Abou-Elenein Hassan S, Attalla Ehab M, Ammar H, Eldesoky Ismail, Farouk Mohamed, Zaghloul Mohamed S
Department of Radiotherapy, Children's Cancer Hospital, Egypt.
J Med Phys. 2011 Oct;36(4):205-12. doi: 10.4103/0971-6203.89969.
The improvement in conformal radiotherapy techniques enables us to achieve steep dose gradients around the target which allows the delivery of higher doses to a tumor volume while maintaining the sparing of surrounding normal tissue. One of the reasons for this improvement was the implementation of intensity-modulated radio therapy (IMRT) by using linear accelerators fitted with multi-leaf collimator (MLC), Tomo therapy and Rapid arc. In this situation, verification of patient set-up and evaluation of internal organ motion just prior to radiation delivery become important. To this end, several volumetric image-guided techniques have been developed for patient localization, such as Siemens OPTIVUE/MVCB and MVision megavoltage cone beam CT (MV-CBCT) system. Quality assurance for MV-CBCT is important to insure that the performance of the Electronic portal image device (EPID) and MV-CBCT is suitable for the required treatment accuracy. In this work, the commissioning and clinical implementation of the OPTIVUE/MVCB system was presented. The geometry and gain calibration procedures for the system were described. The image quality characteristics of the OPTIVUE/MVCB system were measured and assessed qualitatively and quantitatively, including the image noise and uniformity, low-contrast resolution, and spatial resolution. The image reconstruction and registration software were evaluated. Dose at isocenter from CBCT and the EPID were evaluated using ionization chamber and thermo-luminescent dosimeters; then compared with that calculated by the treatment planning system (TPS- XiO 4.4). The results showed that there are no offsets greater than 1 mm in the flat panel alignment in the lateral and longitudinal direction over 18 months of the study. The image quality tests showed that the image noise and uniformity were within the acceptable range, and that a 2 cm large object with 1% electron density contrast can be detected with the OPTIVUE/MVCB system with 5 monitor units (MU) protocol. The registration software was accurate within 2 mm in the anterior-posterior, left-right, and superior-inferior directions. The additional dose to the patient from MV-CBCT study set with 5 MU at the isocenter of the treatment plan was 5 cGy. For Electronic portal image device (EPID) verification using two orthogonal images with 2 MU per image the additional dose to the patient was 3.8 cGy. These measured dose values were matched with that calculated by the TPS-XiO, where the calculated doses were 5.2 cGy and 3.9 cGy for MVCT and EPID respectively.
适形放射治疗技术的进步使我们能够在靶区周围实现陡峭的剂量梯度，这使得在保护周围正常组织的同时能够向肿瘤体积输送更高的剂量。这种进步的原因之一是通过使用配备多叶准直器（MLC）的直线加速器、断层放疗和容积弧形调强放疗（Rapid arc）来实施调强放射治疗（IMRT）。在这种情况下，在放疗前对患者摆位进行验证以及评估体内器官运动变得至关重要。为此，已经开发了几种用于患者定位的容积图像引导技术，例如西门子OPTIVUE/MVCB和MVision兆伏级锥形束CT（MV-CBCT）系统。MV-CBCT的质量保证对于确保电子射野影像装置（EPID）和MV-CBCT的性能适合所需的治疗精度很重要。在这项工作中，介绍了OPTIVUE/MVCB系统的调试和临床应用。描述了该系统的几何结构和增益校准程序。对OPTIVUE/MVCB系统的图像质量特征进行了定性和定量测量及评估，包括图像噪声和均匀性、低对比度分辨率以及空间分辨率。对图像重建和配准软件进行了评估。使用电离室和热释光剂量计评估了CBCT和EPID在等中心处的剂量；然后将其与治疗计划系统（TPS-XiO 4.4）计算的剂量进行比较。结果表明，在为期18个月的研究中，平板探测器在横向和纵向方向上的对准偏移不超过1毫米。图像质量测试表明，图像噪声和均匀性在可接受范围内，并且使用OPTIVUE/MVCB系统采用5个监测单位（MU）的方案可以检测到电子密度对比度为1%的2厘米大的物体。配准软件在前后、左右和上下方向上的精度在2毫米以内。在治疗计划等中心处使用5个MU的MV-CBCT研究设置给患者增加的剂量为5厘戈瑞。对于使用每幅图像2个MU的两幅正交图像进行电子射野影像装置（EPID）验证，给患者增加的剂量为3.8厘戈瑞。这些测量的剂量值与TPS-XiO计算的剂量值相符，其中MVCT和EPID计算的剂量分别为5.2厘戈瑞和3.9厘戈瑞。
