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Related Experiment Videos

Output ratio in air for MLC shaped irregular fields.

Timothy C Zhu1, Bengt E Bjärngard, Ying Xiao

  • 1Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. tzhu@mail.med.upenn.edu

Medical Physics
|October 19, 2004
PubMed
Summary

An algorithm was developed to calculate the output ratio in air (Sc) for irregular fields shaped by multileaf collimators (MLC). This method accurately predicts Sc, crucial for monitor unit calculations in radiation therapy.

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Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Radiotherapy Treatment Planning

Background:

  • Accurate monitor unit (MU) calculation is essential for precise radiotherapy.
  • The output ratio in air (Sc) for irregular fields shaped by multileaf collimators (MLC) is critical for MU calculations.
  • Existing methods may require detailed head geometry, limiting their applicability.

Purpose of the Study:

  • To develop and validate an algorithm for calculating Sc for MLC-shaped irregular fields.
  • To assess the impact of MLC type and position on head-scatter correction factor (HCF).
  • To compare Sc values across different MLC configurations and accelerator models.

Main Methods:

  • Developed an empirical model algorithm projecting MLC leaf positions to the isocenter plane.

Related Experiment Videos

  • Compared calculations for three MLC types: replacing inner jaws, replacing outer jaws, and tertiary attachment.
  • Investigated head-scatter correction factor (HCF) for MLC-shaped irregular fields versus encompassing rectangular fields.
  • Analyzed collimator exchange effects for MLC-shaped and jaw-shaped rectangular fields.
  • Evaluated Sc variations across Elekta, Siemens, and Varian accelerators for identical MLC fields.

    • Main Results:

    • The algorithm calculates Sc without needing exact head geometry.
    • MLC leaf positions significantly different from secondary collimators can cause up to 5% difference in HCF.
    • No collimator exchange effect was observed for MLC-shaped rectangular fields with fixed secondary collimators.
    • Sc values for the same irregular field varied up to 4% across different accelerator models (Elekta, Siemens, Varian).
    • Calculated Sc values agreed with measurements within 1.2% on and off the central axis.

    Conclusions:

    • The developed algorithm provides accurate Sc calculations for MLC-shaped fields, independent of detailed head geometry.
    • MLC configuration and accelerator model influence Sc values, necessitating model-specific considerations.
    • The algorithm's accuracy within 1.2% supports its use in clinical radiotherapy treatment planning.