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MLC positioning verification for small fields: a new investigation into automatic EPID-based verification methods


Multileaf-collimator (MLC) defined small fields in radiotherapy are used in high dose, ultra-conformal techniques such as stereotactic radiotherapy and stereotactic radiosurgery. Proximity to critical structures and irreversible damage arising from inaccurate delivery mean that correct positioning of the MLC system is of the utmost importance. Some of the existing techniques for MLC positioning quality assurance make use of electronic portal imaging device (EPID) images. However, conventional collimation verification algorithms based on the full width at half maximum (FWHM) fail when applied to small field images acquired by an EPID due to overlapping aperture penumbrae, lateral electron disequilibrium and radiation source occlusion. The objective of this study was to investigate sub-pixel edge detection and other techniques with the aim of developing an automatic and autonomous EPID-based method suitable for MLC positional verification of small static fields with arbitrary shapes. Methods investigated included derivative interpolation, Laplacian of Gaussian (LoG) and an algorithm based on the partial area effect hypothesis. None of these methods were found to be suitable for MLC positioning verification in small field conditions. A method is proposed which uses a manufacturer-specific empirically modified FWHM algorithm which shows improvement over the conventional techniques in the small field size range. With a measured mean absolute difference from planned position for Varian linacs of 0.01 ± 0.26 mm, compared with the erroneous FWHM value of 0.70 ± 0.51 mm. For Elekta linacs the proposed algorithm returned 0.26 ± 0.25 mm, in contrast to the FWHM result of 1.79 ± 1.07 mm.

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  1. 1.

    Yang Y, Xing L (2004) Quantitative measurement of MLC leaf displacements using an electronic portal image device. Phys Med Biol 49(8):1521

  2. 2.

    Mamalui-Hunter M, Li H, Low DA (2008) MLC quality assurance using EPID: a fitting technique with subpixel precision. Med Phys 35(6):2347–2355

  3. 3.

    Samant SS, Zheng W, Parra NA, Chandler J, Gopal A, Wu J, Jain J, Zhu Y, Sontag M (2002) Verification of multileaf collimator leaf positions using an electronic portal imaging device. Med Phys 29(12):2900–2912

  4. 4.

    Eilertsen K (1997) Automatic detection of single MLC leaf positions with corrections for penumbral effects and portal imager dose rate characteristics. Phys Med Biol 42(2):313

  5. 5.

    Mohammadi M, Bezak E (2006) Evaluation of MLC leaf positioning using a scanning liquid ionization chamber EPID. Phys Med Biol 52(1):N21

  6. 6.

    Lebron S, Yan G, Li J, Lu B, Liu C (2017) A universal parameterized gradient-based method for photon beam field size determination. Med Phys 44(11):5627–5637

  7. 7.

    Rowshanfarzad P, Sabet M, Barnes MP, O’Connor DJ, Greer PB (2012) EPID-based verification of the MLC performance for dynamic IMRT and VMAT. Med Phys 39(10):6192–6207

  8. 8.

    Rowshanfarzad P, Sabet M, O’Connor DJ, Greer PB (2011) Isocenter verification for linac-based stereotactic radiation therapy: review of principles and techniques. J Appl Clin Med Phys 12(4):185–195

  9. 9.

    Klein EE, Hanley J, Bayouth J, Yin F-F, Simon W, Dresser S, Serago C, Aguirre F, Ma L, Arjomandy B, Liu C, Sandin C, Holmes T (2009) Task Group 142 report: quality assurance of medical accelerators. Med Phys 36(9):4197–4212

  10. 10.

    Benedict SH, Yenice KM, Followill D, Galvin JM, Hinson W, Kavanagh B, Keall P, Lovelock M, Meeks S, Papiez L (2010) Stereotactic body radiation therapy: the report of AAPM Task Group 101. Med Phys 37(8):4078–4101

  11. 11.

    Charles P, Cranmer-Sargison G, Thwaites DI, Crowe S, Kairn T, Knight R, Kenny J, Langton CM, Trapp J (2014) A practical and theoretical definition of very small field size for radiotherapy output factor measurements. Med Phys.

  12. 12.

    Kairn T, Asena A, Charles P, Hill B, Langton CM, Middlebrook N, Moylan R, Trapp J (2015) Field size consistency of nominally matched linacs. Australas Phys Eng Sci Med 38(2):289–297

  13. 13.

    Alber M, Broggi S, De Wagter C, Eichwurzel I, Engstrom P, Fiorino C, Georg D, Hartmann G, Knoos T, Leal A et al (2008) Guidelines for the verification of IMRT. In: ESTRO booklet 9. Brussels

  14. 14.

    Chui C-S, Spirou S, LoSasso T (1996) Testing of dynamic multileaf collimation. Med Phys 23(5):635–641

  15. 15.

    Neal B, Ahmed M, Kathuria K, Watkins T, Wijesooriya K, Siebers J (2016) A clinically observed discrepancy between image-based and log-based MLC positions. Med Phys 43(6 Part 1):2933–2935

  16. 16.

    Aspradakis MM, Byrne JP, Palmans H, Duane S, Conway J, Warrington AP, Rosser K (2010) Small field MV photon dosimetry. IPEM report 103, IPEM

  17. 17.

    IAEA (2017) Dosimetry of small static fields used in external beam radiotherapy. Technical reports series. International Atomic Energy Agency, Vienna

  18. 18.

    Andreo P (2017) The physics of small megavoltage photon beam dosimetry. Radiother Oncol 126(2):205–213

  19. 19.

    Das IJ, Ding GX, Ahnesjo A (2008) Small fields: Nonequilibrium radiation dosimetry. Med Phys 35(1):206–215

  20. 20.

    LoSasso T (2008) IMRT delivery performance with a varian multileaf collimator. Int J Radiat Oncol Biol Phys 71(1):S85–S88

  21. 21.

    Clews L, Greer PB (2009) An EPID based method for efficient and precise asymmetric jaw alignment quality assurance. Med Phys 36(12):5488–5496

  22. 22.

    Greer PB, Cadman P, Lee C, Bzdusek K (2009) An energy fluence-convolution model for amorphous silicon EPID dose prediction. Med Phys 36(2):547–555

  23. 23.

    Du W, Gao S, Wang X, Kudchadker RJ (2012) Quantifying the gantry sag on linear accelerators and introducing an MLC-based compensation strategy. Med Phys 39(4):2156–2162

  24. 24.

    Rowshanfarzad P, McGarry CK, Barnes MP, Sabet M, Ebert MA (2014) An EPID-based method for comprehensive verification of gantry, EPID and the MLC carriage positional accuracy in Varian linacs during arc treatments. Radiat Oncol 9(1):249

  25. 25.

    Rowshanfarzad P, Sabet M, O’Connor DJ, Greer PB (2011) Verification of the linac isocenter for stereotactic radiosurgery using cine-EPID imaging and arc delivery. Med Phys 38(7):3963–3970

  26. 26.

    Fuangrod T, Woodruff HC, Rowshanfarzad P, O’Connor DJ, Middleton RH, Greer PB (2014) An independent system for real-time dynamic multileaf collimation trajectory verification using EPID. Phys Med Biol 59(1):61–81

  27. 27.

    Gao Z, Szanto J, Gerig L (2007) Using multileaf collimator interleaf leakage to extract absolute spatial information from electronic portal imaging device images. J Appl Clin Med Phys 8(1):1–9

  28. 28.

    Prince SJD (2012) Computer vision: models, learning, and inference. Cambridge University Press, Cambridge

  29. 29.

    Ding L, Goshtasby A (2001) On the Canny edge detector. Pattern Recognit 34(3):721–725

  30. 30.

    Canny J (1986) A computational approach to edge detection. IEEE Trans Pattern Anal Mach Intell 6:679–698

  31. 31.

    Bijhold J, Gilhuijs KGA, Van Herk M, Meertens H (1991) Radiation field edge detection in portal images. Phys Med Biol 36(12):1705

  32. 32.

    Trujillo-Pino A, Krissian K, Alemán-Flores M, Santana-Cedrés D (2013) Accurate subpixel edge location based on partial area effect. Image Vis Comput 31(1):72–90

  33. 33.

    Szeliski R (2010) Computer vision: algorithms and applications. Springer, New York

  34. 34.

    Bushberg JT, Boone JM (2011) The essential physics of medical imaging. Lippincott Williams & Wilkins, Philadelphia

  35. 35.

    Vial P, Oliver L, Greer PB, Baldock C (2006) An experimental investigation into the radiation field offset of a dynamic multileaf collimator. Phys Med Biol 51(21):5517

  36. 36.

    Bergman AM, Gete E, Duzenli C, Teke T (2014) Monte Carlo modeling of HD120 multileaf collimator on Varian TrueBeam linear accelerator for verification of 6X and 6X FFF VMAT SABR treatment plans. J Appl Clin Med Phys 15(3):148–163

  37. 37.

    Bakhtiari M, Kumaraswamy L, Bailey D, De Boer S, Malhotra H, Podgorsak M (2011) Using an EPID for patient-specific VMAT quality assurance. Med Phys 38(3):1366–1373

  38. 38.

    Ding GX, Duggan DM, Coffey CW (2006) Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods. Phys Med Biol 51(10):2549

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The authors wish to gratefully acknowledge Prof. Martin Ebert for his helpful comments during the research meetings.

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Correspondence to Joshua Hiatt.

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Hiatt, J., Mukwada, G., Barnes, M. et al. MLC positioning verification for small fields: a new investigation into automatic EPID-based verification methods. Australas Phys Eng Sci Med 41, 945–955 (2018).

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  • Small field MLC EPID-based verification
  • SRT quality assurance
  • Sub-pixel edge detection