Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA.
Med Phys. 2012 Apr;39(4):2156-62. doi: 10.1118/1.3697528.
Gantry sag is one of the well-known sources of mechanical imperfections that compromise the spatial accuracy of radiation dose delivery. The objectives of this study were to quantify the gantry sag on multiple linear accelerators (linacs), to investigate a multileaf collimator (MLC)-based strategy to compensate for gantry sag, and to verify the gantry sag and its compensation with film measurements.
The authors used the Winston-Lutz method to measure gantry sag on three Varian linacs. A ball bearing phantom was imaged with megavolt radiation fields at 10° gantry angle intervals. The images recorded with an electronic portal imaging device were analyzed to derive the radiation isocenter and the gantry sag, that is, the superior-inferior wobble of the radiation field center, as a function of the gantry angle. The authors then attempted to compensate for the gantry sag by applying a gantry angle-specific correction to the MLC leaf positions. The gantry sag and its compensation were independently verified using film measurements.
Gantry sag was reproducible over a six-month measurement period. The maximum gantry sag was found to vary from 0.7 to 1.0 mm, depending on the linac and the collimator angle. The radiation field center moved inferiorly (i.e., away from the gantry) when the gantry was rotated from 0° to 180°. After the MLC leaf position compensation was applied at 90° collimator angle, the maximum gantry sag was reduced to <0.2 mm. The film measurements at gantry angles of 0° and 180° verified the inferior shift of the radiation fields and the effectiveness of MLC compensation.
The results indicate that gantry sag on a linac can be quantitatively measured using a simple phantom and an electronic portal imaging device. Reduction of gantry sag is feasible by applying a gantry angle-specific correction to MLC leaf positions at 90° collimator angle.
龙门倾斜是影响放射剂量空间精度的机械缺陷之一。本研究的目的是定量测量多台直线加速器（linacs）的龙门倾斜，研究一种基于多叶准直器（MLC）的补偿策略，并通过胶片测量验证龙门倾斜及其补偿。
作者使用 Winston-Lutz 方法测量了三台瓦里安直线加速器的龙门倾斜。使用兆伏辐射场在 10°的龙门角度间隔对球轴承模体进行成像。使用电子射野影像装置记录的图像进行分析，得出辐射等中心和龙门倾斜，即辐射场中心的上下摆动，作为龙门角度的函数。然后，作者试图通过对 MLC 叶片位置应用特定于龙门角度的校正来补偿龙门倾斜。通过胶片测量独立验证了龙门倾斜及其补偿。
在六个月的测量期间，龙门倾斜具有可重复性。发现龙门倾斜最大值在 0.7 至 1.0 毫米之间变化，具体取决于直线加速器和准直器角度。当龙门从 0°旋转到 180°时，辐射场中心向下移动（即远离龙门）。在 90°准直器角度应用 MLC 叶片位置补偿后，最大龙门倾斜减小到 <0.2 毫米。0°和 180°的龙门角度的胶片测量验证了辐射场的下偏和 MLC 补偿的有效性。
结果表明，使用简单的模体和电子射野影像装置可以定量测量直线加速器上的龙门倾斜。通过在 90°准直器角度对 MLC 叶片位置应用特定于龙门角度的校正，可以实现龙门倾斜的减小。
