Bertelsen Anders, Nielsen Morten, Westberg Jonas, Jensen Henrik R, Brink Carsten
Radiofysisk Laboratorium, Odense Universitetshospital, Odense C, Denmark.
Acta Oncol. 2009;48(2):259-66. doi: 10.1080/02841860802251542.
During external radiotherapy daily or even weekly image verification of the patient position might be problematic due to the resulting workload. Therefore it has been customary to perform image verification only at the first treatment fraction. In this study it is investigated whether the patient position uncertainty at the initial three treatment fractions is representative for the uncertainty throughout the treatment course.
Seventy seven patients were treated using Elekta Synergy accelerators. The patients were immobilized during treatment by use of a customized VacFix bag and a mask of AquaPlast. Cone beam CT (CBCT) scans were performed at fractions 1, 2, and 3 and at the 10th and 20th treatment fractions. Displacements in patient position, translational and rotational, have been measured by an image registration of the CBCT and the planning CT scan. The displacements data are evaluated retrospectively and the effect of Action Level (AL) image verification protocols based on sessions 1, 2, 3 are simulated. The resulting overall patient position uncertainties of the different protocols are evaluated at the 10th and 20th fractions.
The differences between the addressed protocols are shown to be very small compared to the overall increase in patient position uncertainty during the treatment course. Thus the main problem in achieving the smallest possible uncertainty for the overall treatment is not the selection of 'the best' image verification protocol for the initial three fractions. The main challenge is that the overall patient position uncertainty increases during the treatment course. Information about the patient position during the first three fractions is therefore not representative for the overall patient position. For these types of patients and immobilization equipment it would consequently be an advantage to reduce the number of image verification sessions during the initial fractions and then compensate with additional imaging sessions during the remaining treatment course.
在体外放射治疗期间，由于工作量的原因，每天甚至每周对患者体位进行图像验证可能存在问题。因此，惯例是仅在首次治疗分次时进行图像验证。在本研究中，调查了最初三个治疗分次时患者体位的不确定性是否代表整个治疗过程中的不确定性。
77例患者使用医科达Synergy加速器进行治疗。在治疗期间，患者通过使用定制的VacFix袋和水凝胶面罩进行固定。在第1、2和3分次以及第10和第20治疗分次时进行锥形束CT（CBCT）扫描。通过CBCT与计划CT扫描的图像配准测量患者体位的平移和旋转位移。对位移数据进行回顾性评估，并模拟基于第1、2、3次扫描的行动水平（AL）图像验证方案的效果。在第10和第20分次时评估不同方案产生的总体患者体位不确定性。
与治疗过程中患者体位不确定性的总体增加相比，所讨论的方案之间的差异非常小。因此，实现总体治疗中尽可能小的不确定性的主要问题不是为最初三个分次选择“最佳”图像验证方案。主要挑战是在治疗过程中患者体位的总体不确定性增加。因此，关于最初三个分次期间患者体位的信息不能代表患者的总体体位。对于这类患者和固定设备，因此在最初分次期间减少图像验证次数，然后在剩余治疗过程中通过额外的成像次数进行补偿将是有利的。
