Shen Sui, Duan Jun, Fiveash John B, Brezovich Ivan A, Plant Brian A, Spencer Sharon A, Popple Richard A, Pareek Prem N, Bonner James A
Department of Radiation Oncology, University of Alabama at Birmingham, Birmingham, Alabama 35233, USA.
Med Phys. 2003 Dec;30(12):3196-205. doi: 10.1118/1.1626121.
The capability of a commercial respiratory gating system based on video tracking of reflective markers to reduce motion-induced CT planning and treatment errors was evaluated. Spherical plastic shells (2.8-82 cm3), simulating the gross target volume (GTV), were placed in a water-filled body phantom that was moved sinusoidally along the longitudinal axis of the CT scanner and the accelerator for +/- 1 cm at 15-30 cycle/min. During gated CT imaging, the x-ray exposure was initiated by the gating system shortly before the end of expiration (so that the imaging time would be centered at the end of expiration); it was terminated by the scanner after completion of each slice. In nongated CT images, the target appeared distorted and often broken up. GTVs volume errors ranged 16%-110% in axial scans, and 7%-36% in spiral scans. In gated CT images, the spheres appeared 3 and 5 mm longer than their actual diameters (volume errors 2%-16%), at the respective respiration rates of 15 and 20 cycles/min. At 30 cycles/min the target appeared 1 cm longer, and volume error ranged 25%-53%. During treatment, gating kept the beam on for a duration equal to the CT acquisition time of 1 s/slice. The difference in positional errors between gated CT and portal films was 1 mm, regardless the size of residual motion errors. Because of the potential of suboptimal placement of the gating window between CT imaging and treatment, an extra 1.5-2.5 mm safety margin can be added regardless of the size of residual motion error. For respiratory rates > or = 30 cycles/min, the effectiveness of gating is limited by large residual motion in the 1 s CT acquisition time.
对基于反射标记视频跟踪的商用呼吸门控系统减少运动诱导的CT计划和治疗误差的能力进行了评估。模拟大体靶体积（GTV）的球形塑料壳（2.8 - 82 cm³）被放置在充满水的人体模体中，该模体沿CT扫描仪和加速器的纵轴以15 - 30次/分钟的频率进行±1 cm的正弦运动。在门控CT成像期间，门控系统在呼气即将结束前不久启动X射线曝光（以便成像时间以呼气结束为中心）；扫描完成每一层后由扫描仪终止曝光。在非门控CT图像中，靶区出现变形且常常破碎。在轴向扫描中，GTV体积误差范围为16% - 110%，在螺旋扫描中为7% - 36%。在门控CT图像中，在呼吸频率分别为15次/分钟和20次/分钟时，球体看起来比其实际直径长3 mm和5 mm（体积误差为2% - 16%）。在30次/分钟时，靶区看起来长1 cm，体积误差范围为25% - 53%。在治疗期间，门控使射束开启的持续时间等于CT每层采集时间1 s。门控CT与射野片之间的位置误差差异为1 mm，与残余运动误差的大小无关。由于在CT成像和治疗之间门控窗口放置可能不理想，无论残余运动误差大小，均可额外增加1.5 - 2.5 mm的安全 margins。对于呼吸频率≥30次/分钟，门控的有效性受到1 s CT采集时间内较大残余运动的限制。
