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Output ratios in a miniphantom for asymmetric fields shaped by a multileaf collimator
D Georg1, C Garibaldi, A Dutreix
1Radiotherapy Department, University Hospital Gasthuisberg, Leuven, Belgium.
Physics in Medicine and Biology
|December 12, 1997
Summary
This study extends a formalism to accurately calculate radiation output for asymmetric fields generated by a GE multileaf collimator (MLC). The enhanced method precisely predicts output ratios, crucial for precise radiation therapy delivery.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Physics
Background:
- The GE multileaf collimator (MLC) enables the creation of 'double' asymmetric radiation fields.
- Existing formalisms for calculating output ratios in miniphantoms were limited to fields including the collimator's rotational axis.
Purpose of the Study:
- To extend a previously proposed formalism for calculating output ratios in miniphantoms.
- To accurately calculate output ratios for asymmetric fields, including those with overlapping central leaves or jaws at the collimator axis.
Main Methods:
- Introduction of 'reduced coordinates' to extend the existing formalism.
- Application of the extended formalism to asymmetric square, rectangular, and irregular fields.
- Validation using miniphantom and film measurements for 6, 18, and 25 MV photon beams.
Main Results:
- Measured and calculated output ratios agreed within 1% for all field types and energies.
- Off-axis ratios derived from miniphantom and film measurements agreed within 1% (up to 1.3% at 18 MV).
- Calculated products of output ratios and off-axis ratios showed agreement within 1% (up to 1.6% for 18 MV).
Conclusions:
- The extended formalism accurately calculates output ratios for asymmetric fields, improving precision in radiation therapy.
- The method is validated across various field shapes and energies, demonstrating its robustness.
- This work enhances the capability to deliver precise radiation doses using advanced MLC configurations.