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Evaluating the Dosimetric Accuracy of Small Gating Windows in Radiotherapy

W Short*, N Sperling , University of Toledo Medical Center, Toledo, OH


(Wednesday, 8/1/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 4

Purpose: To evaluate the effect of small gating windows on the treatment accuracy of gated VMAT radiotherapy in thoracic and abdominal lesions. The aim is to contrast the measured dose distribution with the treatment planning system planned dose distribution using clinically available absolute and relative dose plan quality assurance tools.

Methods: Using a Varian Edge, five clinically accepted VMAT plans were delivered to two static phantoms: a commercial cylindrical diode array with a micro-ionization chamber, and a custom cylindrical acrylic phantom wrapped with radiochromic film. The measured dose was then compared with the planned dose for each of five different beam-on windows, 100%, 70%, 50%, 30%, and 20%. The gating signal was generated using a programmable phantom at 15 BPM in combination with the Varian RPM system. A gamma analysis of 1mm/1%, 0.5mm/0.5%, and the percent difference of 1% were used to compare the measured treatments with the planned treatment doses determined by the treatment planning system.

Results: The max deviation from the ungated treatment was 1.8, 0.8, and 1.3% for the 1mm/1%, 0.5mm/0.5%, and 1% DTA. Average point doses were slightly higher than the ungated treatment and deviated the most compared to the expected dose for the two larger gating windows, but the smaller gating windows were slightly less than the ungated plan. Treatment time increased by an average factor of 7 for the smallest gating window.

Conclusion: Preliminary data supports the use of small gating windows on a Varian Edge, suggesting there is minimal difference between gating levels. However, clinical treatment times are significantly longer, which may lead to an increase of intra-fraction patient motion.


Gating, Organ Motion, Treatment Techniques


TH- External beam- photons: Motion management (intrafraction)

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