A practical and theoretical definition of very small field size for radiotherapy output factor measurements.
OBJECTIVE
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.
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia.
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Charles
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.
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Cranmer-Sargison
Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia.
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Thwaites
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia.
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Crowe
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.
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Kairn
Premion, The Wesley Medical Centre, Suite 1, 40 Chasely St, Auchenflower, Brisbane, QLD 4066, Australia.
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Knight
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.
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Kenny
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia.
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Langton
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia.
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Trapp
D004583:Electrons; D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons / Q000627:therapeutic use; D011874:Radiometry; D011878:Radiotherapy / Q000295:instrumentation / Q000379:methods*