Sci—Thur PM: Planning & Delivery — 05: Evaluation of dose difference between VMAT plans with and without jaw tracking



The goal of this study is to quantify the dose difference between VMAT plans calculated with and without jaw tracking. In this study the sites were chosen so that there would be jaw tracking in the direction perpendicular to the leaf motion (Y jaws). For VMAT plans without jaw tracking in the Y direction there is additional dose (over the leaf transmission) leaking through abutting leaves that can't be moved out of the field and therefore move across the treatment field during delivery. VMAT plans for four head and neck patients with concurrent boost and three pelvis patients with concurrent prostate boost were generated using jaw tracking. A code was written in Matlab to convert each VMAT plan with jaw tracking (JT plan) to a VMAT plan with static jaws (SJ plan). The ST plan dose distribution was then recalculated and compared to the JT plan dose. VMAT plans with static jaws leave an additional dose trail compared to VMAT plans with jaw tracking. Between 6.7 and 230 cc of the SJ plans received an additional 2% of the prescription dose when compared to the JT plans and 0.5 to 30.1 cc received an additional 4% of the prescription dose. The additional dose trail left by the 2 arcs VMAT plans was less than the 1 arc VMAT for most plans presented in this study. This additional dose is given to normal tissues and/or critical structures surrounding the PTV.