SU‐E‐J‐39: Effect of Jaw Size in MVCT On Image Quality and Dose in Helical Tomotherapy

Abstract

Purpose: The jaw size for the TomoTherapy Hi‐Art II (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye. Methods: MVCT image quality (image noise, uniformity, contrast linearity, high‐contrast resolution, and full width at half‐maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements. Results: The change in jaw size from J4 to J1 produced no change or only a slight improvement in image noise, uniformity, contrast linearity, and high‐contrast resolution. However, the full‐width at half‐maximum reduced from approximately 6.7 at J4 to 4.1 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69∼2.32 cGy (peripheral: 0.83∼2.49 cGy) at J4 to 0.85∼2.81 cGy (peripheral: 1.05∼2.86 cGy) at J1. The measured lens dose increased from 0.92∼3.36 cGy at J4 to 1.06∼3.91 cGy at J1. Conclusion: The change in jaw size can be expected to accelerate treatment planning and improve image registration accuracy. An additional dose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in imaging dose over that with J4 should be provided with J1. This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (Grant No. 2009‐00420) and program of Basic Atomic Energy Research Institute (BAERI) which is a part of the Nuclear R and D programs (No. 2012000486) funded by the MEST.