SU-F-T-532: Wobble: A New Metric to Quantify Dynamic-Jaw VMAT Delivery




Dynamic-jaw tracking minimizes the unblocked area to account for changes in the target's projection as a function of gantry angle. To more fully quantify the benefit of dynamic-jaw tracking, we have introduced a “wobble” metric that accounts for both target offset and eccentricity as opposed to a jaw factor which only accounts for the increase in blocked area


The wobble is the unblocked area divided by target crosssectional area, and incorporates target offset and eccentricity. The jaw factor is simply the ratio of unblocked area with and without dynamic-jaws. Forty patients treated to different sites were replanned with dynamic-jaw tracking in Eclipse. The majority had multiple arcs and reduced-field boosts resulting in 151 fields. We quantified the relationship between wobble and jaw factor.


The highest jaw factor measured was 0.9692, for a prostate case, and corresponded to a wobble of 0.355. The lowest jaw factor was 0.4722, for a rectal case, and corresponded to a wobble of 2.410. The median jaw factor was 0.7917. The wobble values ranged from 0.2145 to 2.575. The minimum value of wobble was found in a PTV containing prostate-only site while the maximum value of wobble was found in a PTV containing prostate, seminal vesicle, and lymph nodes. The median wobble was 1.013. For all sites other than brain, there is approximately 20% reduction in jaw factor per 1 unit of wobble. For brain, there was 30% jaw factor reduction per unit of wobble.


The wobble metric was introduced to fully quantify dynamic-jaw tracking and potential dose reduction. There was good correlation observed between wobble and jaw factor. Increasing wobble indicates either large offset or more eccentricity and the potential for substantial dose reduction with dynamic tracking.