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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
Summary
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.
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.