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Related Experiment Videos

Dosimetric effect of collimating jaws for small multileaf collimated fields.

James C L Chow1, Melanie Seguin, Andrew Alexander

  • 1Medical Physics Department, Grand River Regional Cancer Center, Grand River Hospital, PO. Box 9056, 835 King Street West, Kitchener, Ontario N2G 1G3, Canada. james.chow@grhosp.on.ca

Medical Physics
|April 21, 2005
PubMed
Summary

Dosimetric effects from multileaf collimator (MLC) jaw positioning were studied in small radiation fields. Moving the jaw away from leaf-ends increases output and penumbra, crucial for intensity-modulated radiotherapy (IMRT) accuracy.

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Area of Science:

  • Medical Physics
  • Radiation Oncology

Background:

  • Intensity-modulated radiotherapy (IMRT) requires precise dose calculations, especially for small fields.
  • Multileaf collimators (MLCs) and jaws interact in ways that can affect dose delivery, particularly at field edges.
  • Conventional treatment planning systems often neglect dosimetric effects from jaw-leaf interactions.

Purpose of the Study:

  • To investigate the dosimetric impact of jaw positioning relative to MLC leaf-ends in small radiation fields (0.5x0.5 to 2x2 cm2).
  • To quantify the contribution of jaw-end, leaf-end, and inter/intraleaf leakage to dose measurements.
  • To evaluate the influence of these effects on percentage depth dose, beam profile, and output.

Main Methods:

  • Measurements were performed on a Varian 21 EX linear accelerator using a 6 MV photon beam and a 120-leaf MLC.

Related Experiment Videos

  • The jaw position was varied at different distances from the MLC leaf-ends in the side-edge of small fields.
  • Percentage depth dose, beam profile, and output were measured for various jaw positions and field sizes.

    • Main Results:

    • Moving the jaw away from the leaf-ends increased output and penumbra width for small fields.
    • These increases were most significant for the smallest field size (0.5x0.5 cm2).
    • The rate of increase in output and penumbra changed rapidly when the jaw-end was 1-2 cm from the leaf-end.

    Conclusions:

    • Jaw positioning significantly impacts dosimetric measurements in small IMRT fields.
    • These effects, often ignored by treatment planning systems, necessitate careful consideration during commissioning.
    • Measurements during IMRT commissioning are essential for physicists to verify treatment planning system accuracy regarding jaw and leaf transmission effects.