Berbeco Ross I, Jiang Steve B, Sharp Gregory C, Chen George T, Mostafavi Hassan, Shirato Hiroki
Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA.
Phys Med Biol. 2004 Jan 21;49(2):243-55. doi: 10.1088/0031-9155/49/2/005.
The design of an integrated radiotherapy imaging system (IRIS), consisting of gantry mounted diagnostic (kV) x-ray tubes and fast read-out flat-panel amorphous-silicon detectors, has been studied. The system is meant to be capable of three main functions: radiographs for three-dimensional (3D) patient set-up, cone-beam CT and real-time tumour/marker tracking. The goal of the current study is to determine whether one source/panel pair is sufficient for real-time tumour/marker tracking and, if two are needed, the optimal position of each relative to other components and the isocentre. A single gantry-mounted source/imager pair is certainly capable of the first two of the three functions listed above and may also be useful for the third, if combined with prior knowledge of the target's trajectory. This would be necessary because only motion in two dimensions is visible with a single imager/source system. However, with previously collected information about the trajectory, the third coordinate may be derived from the other two with sufficient accuracy to facilitate tracking. This deduction of the third coordinate can only be made if the 3D tumour/marker trajectory is consistent from fraction to fraction. The feasibility of tumour tracking with one source/imager pair has been theoretically examined here using measured lung marker trajectory data for seven patients from multiple treatment fractions. The patients' selection criteria include minimum mean amplitudes of the tumour motions greater than 1 cm peak-to-peak. The marker trajectory for each patient was modelled using the first fraction data. Then for the rest of the data, marker positions were derived from the imager projections at various gantry angles and compared with the measured tumour positions. Our results show that, due to the three dimensionality and irregular trajectory characteristics of tumour motion, on a fraction-to-fraction basis, a 'monoscopic' system (single source/imager) is inadequate for consistent real-time tumour tracking, even with prior knowledge. We found that, among the seven patients studied with peak-to-peak marker motion greater than 1 cm, five cases have mean localization errors greater than 2 mm and two have mean errors greater than 3 mm. Because of this uncertainty associated with a monoscopic system, two source/imager pairs are necessary for robust 3D target localization. Dual orthogonal x-ray source/imager pairs mounted on the linac gantry are chosen for the IRIS. We further studied the placement of the x-ray sources/panel based on the geometric specifications of the Varian 21EX Clinac. The best configuration minimizes the localization error while maintaining a large field of view and avoiding collisions with the floor/ceiling or couch.
我们研究了一种集成放射治疗成像系统（IRIS）的设计，该系统由安装在机架上的诊断（千伏）X射线管和快速读出的平板非晶硅探测器组成。该系统旨在具备三项主要功能：用于三维（3D）患者定位的射线照片、锥形束CT以及实时肿瘤/标记物跟踪。当前研究的目标是确定一对源/平板是否足以进行实时肿瘤/标记物跟踪，如果需要两对，那么每对相对于其他组件和等中心的最佳位置。单个安装在机架上的源/成像器对肯定能够实现上述三项功能中的前两项，如果结合目标轨迹的先验知识，也可能对第三项功能有用。这是必要的，因为单个成像器/源系统只能看到二维运动。然而，利用先前收集的关于轨迹的信息，可以从另外两个坐标足够精确地推导出第三个坐标，以方便跟踪。只有当3D肿瘤/标记物轨迹在分次治疗之间保持一致时，才能进行第三个坐标的推导。在此，我们使用来自七名患者多个治疗分次的测量肺部标记物轨迹数据，从理论上检验了使用一对源/成像器进行肿瘤跟踪的可行性。患者的选择标准包括肿瘤运动的最小平均峰峰值幅度大于1厘米。使用第一个分次的数据对每名患者的标记物轨迹进行建模。然后对于其余数据，从不同机架角度的成像器投影中得出标记物位置，并与测量的肿瘤位置进行比较。我们的结果表明，由于肿瘤运动的三维性和不规则轨迹特征，在分次治疗的基础上，即使有先验知识，“单视”系统（单个源/成像器）也不足以进行一致的实时肿瘤跟踪。我们发现，在研究的七名标记物峰峰值运动大于1厘米的患者中，五例的平均定位误差大于2毫米，两例的平均误差大于3毫米。由于单视系统存在这种不确定性，因此需要两对源/成像器才能实现可靠的3D目标定位。IRIS选择了安装在直线加速器机架上的双正交X射线源/成像器对。我们还根据瓦里安21EX直线加速器的几何规格进一步研究了X射线源/平板的放置。最佳配置在保持大视野并避免与地板/天花板或治疗床碰撞的同时，将定位误差最小化。
