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

  • Joshua HiattEmail author
  • Godfrey Mukwada
  • Michael Barnes
  • Hans Lynggaard Riis
  • Du Huynh
  • Pejman Rowshanfarzad
Scientific Paper


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.


Small 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.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.


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Copyright information

© Australasian College of Physical Scientists and Engineers in Medicine 2018

Authors and Affiliations

  1. 1.Department of Radiation OncologyLiverpool & Macarthur Cancer Therapy CentresLiverpoolAustralia
  2. 2.Department of Radiation OncologySir Charles Gairdner HospitalNedlandsAustralia
  3. 3.Department of Radiation OncologyCalvary Mater Newcastle HospitalNewcastleAustralia
  4. 4.University of NewcastleNewcastleAustralia
  5. 5.Radiofysisk LaboratoriumOdense University HospitalOdenseDenmark
  6. 6.School of Physics, Mathematics and Computing, Faculty of Engineering and Mathematical SciencesThe University of Western AustraliaCrawleyAustralia

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