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Developing QA Procedures for Gated VMAT SABR Treatments

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F Viel

F Viel1*, E Gete2, C Duzenli1,2, R Lee2, (1) University of British Columbia, Vancouver, BC, (2) BC Cancer Agency Vancouver Centre, Vancouver, BC,

SU-D-105-7 Sunday 2:05PM - 3:00PM Room: 105

Purpose: To develop a QA procedure for gated VMAT SABR liver cancer treatments and investigate the gating parameters for acceptable plan delivery in terms of the dose to a moving volume and treatment delivery time.
Methods: 10 patient plans for VMAT SABR liver were created using the Eclipseâ„¢ TPS. The verification plans were then transferred to a CT-scanned Quasarâ„¢ phantom and delivered on a TrueBeamâ„¢ linac using a 10FFF beam and Varian's RPM system for respiratory gating. Two kinds of breathing patterns were used: free breathing (FB) and an interrupted (~5s pause) end of exhale coached breathing (CB) pattern. Ion chamber and Gafchromicâ„¢ film measurements were acquired for a gated delivery while the phantom moved under the described breathing patterns and a non-gated, stationary phantom delivery. The gate window was set to obtain a range of residual target motion from 2-10mm.
Results: Preliminary chamber measurements indicate that the dose to the center of the PTV can vary considerably under gated delivery compared to the static case. The effect can be significant for free breathing; ~4-12% over the selected range of residual target motion. The agreement was more consistent with CB pattern at ~1-4%. Gamma analysis (3%, 3mm) showed an agreement above 99.74% for all gated deliveries compared to the static delivery. The treatment time with a gate width of 2 mm was ~265s for the CB pattern compared to ~740s under a typical FB pattern. A non-gated delivery of the same plan took ~100s.
Conclusion: Gated VMAT treatments have been delivered successfully to a motorized phantom. FB patterns contain considerable variability and it is difficult to achieve acceptable results even with very small gate windows. However, a CB pattern combined with a sufficiently small gate, resulted in acceptable dose distributions that can be delivered in a reasonable amount of time.

Funding Support, Disclosures, and Conflict of Interest: Francis Viel received funding from the Natural Sciences and Engineering Research Council of Canada. This work has been supported by the Varian Research Collaborations Program.

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