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Suitability of using multileaf collimator (MLC) for photon field matching
1Medical Physics and Radiation Oncology, Ottawa Regional Cancer Center, Ottawa, Ontario, Canada. ckwok@ottawahospital.on.ca
Summary
Siemens double-focused multileaf collimator (MLC) field matching closely resembles independent jaws but with a non-uniform match line. Adjusting MLC leaf gap-overlap and using intrafraction feathering significantly reduces dose variation for improved photon field matching.
Area of Science:
- Medical Physics
- Radiation Oncology
Background:
- Accurate photon field matching is crucial in radiation therapy to ensure precise dose delivery.
- Multileaf collimators (MLCs) are widely used for beam shaping, but their suitability for field matching requires careful evaluation.
Purpose of the Study:
- To assess the suitability of a Siemens double-focused multileaf collimator (MLC) for photon field matching.
- To investigate dose variations and optimize matching techniques using MLCs.
Main Methods:
- Film-based dosimetry was employed for precise dose measurements.
- Theraplan Plus treatment planning system (TPS) was used for planning comparisons and further investigations.
- Evaluated field matching using independent jaws and MLCs, analyzing dose changes with varying leaf gap-overlap.
Main Results:
- MLC field matching closely mimicked independent jaws, with a notable non-uniform match line due to leaf pair gap-overlap.
- Dose change was linearly dependent on MLC leaf pair gap-overlap, measuring 11 ± 1%/mm.
- Intrafraction "feathering" (junction movement) reduced dose variation: once to 6.5 ± 1%/mm, twice to 4 ± 1%/mm.
Conclusions:
- MLC positioning error tolerances of ±2 mm may be too large for accurate photon field matching.
- A combination of tighter tolerances (±1 mm) and intrafraction junction moves can minimize dose heterogeneity within the planning target volume (PTV) to -5% to +7%.