Rangaraj Dharanipathy, Palaniswaamy Geethpriya, Papiez Lech
Department of Radiation Oncology, Washington University, St. Louis, Missouri 63110, USA.
Med Phys. 2008 Aug;35(8):3765-78. doi: 10.1118/1.2952775.
The goal of this article is to present the algorithm for DMLC leaf control capable of delivering IMRT to tumors that experience motion in two dimensions in the beams eye view (BEV) plane. The generic, two-dimensional (2D) motion of the projection of the rigid target on BEV plane can be divided into two components. The first component describes the motion of the projection of the target along the x axis (parallel to the MLC leaf motions) and the other describes the motion of the target projection on the y axis (perpendicular to the leaf motion direction). First, time optimal leaf trajectories are calculated independently for each leaf pair of the MLC assembly to compensate the x-axis component of the 2D motion of the target on the BEV. These leaf trajectories are then synchronized following the mid time (MT) synchronization procedure. To compensate for the y-axis component of the motion of the target projection on the BEV plane, the procedure of "switching" leaf pair trajectories in the upward (or downward) direction is executed when the target's BEV projection moves upward (or downward) from its equilibrium position along the y axis. When the intensity function is a 2D histogram, the error between the intended and delivered intensity in 2D DMLC IMRT delivery will depend on the shape of the intensity map and on the MLC physical constraint (leaf width and maximum admissible leaf speed). The MT synchronization of leaf trajectories decreases the impact of above constraints on the error in 2D DMLC IMRT intensity map delivery. The proof is provided, that if hardware constraints in the 2D DMLC IMRT delivery strategy are removed, the errors between planned and delivered 2D intensity maps are entirely eliminated. Examples of 2D DMLC IMRT delivery to rigid targets moving along elliptical orbits on BEV planes are calculated and analyzed for 20 clinical fluence maps. The comparisons between the intensity delivered without motion correction, with motion correction along x axis only, and with motion correction for full 2D motion of the target are calculated and quantitatively evaluated. The fluence maps were normalized to 100 MU and the rms difference between the desired and delivered fluence was 12 MU for no motion compensation, 11.18 MU for 1D compensation, and 4.73 MU for 2D motion compensations. The advantage of correcting for full 2D motion of target projected on the BEV plane is demonstrated.
本文的目标是提出一种用于动态多叶准直器（DMLC）叶片控制的算法，该算法能够对在射野方向观（BEV）平面内经历二维运动的肿瘤进行调强放射治疗（IMRT）。刚性靶区在BEV平面上投影的一般二维运动可分为两个分量。第一个分量描述靶区投影沿x轴（平行于MLC叶片运动方向）的运动，另一个分量描述靶区投影沿y轴（垂直于叶片运动方向）的运动。首先，针对MLC组件的每对叶片独立计算时间最优叶片轨迹，以补偿BEV平面上靶区二维运动的x轴分量。然后，按照中间时间（MT）同步程序对这些叶片轨迹进行同步。为了补偿BEV平面上靶区投影运动的y轴分量，当靶区的BEV投影沿y轴从其平衡位置向上（或向下）移动时，执行叶片对轨迹在向上（或向下）方向的“切换”程序。当强度函数为二维直方图时，二维DMLC IMRT放疗中预期强度与实际输出强度之间的误差将取决于强度图的形状以及MLC的物理约束（叶片宽度和最大允许叶片速度）。叶片轨迹的MT同步减少了上述约束对二维DMLC IMRT强度图放疗误差的影响。文中证明，如果去除二维DMLC IMRT放疗策略中的硬件约束，则计划的和实际输出的二维强度图之间的误差将完全消除。针对20个临床注量图，计算并分析了在BEV平面上沿椭圆轨道运动的刚性靶区的二维DMLC IMRT放疗示例。计算并定量评估了无运动校正、仅沿x轴进行运动校正以及对靶区的完整二维运动进行运动校正时所输出的强度之间的比较。注量图归一化为100 MU，无运动补偿时期望注量与实际输出注量之间的均方根差为12 MU，一维补偿时为11.18 MU，二维运动补偿时为4.73 MU。结果表明了对BEV平面上靶区的完整二维运动进行校正的优势。
