MLC positioning verification for small fields: a new investigation into automatic EPID-based verification methods
- 79 Downloads
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.
KeywordsSmall field MLC EPID-based verification SRT quality assurance Sub-pixel edge detection
The authors wish to gratefully acknowledge Prof. Martin Ebert for his helpful comments during the research meetings.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
- 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. BrusselsGoogle Scholar
- 16.Aspradakis MM, Byrne JP, Palmans H, Duane S, Conway J, Warrington AP, Rosser K (2010) Small field MV photon dosimetry. IPEM report 103, IPEMGoogle Scholar
- 17.IAEA (2017) Dosimetry of small static fields used in external beam radiotherapy. Technical reports series. International Atomic Energy Agency, ViennaGoogle Scholar
- 33.Szeliski R (2010) Computer vision: algorithms and applications. Springer, New YorkGoogle Scholar
- 34.Bushberg JT, Boone JM (2011) The essential physics of medical imaging. Lippincott Williams & Wilkins, PhiladelphiaGoogle Scholar