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Related Experiment Videos

Indexing artefacts using a tertiary collimator and a method to avoid them.

P Cooper1, R I MacKay, P C Williams

  • 1North Western Medical Physics, Christie Hospital NHS Trust, Manchester, UK. phill.cooper@physics.cr.man.ac.uk

Physics in Medicine and Biology
|August 8, 2002
PubMed
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A novel slotted grid collimator improves radiation field resolution but can cause dose artefacts. An alternative arrangement is proposed to avoid these issues, enhancing treatment accuracy.

Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Radiotherapy Technology

Background:

  • A tertiary, slotted grid collimator was developed to enhance spatial resolution of radiation fields produced by multileaf collimators (MLC).
  • This device irradiates through slots, creating high-dose strips, requiring patient indexing (rotation) for complete field coverage.
  • Rotation of the tertiary collimator can introduce artefacts due to MLC leaf positioning and radiation source characteristics.

Purpose of the Study:

  • To investigate artefacts in radiation field definition caused by the rotation of a tertiary, slotted grid collimator.
  • To quantify the magnitude of overdose and underdose effects resulting from slot-to-MLC leaf junction proximity and leaf edge penumbra.
  • To propose and demonstrate an alternative arrangement that mitigates these artefacts.

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Main Methods:

  • Utilized a prototype slotted grid collimator aligned with a multileaf collimator (MLC).
  • Performed dose measurements using a mini-diode for different slot widths (2.5 mm and 5.0 mm) under varying MLC leaf positions.
  • Analyzed artefacts arising from slot edge proximity to MLC leaf junctions, finite source size, and leaf edge penumbra.

Main Results:

  • Unwanted overdose due to partial shielding by neighboring MLC leaves did not exceed 8% for either slot width.
  • Significant underdose was observed, particularly with the 2.5 mm slot, where peak intensity reached only 41% of the central dose.
  • Artefacts were linked to the unavoidable arrangement where slot spacing matched MLC leaf spacing.

Conclusions:

  • The rotation of a tertiary slotted grid collimator can lead to significant underdose artefacts, compromising radiation therapy precision.
  • The identified artefacts are primarily caused by the interplay between slot geometry, MLC leaf positioning, and beam penumbra.
  • An alternative collimator arrangement has been proposed and demonstrated to effectively eliminate these detrimental artefacts, improving treatment accuracy.