Recently,jaw size was changed from 4 mm aperture (J4) to 1 mm (J1) in TomoTherapy Hi-Art Ⅱ(R) (TomoTherapy Inc.,Madison,WI) to improve the longitudinal (IEC-Y) resolution of megavoltage computed tomography (MVCT) images.This study attempted to conduct a comparative analysis on the effect of jaw aperture size of MVCT on image quality and dose.Also,this study investigated the clinical effect of jaw aperture size on highly radiosensitive lens dose.MVCT image quality test (image noise,uniformity,contrast linearity,and spatial resolution) and multiple scan average dose (MSAD) were measured to examine the effect of change in jaw aperture size.A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure exposed lens dose (cGy).For image quality,the change in jaw aperture size did not have a significant effect on image noise,uniformity,contrast linearity,and spatial resolution although showing slight improvement of image quality.FHWM was 6.7 mm and 4.1 mm in J4 and J1,respectively,which represents enhancement in the longitudinal resolution of MVCT image.MSAD of center point was approximately 0.69-2.32 cGy (peripheral: 0.83- 2.48 cGy) in J4,and 0.85-2.81 cGy (peripheral: 1.05-2.86 cGy) in J1.The measured lens dose using PLDs ranged 0.92-3.36 cGy and 1.06-3.91 cGy in J4 and J1,respectively,indicating dose increase for the narrower jaw aperture size.An additional dose of 3.86 cGy,1.92 cGy,and 1.22 cGy was delivered for a pitch of 1,2,and 3,respectively during MVCT scan per fraction in head and neck treatment plans.Therefore,15% increase in imaging dose with 1 mm aperture jaw should be allowed for compared to that with previous 4 mm aperture jaw.
MVCT Image quality MSAD Lens PLD
J.H. Jung J.W. Yoon T.S. Suh K.H. Cho S.K. Moon Y.H. Kim C.K. Min W.C. Kim E.S. Kim A.R. Chang T.H. Kim
Department of Biomedical Engineering, The Catholic University of Korea, Seoul, Korea; Research Insti Department of Biomedical Engineering, The Catholic University of Korea, Seoul, Korea; Research Insti Department of Radiation Oncology, Bucheon Hospital, The Soonchunhyang University of Korea, Gyeonggi, Department of Radiation Oncology, Chenan Hospital, The Soonchunhyang University of Korea, Chungnam, Department of Radiation Oncology, Seoul Hospital, The Soonchunhyang University of Korea, Seoul, Kore