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A practical and theoretical definition of very small field size for radiotherapy output factor measurements
2. R. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjäll, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S. Vatnitsky, “A new formalism for reference dosimetry of small and nonstandard fields,” Med. Phys. 35, 5179–5187 (2008).
3. M. Aspradakis, J. Byrne, H. Palmans, J. Conway, K. Rosser, J. Warrington, and S. Duane, “Small field MV photon dosimetry,” IPEM Report No. 103 (Institute of Physics and Engineering in Medicine, York, 2010).
4. G. Cranmer-Sargison, P. H. Charles, J. V. Trapp, and D. I. Thwaites, “A methodological approach to reporting corrected small field relative outputs,” Radiother. Oncol. 109, 350–355 (2013).
5. C. McKerracher and D. I. Thwaites, “Head scatter factors for small MV photon fields. Part I: A comparison of phantom types and methodologies,” Radiother. Oncol. 85, 277–285 (2007).
8. G. Cranmer-Sargison, S. Weston, N. P. Sidhu, and D. I. Thwaites, “Experimental small field 6 MV output ratio analysis for various diode detector and accelerator combinations,” Radiother. Oncol. 100, 429–435 (2011).
9. H. Bouchard, J. Seuntjens, and I. Kawrakow, “A Monte Carlo method to evaluate the impact of positioning errors on detector response and quality correction factors in nonstandard beams,” Phys. Med. Biol. 56, 2617–2634 (2011).
10. G. Cranmer-Sargison, P. Z. Liu, S. Weston, N. Suchowerska, and D. I. Thwaites, “Small field dosimetric characterization of a new 160-leaf MLC,” Phys. Med. Biol. 58, 7343–7354 (2013).
12. A. J. Scott, S. Kumar, A. E. Nahum, and J. D. Fenwick, “Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields,” Phys. Med. Biol. 57, 4461–4476 (2012).
13. H. Bouchard, J. Seuntjens, J.-F. Carrier, and I. Kawrakow, “Ionization chamber gradient effects in nonstandard beam configurations,” Med. Phys. 36, 4654–4663 (2009).
14. J. D. Fenwick, S. Kumar, A. J. Scott, and A. E. Nahum, “Using cavity theory to describe the dependence on detector density of dosimeter response in non-equilibrium small fields,” Phys. Med. Biol. 58, 2901–2923 (2013).
15. T. S. Underwood, H. C. Winter, M. A. Hill, and J. D. Fenwick, “Detector density and small field dosimetry: Integral versus point dose measurement schemes,” Med. Phys. 40, 082102 (16pp.) (2013).
16. P. H. Charles, S. Crowe, T. Kairn, R. T. Knight, B. Hill, J. Kenny, C. M. Langton, and J. V. Trapp, “Monte Carlo-based diode design for correction-less small field dosimetry,” Phys. Med. Biol. 58, 4501–4512 (2013).
17. P. H. Charles, S. B. Crowe, T. Kairn, J. Kenny, J. Lehmann, J. Lye, L. Dunn, B. Hill, R. T. Knight, C. M. Langton, and J. V. Trapp, “The effect of very small air gaps on small field dosimetry,” Phys. Med. Biol. 57, 6947–6960 (2012).
18. G. Cranmer-Sargison, S. Weston, J. A. Evans, N. P. Sidhu, and D. I. Thwaites, “Monte Carlo modelling of diode detectors for small field MV photon dosimetry: Detector model simplification and the sensitivity of correction factors to source parameterization,” Phys. Med. Biol. 57, 5141–5153 (2012).
19. F. Crop, N. Reynaert, G. Pittomvils, L. Paelinck, C. De Wagter, L. Vakaet, and H. Thierens, “The influence of small field sizes, penumbra, spot size and measurement depth on perturbation factors for microionization chambers,” Phys. Med. Biol. 54, 2951–2969 (2009).
20. T. S. Underwood, H. C. Winter, M. A. Hill, and J. D. Fenwick, “Mass-density compensation can improve the performance of a range of different detectors under non-equilibrium conditions,” Phys. Med. Biol. 58, 8295–8310 (2013).
21. E. Pantelis, A. Moutsatsos, K. Zourari, L. Petrokokkinos, L. Sakelliou, W. Kilby, C. Antypas, P. Papagiannis, P. Karaiskos, E. Georgiou, and I. Seimenis, “On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the correction factors for microchamber and diode detectors,” Med. Phys. 39, 4875–4885 (2012).
22. A. Ralston, P. Liu, K. Warrener, D. McKenzie, and N. Suchowerska, “Small field diode correction factors derived using an air core fibre optic scintillation dosimeter and EBT2 film,” Phys. Med. Biol. 57, 2587–2602 (2012).
23. C. Bassinet, C. Huet, S. Derreumaux, G. Brunet, M. Chea, M. Baumann, T. Lacornerie, S. Gaudaire-Josset, F. Trompier, P. Roch, G. Boisserie, and I. Clairand, “Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife and linear accelerators equipped with microMLC and circular cones,” Med. Phys. 40, 071725 (13pp.) (2013).
24. G. Cranmer-Sargison, S. Weston, J. A. Evans, N. P. Sidhu, and D. I. Thwaites, “Implementing a newly proposed Monte Carlo based small field dosimetry formalism for a comprehensive set of diode detectors,” Med. Phys. 38, 6592–6602 (2011).
25. P. Francescon, S. Cora, and N. Satariano, “Calculation of for several small detectors and for two linear accelerators using Monte Carlo simulations,” Med. Phys. 38, 6513–6527 (2011).
26. P. Francescon, W. Kilby, N. Satariano, and S. Cora, “Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the CyberKnife system,” Phys. Med. Biol. 57, 3741–3758 (2012).
28. D. T. B. P. Andreo, K. Hohlfeld, M. S. Huq, T. Kanai, F. Laitano, and V. a. S. V. G. Smyth, “Absorbed dose determination in external beam radiotherapy: An international code of practice for dosimetry based on standards of absorbed dose to water,” IAEA TRS-398 (International Atomic Energy Agency, Vienna, 2000).
29. E. E. Klein, J. Hanley, J. Bayouth, F.-F. Yin, W. Simon, S. Dresser, C. Serago, F. Aguirre, L. Ma, B. Arjomandy, C. Liu, C. Sandin, and T. Holmes, “Task Group 142 report: Quality assurance of medical accelerators,” Med. Phys. 36, 4197–4212 (2009).
30. D. W. O. Rogers, B. A. Faddegon, G. X. Ding, C. M. Ma, and J. We, “BEAM: A Monte Carlo code to simulate radiotherapy treatment units,” Med. Phys. 22, 503–524 (1995).
31. T. Kairn, T. Aland, R. D. Franich, P. N. Johnston, M. B. Kakakhel, J. Kenny, R. T. Knight, C. M. Langton, D. Schlect, M. L. Taylor, and J. V. Trapp, “Adapting a generic BEAMnrc model of the BrainLAB m3 micro-multileaf collimator to simulate a local collimation device,” Phys. Med. Biol. 55, N451–N463 (2010).
32. T. Kairn, J. Kenny, S. B. Crowe, A. L. Fielding, R. D. Franich, P. N. Johnston, R. T. Knight, C. M. Langton, D. Schlect, and J. V. Trapp, “Technical Note: Modeling a complex micro-multileaf collimator using the standard BEAMnrc distribution,” Med. Phys. 37, 1761–1767 (2010).
33. I. Kawrakow, E. Mainegra-Hing, F. Tessier, and B. R. B. Walters, “The EGSnrc C ++library,” NRC Report PIRS898 (rev A) (National Research Council, Ottowa, 2009).
34.ISO, “Guide to expression of uncertainty in measurement,” Technical Report Guide 98 (International Organization of Standardization, Geneva, 1995).
35. F. M. Khan, The Physicis of Radiation Therapy, 3rd ed. (Lippincott Williams and Wilkins, Philadelphia, 2003).
36. A. J. D. Scott, A. E. Nahum, and J. D. Fenwick, “Monte Carlo modeling of small photon fields: Quantifying the impact of focal spot size on source occlusion and output factors, and exploring miniphantom design for small-field measurements,” Med. Phys. 36, 3132–3143 (2009).
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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.
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.
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.
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.
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