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Title: A practical and theoretical definition of very small field size for radiotherapy output factor measurements

Abstract

Purpose: This work introduces the concept of very small field size. Output factor (OPF) measurements at these field sizes require extremely careful experimental methodology including the measurement of dosimetric field size at the same time as each OPF measurement. Two quantifiable scientific definitions of the threshold of very small field size are presented. Methods: A practical definition was established by quantifying the effect that a 1 mm error in field size or detector position had on OPFs and setting acceptable uncertainties on OPF at 1%. Alternatively, for a theoretical definition of very small field size, the OPFs were separated into additional factors to investigate the specific effects of lateral electronic disequilibrium, photon scatter in the phantom, and source occlusion. The dominant effect was established and formed the basis of a theoretical definition of very small fields. Each factor was obtained using Monte Carlo simulations of a Varian iX linear accelerator for various square field sizes of side length from 4 to 100 mm, using a nominal photon energy of 6 MV. Results: According to the practical definition established in this project, field sizes ≤15 mm were considered to be very small for 6 MV beams for maximal field size uncertaintiesmore » of 1 mm. If the acceptable uncertainty in the OPF was increased from 1.0% to 2.0%, or field size uncertainties are 0.5 mm, field sizes ≤12 mm were considered to be very small. Lateral electronic disequilibrium in the phantom was the dominant cause of change in OPF at very small field sizes. Thus the theoretical definition of very small field size coincided to the field size at which lateral electronic disequilibrium clearly caused a greater change in OPF than any other effects. This was found to occur at field sizes ≤12 mm. Source occlusion also caused a large change in OPF for field sizes ≤8 mm. Based on the results of this study, field sizes ≤12 mm were considered to be theoretically very small for 6 MV beams. Conclusions: Extremely careful experimental methodology including the measurement of dosimetric field size at the same time as output factor measurement for each field size setting and also very precise detector alignment is required at field sizes at least ≤12 mm and more conservatively≤15 mm for 6 MV beams. These recommendations should be applied in addition to all the usual considerations for small field dosimetry, including careful detector selection.« less

Authors:
; ;  [1];  [2];  [3];  [4];  [5];  [6]
+ Show Author Affiliations
  1. School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001 (Australia)
  2. Department of Medical Physics, Saskatchewan Cancer Agency, 20 Campus Dr., Saskatoon, Saskatchewan S7L 3P6, Canada and Academic Unit of Medical Physics, Faculty of Medicine and Health, University of Leeds, 8.001 Worsley Building, Leeds LS2 9JT (United Kingdom)
  3. Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006 (Australia)
  4. School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia and Premion, The Wesley Medical Centre, Suite 1, 40 Chasely St, Auchenflower, Brisbane, QLD 4066 (Australia)
  5. Premion, The Wesley Medical Centre, Suite 1, 40 Chasely St, Auchenflower, Brisbane, QLD 4066 (Australia)
  6. The Australian Clinical Dosimetry Service, Australian Radiation Protection and Nuclear Safety Agency, 619 Lower Plenty Road, Yallambie, VIC 3085, Australia and Radiation Oncology Queensland, St Andrew's Toowoomba Hospital, Toowoomba, QLD 4350 (Australia)
Publication Date:
OSTI Identifier:
22250832
Resource Type:
Journal Article
Journal Name:
Medical Physics
Additional Journal Information:
Journal Volume: 41; Journal Issue: 4; Other Information: (c) 2014 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0094-2405
Country of Publication:
United States
Language:
English
Subject:
61 RADIATION PROTECTION AND DOSIMETRY; 62 RADIOLOGY AND NUCLEAR MEDICINE; ALIGNMENT; BEAM POSITION; BEAM PRODUCTION; COMPUTERIZED SIMULATION; DOSIMETRY; ERRORS; LINEAR ACCELERATORS; MONTE CARLO METHOD; PHANTOMS; RADIOTHERAPY

Citation Formats

Charles, P. H., E-mail: p.charles@qut.edu.au, Crowe, S. B., Langton, C. M., Trapp, J. V., Cranmer-Sargison, G., Thwaites, D. I., Kairn, T., Knight, R. T., and Kenny, J. A practical and theoretical definition of very small field size for radiotherapy output factor measurements. United States: N. p., 2014. Web. doi:10.1118/1.4868461.
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Charles, P. H., E-mail: p.charles@qut.edu.au, Crowe, S. B., Langton, C. M., Trapp, J. V., Cranmer-Sargison, G., Thwaites, D. I., Kairn, T., Knight, R. T., & Kenny, J. A practical and theoretical definition of very small field size for radiotherapy output factor measurements. United States. https://doi.org/10.1118/1.4868461
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Charles, P. H., E-mail: p.charles@qut.edu.au, Crowe, S. B., Langton, C. M., Trapp, J. V., Cranmer-Sargison, G., Thwaites, D. I., Kairn, T., Knight, R. T., and Kenny, J. 2014. "A practical and theoretical definition of very small field size for radiotherapy output factor measurements". United States. https://doi.org/10.1118/1.4868461.
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@article{osti_22250832,
title = {A practical and theoretical definition of very small field size for radiotherapy output factor measurements},
author = {Charles, P. H., E-mail: p.charles@qut.edu.au and Crowe, S. B. and Langton, C. M. and Trapp, J. V. and Cranmer-Sargison, G. and Thwaites, D. I. and Kairn, T. and Knight, R. T. and Kenny, J.},
abstractNote = {Purpose: This work introduces the concept of very small field size. Output factor (OPF) measurements at these field sizes require extremely careful experimental methodology including the measurement of dosimetric field size at the same time as each OPF measurement. Two quantifiable scientific definitions of the threshold of very small field size are presented. Methods: A practical definition was established by quantifying the effect that a 1 mm error in field size or detector position had on OPFs and setting acceptable uncertainties on OPF at 1%. Alternatively, for a theoretical definition of very small field size, the OPFs were separated into additional factors to investigate the specific effects of lateral electronic disequilibrium, photon scatter in the phantom, and source occlusion. The dominant effect was established and formed the basis of a theoretical definition of very small fields. Each factor was obtained using Monte Carlo simulations of a Varian iX linear accelerator for various square field sizes of side length from 4 to 100 mm, using a nominal photon energy of 6 MV. Results: According to the practical definition established in this project, field sizes ≤15 mm were considered to be very small for 6 MV beams for maximal field size uncertainties of 1 mm. If the acceptable uncertainty in the OPF was increased from 1.0% to 2.0%, or field size uncertainties are 0.5 mm, field sizes ≤12 mm were considered to be very small. Lateral electronic disequilibrium in the phantom was the dominant cause of change in OPF at very small field sizes. Thus the theoretical definition of very small field size coincided to the field size at which lateral electronic disequilibrium clearly caused a greater change in OPF than any other effects. This was found to occur at field sizes ≤12 mm. Source occlusion also caused a large change in OPF for field sizes ≤8 mm. Based on the results of this study, field sizes ≤12 mm were considered to be theoretically very small for 6 MV beams. Conclusions: Extremely careful experimental methodology including the measurement of dosimetric field size at the same time as output factor measurement for each field size setting and also very precise detector alignment is required at field sizes at least ≤12 mm and more conservatively≤15 mm for 6 MV beams. These recommendations should be applied in addition to all the usual considerations for small field dosimetry, including careful detector selection.},
doi = {10.1118/1.4868461},
url = {https://www.osti.gov/biblio/22250832}, journal = {Medical Physics},
issn = {0094-2405},
number = 4,
volume = 41,
place = {United States},
year = {Tue Apr 15 00:00:00 EDT 2014},
month = {Tue Apr 15 00:00:00 EDT 2014}
}
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Journal Article:
https://doi.org/10.1118/1.4868461
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