SU-E-T-733: The Effects of Duty Cycle On Gated VMAT Delivery

Authors

  • Song C,

    1. St. Luke's Hospital, Bethlehem, PA
    2. Thomas Jefferson University, Philadelphia, PA
    3. St. Luke's Hospital, Bethlehem, PA
    4. St. Luke's Hospital, Bethlehem, PA
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  • Xue C,

    1. St. Luke's Hospital, Bethlehem, PA
    2. Thomas Jefferson University, Philadelphia, PA
    3. St. Luke's Hospital, Bethlehem, PA
    4. St. Luke's Hospital, Bethlehem, PA
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  • Li R,

    1. St. Luke's Hospital, Bethlehem, PA
    2. Thomas Jefferson University, Philadelphia, PA
    3. St. Luke's Hospital, Bethlehem, PA
    4. St. Luke's Hospital, Bethlehem, PA
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  • Xue T

    1. St. Luke's Hospital, Bethlehem, PA
    2. Thomas Jefferson University, Philadelphia, PA
    3. St. Luke's Hospital, Bethlehem, PA
    4. St. Luke's Hospital, Bethlehem, PA
    Search for more papers by this author

Abstract

Purpose:

When delivering gated VMAT treatments, the gantry moves back and forth after each beam interruption. There are more beam interruptions with smaller duty cycles. This is a study of the effects of gating and duty cycle on the dose output from the treatment machine.

Methods:

5 previously-treated SBRT lung patients with 6X FFF RapidArc plans and 4 previously-treated lung patients with 6X RapidArc plans are used for this study. The treatment machine is a TrueBeam STx with HD-MLC. A sinusoidal motion generator is used to gate the beam. A MapCheck, and a 0.6 cc ion chamber sandwiched in solid water phantom are used for dose measurements. The MapCheck is used to measure non-gated, gated with 50% and 30% duty cycles. The ion chamber is used to measure non-gated and gated with 30% duty cycles.

Results:

For the 5 SBRT lung plans with 6X FFF, the average percentage of diodes with dose difference less than 0.1% is 99.5% between non-gated and 50% duty cycle, and 99.7% between non-gated and 30% duty cycle. For the 4 lung plans with 6X, the average percentage of diodes with dose difference less than 0.1% is 99.5% between non-gated and 50% duty cycle, and 98.9% between non-gated and 30% duty cycle. Using the ion chamber, the measured dose difference between non-gated and 30% duty cycle is within 0.1% for all 10 SBRT arcs and within 0.2% for all 10 non-SBRT arcs.

Conclusion:

On a TrueBeam machine, the VMAT dose delivery difference is within 0.2% between non-gated and gated treatments. This difference decreases with the increase of duty cycle, also decreases with the decrease of target volume.

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