Program Information
Clinical Implementation of Deep Inspiration Breath Hold VMAT On An Elekta Versa Accelerator
J Matney*, L Rankine, R Woods, S Das , University of North Carolina, Chapel Hill, NC
Presentations
MO-F-CAMPUS-JT-3 (Monday, July 31, 2017) 4:30 PM - 5:30 PM Room: Joint Imaging-Therapy ePoster Theater
Purpose: The Elekta Versa fast-tuning magnetron design can delay beam-on for up to 4 seconds, thus such accelerators may be better suited for deep inspiration breath hold (DIBH) instead of free-breathing gating. This study characterized the performance of Elekta Versa DIBH gated volumetric modulated arc therapy (VMAT).
Methods: We investigated ramp-up-ramp-down dosimetric effects by delivering a set number of monitor units to an ion chamber and gating with a range of beam on/off times to simulate potential gating periods. A series of gated lung and breast VMAT deliveries were measured with an ArcCheck (Sun Nuclear, Melbourne, FL). Delivery performance was evaluated by comparing measured vs. planned dose using 3%/3mm gamma analysis for non-gated and gated deliveries.
Results: Compared to non-gated delivery, quickly gating a static field showed minor reduction in measured dose: 0.6% at 1 second on/off gate. However, if the accelerator returns to standby mode (after ~6.5 seconds gate-off), there is a 3-4 second delay in beam-on. Gating the beam from standby reduced the measured dose by 2.2% and 0.5% for 5 and 10 second gating windows, respectively. VMAT gating reduced the non-gated (>97% at 3%/3mm criteria) gamma index pass rates by 0.5-2.4% during 5 second windows, 0.3-1% during 10 second windows. The VMAT DIBH technique has been clinically implemented at our institution to treat patients with left-sided breast/chestwall disease.
Conclusion: The Elekta Versa can perform gated DIBH VMAT at dosimetric quality comparable to non-gated VMAT. When the breath hold length is sufficiently long (>10s), gating does not significantly (<1%) affect the measured dosimetry. Measurements show a need to obtain a gating window >10s to overcome the beam-start up delay inherent in the Elekta Versa. To limit minor ramp-up effects and increase duty cycle, we recommend patients maintain 15-20 second breath hold period for VMAT DIBH on our institution.
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