 5x10 cm2 at isocenter

• Pole separation = 7.2 cm

Water phantom

PresenterPresentation NotesMathis et al 2014 (Poster - 2014 CSM) Found TLDs to be “not significantly affected by the B field within the experimental uncertainty (6%)”Mathis et al 2014 – AAPM Annual Meeting Abstract – “No significant differences (less than 2% difference) in the performance of TLD, OSLD, or PRESAGE dosimeters due to exposure to a strong magnetic field were observed”Wen et al 2016 - Annual Meeting Abstract – “For TLDs, the ratio of signals with the B field to signals without the B field averaged over three dose levels was 1.003 +/- 0.016…” “uncertainty of 2%”Wang et al. 2016 – AAPM Annual Mtg Abstract - 1.002 for TLDs, statistical uncertainty 3%

Methods and Materials

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All TLDs prepared according to the standard techniques at the UWMRRC

Microcubes:• Irradiated in Virtual WaterTM probes• Four TLDs in each probe

Chips:• Irradiated in Virtual WaterTM paddles• Five TLDs in each paddle

Five probe or paddle measurements for each configuration

PresenterPresentation NotesMathis et al 2014 (Poster - 2014 CSM) Found TLDs to be “not significantly affected by the B field within the experimental uncertainty (6%)”Mathis et al 2014 – AAPM Annual Meeting Abstract – “No significant differences (less than 2% difference) in the performance of TLD, OSLD, or PRESAGE dosimeters due to exposure to a strong magnetic field were observed”Wen et al 2016 - Annual Meeting Abstract – “For TLDs, the ratio of signals with the B field to signals without the B field averaged over three dose levels was 1.003 +/- 0.016…” “uncertainty of 2%”Wang et al. 2016 – AAPM Annual Mtg Abstract - 1.002 for TLDs, statistical uncertainty 3%

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Simulations were performed in GEANT41 v10.04 patch 1 on the UWMRRC computing cluster

Elekta Precise® phase space data for 6 MV and 10 MV photon beams from the IAEA repository

Methods and Materials

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Chips Cubes

Methods and Materials

Results

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Measured TLD chip 𝐷𝐷𝑇𝑇𝐿𝐿𝐿𝐿𝐷𝐷𝑤𝑤 0 𝑇𝑇

1.29 𝑇𝑇values are within two sigma of unity

All simulated values within two sigma of unity

All simulated values within two sigma of measured values

Measured 8 MV Simulated 6 MV Simulated 10 MV

TLD Form𝐷𝐷𝑇𝑇𝐿𝐿𝐷𝐷𝐷𝐷𝑤𝑤 0 𝑇𝑇

1.29 𝑇𝑇

1σ𝐷𝐷𝑇𝑇𝐿𝐿𝐷𝐷𝐷𝐷𝑤𝑤 0 𝑇𝑇

1.29 𝑇𝑇

1σ𝐷𝐷𝑇𝑇𝐿𝐿𝐷𝐷𝐷𝐷𝑤𝑤 0 𝑇𝑇

1.29 𝑇𝑇

1σ

microcube 0.977 0.8% 0.999 2.2% 1.006 1.9%(1x1x1 mm3)

chip 0.982 (2 Gy) 0.993 (5 Gy)

0.9% (2 Gy) 0.5% (5 Gy) 0.971 1.6% 0.993 1.4%(3x3x1 mm3)

Can the air gap influence the microcube results?

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Nominal air gap surrounding the microcubes

Air gaps have been shown to influence the response of ionization chambers in a magnetic field1

Simulations were performed with various amount of air surrounding the microcubes:

• No air gap• Box (1.1x1.1x4.0) mm3

• Measured TLD cutout dimensions• Measured dimensions scaled by 1.2 (i.e.,

worst case scenario)

Results

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One standard deviation expressed as a percent in parenthesis

No significant change in response with and without a magnetic field present

• Within one standard deviation for both photon energies

Indicates microcube results are not influenced by symmetric air gaps

TLD microcube response versus air gap

Field Strength Energy No air gap Box (1.1x1.1x4.0) mm3 Nominal air gapNominal air gap

scaled by 1.2

0 T6 MV 1.000 (1.8%) 0.985 (1.8%) 1.006 (1.8%) 0.985 (1.8%)

10 MV 1.000 (1.5%) 0.999 (1.5%) 0.998 (1.5%) 0.992 (1.5%)

1.29 T6 MV 1.000 (1.8%) 0.984 (1.8%) 1.000 (1.8%) 0.996 (1.8%)

10 MV 1.000 (1.5%) 0.990 (1.5%) 1.000 (1.5%) 1.015 (1.6%)

Conclusions

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No significant change in the measured TLD chip response per dose in the presence of a 1.29 T magnetic field

• Within two standard deviations

Agreement between simulated and measured results• The change in response can be modeled by Monte Carlo simulations

Air gaps surrounding the microcubes in the Virtual WaterTM probes had no significant effect on the simulated TLD response

• No skewing of microcube results

Acknowledgments

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Markus Meier and Markus Schrader at the PTB for their help operating the linac and magnet

Collaborators:• Dr. Wesley Culberson – University of Wisconsin-Madison • Cliff Hammer – University of Wisconsin-Madison • Dr. Ralf-Peter Kapsch – Physikalisch-Technische Bundesanstalt• Dr. Ulrike Ankerhold – Physikalisch-Technische Bundesanstalt

Customers of the UWADCL, whose patronage supports student research

Results

15

TLD FormMagnetic Field

Strength# of

Probes# of TLDs Average Delivered Dose (Gy)

Average TLD response (nC)

Average Measured / Delivered (nC/Gy)

Effect of 1.29 T Type A (k=1)

microcube 0 T 5 20 2.002 3816.3 1906.2

(1x1x1 mm3) 1.29 T 5 20 1.992 3710.4 1862.4 -2.3% 0.8%

0 T 5 25 2.002 18354.1 9167.6chip 1.29 T 5 25 1.993 17947.6 9006.3 -1.8% 0.9%

(3x3x1 mm3) 0 T 5 25 5.003 46960.7 9386.4

1.29 T 5 25 4.980 46390.8 9321.4 -0.7% 0.5%

Methods and Materials

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17

Introduction

Measurement uncertainty

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TLD chips

Type A Type B CommentsCs-137 Beam uniformity 0 0.5 Taken from spreadsheet (assume normal distribution)Distance from the source 0 0.2 Distance from Cs-137 source. Taken from spreadsheet (assume normal distribution)TLD Reader HV stability 0 0.1 Taken from spreadsheet (assume normal distribution)

TLD Reader linearity 0 0.2 Taken from spreadsheet (assume rectangular distribution)TLD Reader Repeatability 0 0.1 Taken from spreadsheet (assume normal distribution)

TLD sort criteria 0 0.6 Taken from spreadsheet (assume rectangular distribution). This also contains the CF uncertaintyTLD Repeatability 0 0.6 Taken from spreadsheet (assume rectangular distribution)

PTB dose determination 0 T 0 0.4 From PTB. Dose determination without a magnetic field present (k=1)PTB dose determination 1.29 T 0 0.4 From PTB. Dose determination with a magnetic field present (k=1)Dose determination at 1.42 T 0 0.2 From PTB. Calculated using MC with an uncertainty of +/- 0.2%

Monitor chamber uncertainty in Bfield 0 0.2 From PTB. They found the uncertainty in the monitor chamber response increases with the magnet on.TLD positioning uncertainty at PTB 0 0.2 1 mm uncertainty and assume normal distribution. Use inverse square law for now

quadratic sum 0 1.2

2 Gy TLD chips 0.9 05 Gy TLD chips 0.5 0

Quadratic sum 2 Gy chips 1.5 k = 1Quadratic sum 5 Gy chips 1.3 k = 1

Measurement uncertainty

19

TLD cubes

Type A Type B CommentsCs-137 Beam uniformity 0 0.5 Taken from spreadsheet (assume normal distribution)Distance from the source 0 0.2 Distance from Cs-137 source. Taken from spreadsheet (assume normal distribution)TLD Reader HV stability 0 0.1 Taken from spreadsheet (assume normal distribution)

TLD Reader Repeatability 0 0.1 Taken from spreadsheet (assume normal distribution)TLD sort criteria 0 0.6 Taken from spreadsheet (assume rectangular distribution). This also contains the CF uncertainty

TLD Repeatability 0 0.6 Taken from spreadsheet (assume rectangular distribution)PTB dose determination 0 T 0 0.4 From PTB. Dose determination without a magnetic field present (k=1)

PTB dose determination 1.29 T 0 0.4 From PTB. Dose determination with a magnetic field present (k=1)Dose determination at 1.42 T 0 0.2 From PTB. Calculated using MC with an uncertainty of +/- 0.2%

Monitor chamber uncertainty in Bfield 0 0.2 From PTB. They found the uncertainty in the monitor chamber response increases with the magnet on.TLD positioning uncertainty at PTB 0 0.2 1 mm uncertainty and assume normal distribution. Use inverse square law for now

quadratic sum 0 1.2

microcube response 0.8 0

Quadratic sum microCubes 1.4 k = 1

Measurement uncertainty

20

TLD cubes

Type A Type B CommentsCs-137 Beam uniformity 0 0.5 Taken from spreadsheet (assume normal distribution)Distance from the source 0 0.2 Distance from Cs-137 source. Taken from spreadsheet (assume normal distribution)TLD Reader HV stability 0 0.1 Taken from spreadsheet (assume normal distribution)

TLD Reader Repeatability 0 0.1 Taken from spreadsheet (assume normal distribution)TLD sort criteria 0 0.6 Taken from spreadsheet (assume rectangular distribution). This also contains the CF uncertainty

TLD Repeatability 0 0.6 Taken from spreadsheet (assume rectangular distribution)PTB dose determination 0 T 0 0.4 From PTB. Dose determination without a magnetic field present (k=1)

PTB dose determination 1.29 T 0 0.4 From PTB. Dose determination with a magnetic field present (k=1)Dose determination at 1.42 T 0 0.2 From PTB. Calculated using MC with an uncertainty of +/- 0.2%

Monitor chamber uncertainty in Bfield 0 0.2 From PTB. They found the uncertainty in the monitor chamber response increases with the magnet on.TLD positioning uncertainty at PTB 0 0.2 1 mm uncertainty and assume normal distribution. Use inverse square law for now

quadratic sum 0 1.2

microcube response 0.8 0

Quadratic sum microCubes 1.4 k = 1

An investigation of TLD response in a strong magnetic fieldIntroductionIntroductionGoalMethods and MaterialsMethods and MaterialsMethods and MaterialsMethods and MaterialsMethods and MaterialsResultsCan the air gap influence the microcube results?ResultsConclusionsAcknowledgmentsResultsMethods and MaterialsIntroductionMeasurement uncertaintyMeasurement uncertaintyMeasurement uncertainty