Reiner Béatrice, Bownes Peter, Buckley David L, Thwaites David I
Division of Biomedical Imaging, University of Leeds, Leeds, LS2 9JT, UK.
Institute of Radiation Oncology, Hospital Graubuenden, CH-7000 Chur, Switzerland.
J Radiosurg SBRT. 2016;4(1):31-42.
This work evaluates the precision and characteristics of the trigger level of the vacuum surveillance system of eXtend™ on Gamma Knife Perfexion and the effect of the potential displacement on the dose distribution.
A total of 20 individually moulded mouthpieces based on human dental models were used to measure translational shift and rotation until the vacuum surveillance of eXtend interrupted the irradiation. The positional accuracy of the movement was 0.01 mm using a computerized numerically controlled positioning system. Rotation was introduced by peripheral pressure in superior or inferior direction on the mould and was measured with a digital inclinometer. In 10 patients with a large brain metastasis the effect of a potential displacement of the centre of the target was recalculated. Two out of the 10 targets were located near the optic nerve and chiasm. In addition, the potential displacement of the chiasm and the optic nerve due to rotation based on their distance to the centre of rotation (COR, the mouthpiece) was evaluated. For the recalculation of the delivered dose after a displacement, GammaPlan Software® was used. Dose distribution with displacement between ±0.1 mm and ±2.0 mm was simulated for ten targets with volumes between 7.7 cm and 19.3 cm. Shifts were applied to the X (lateral), Y (ventro-dorsal) and Z axis (superior-inferior). Dwell time was kept constant within <0.5% total time and 1% individual dwell time. Evaluated parameters were "minimum dose" (D), "coverage" and "Paddick Conformity Index" (PCI).
The vacuum between mouthpiece and dental model was broken by a mean translation of the mouthpiece of 0.15 mm (SD ±0.05 mm, range 0.05-0.29 mm) or a mean rotation of 0.33º (SD ±0.15º, range 0.05-1.0º). Rotation caused by pressure in the inferior direction was higher with 0.42º (SD ±0.16º) compared to rotation due to pressure in the superior direction with 0.21º (SD ±0.09º). The mean rotation would produce a mean displacement of 0.64 mm for the target, 0.39 mm for the chiasm and 0.33 mm for the optic nerve, for the group of patients studied. The biggest effect of positional deviation was seen in the minimum dose to the target, with 2.5% reduction for a 0.5 mm and 5% for a 1.0 mm shift in the Z-direction respectively.
The trigger level is sensitive and reproducible. A sudden movement by the patient, such as an attempt to get out or correct an uncomfortable position would interrupt irradiation immediately. The dominant effect for a displacement is due to rotation. The resulting displacement depends on the distance between target and COR (magnitude) and the location (direction). Therefore, lesions near chiasm or brainstem, which are closer to the COR, are less affected from a rotation than peripheral targets.
本研究评估伽玛刀Perfexion的eXtend™真空监测系统触发水平的精度和特性，以及潜在位移对剂量分布的影响。
基于人体牙科模型制作了20个单独的口模，用于测量平移和旋转，直至eXtend的真空监测中断照射。使用计算机数控定位系统，运动的位置精度为0.01毫米。通过在口模上施加上下方向的外周压力引入旋转，并使用数字倾角仪进行测量。对10例患有大脑转移瘤的患者，重新计算了靶点中心潜在位移的影响。10个靶点中有2个位于视神经和视交叉附近。此外，根据视交叉和视神经到旋转中心（COR，即口模）的距离，评估了旋转导致的视交叉和视神经的潜在位移。为了重新计算位移后的输出剂量，使用了GammaPlan Software®。对体积在7.7立方厘米至19.3立方厘米之间的10个靶点，模拟了±0.1毫米至±2.0毫米之间的位移的剂量分布。位移应用于X轴（横向）、Y轴（腹背）和Z轴（上下）。驻留时间在总时间的<0.5%和单个驻留时间的1%内保持恒定。评估参数为“最小剂量”（D）、“覆盖范围”和“帕迪克适形指数”（PCI）。
口模与牙科模型之间的真空在口模平均平移0.15毫米（标准差±0.05毫米，范围0.05 - 0.29毫米）或平均旋转0.33°（标准差±0.15°，范围0.05 - 1.0°）时被打破。向下方向压力引起的旋转为0.42°（标准差±0.16°），高于向上方向压力引起的旋转0.21°（标准差±0.09°）。对于所研究的患者组，平均旋转将使靶点产生平均位移0.64毫米，视交叉产生0.39毫米，视神经产生0.33毫米。位置偏差的最大影响出现在靶点的最小剂量上，在Z方向上分别为0.5毫米位移时降低2.5%，1.0毫米位移时降低5%。
触发水平敏感且可重复。患者的突然移动，如试图挣脱或纠正不舒服的姿势，会立即中断照射。位移的主要影响是由于旋转。产生的位移取决于靶点与旋转中心的距离（大小）和位置（方向）。因此，靠近视交叉或脑干（即更靠近旋转中心）的病变，与外周靶点相比，受旋转的影响较小。
