Min Soonki, Choi Young Eun, Kwak Jungwon, Cho Byungchul
Asan Medical Center, University of Ulsan College of Medicine.
J Appl Clin Med Phys. 2014 Jan 8;16(1):4934. doi: 10.1120/jacmp.v16i1.4934.
Patient-specific pretreatment verification of intensity-modulated radiation therapy (IMRT) or volumetric-modulated arc therapy (VMAT) is strongly recommended for all patients in order to detect any potential errors in treatment planning process and machine deliverability, and is thus performed routinely in many clinics. Portal dosimetry is an effective method for this purpose because of its prompt setup, easy data acquisition, and high spatial resolution. However, portal dosimetry cannot be applied to IMRT or VMAT with flattening filter-free (FFF) beams because of the high dose-rate saturation effect of the electronic portal imaging device (EPID). In our current report, we suggest a practical QA method of expanding the conventional portal dosimetry to FFF beams with a QA plan generated by the following three steps: 1) replace the FFF beams with flattening filtered (FF) beams of the same nominal energy; 2) reduce the dose rate to avoid the saturation effect of the EPID detector; and 3) adjust the total MU to match the gantry and MLC leaf motions. Two RapidArc plans with 6 and 10 MV FFF beams were selected, and QA plans were created by the aforementioned steps and delivered. The trajectory log files of TrueBeam obtained during the treatment and during the delivery of QA plan were analyzed and compared. The maximum discrepancies in the expected trajectories between the treatment and QA plans were within 0.002 MU for the MU, 0.06° for the motion of gantry rotation, and 0.006 mm for the positions of the MLC leaves, indicating much higher levels of accuracy compared to the mechanical specifications of the machine. For further validation of the method, direct comparisons of the delivered QA FF beam to the treatment FFF beam were performed using film dosimetry and show that gamma passing rates under 2%/2 mm criteria are 99.0%-100% for the all four arc beams. This method can be used on RapidArc plans with FFF beams without any additional procedure or modifications on the conventional portal dosimetry of IMRT and is, therefore, a practical option for routine clinical use.
强烈建议对所有患者进行特定患者的调强放射治疗（IMRT）或容积调强弧形治疗（VMAT）的治疗前验证，以便检测治疗计划过程和机器输出中的任何潜在误差，因此许多诊所都将其作为常规操作。射野剂量测定法是实现这一目的的有效方法，因为它设置迅速、数据采集容易且空间分辨率高。然而，由于电子射野成像装置（EPID）的高剂量率饱和效应，射野剂量测定法不能应用于无均整器（FFF）束流的IMRT或VMAT。在我们当前的报告中，我们提出了一种实用的质量保证（QA）方法，通过以下三个步骤生成QA计划，将传统的射野剂量测定法扩展到FFF束流：1）用相同标称能量的均整过滤（FF）束流替换FFF束流；2）降低剂量率以避免EPID探测器的饱和效应；3）调整总监测单位（MU）以匹配机架和多叶准直器（MLC）叶片的运动。选择了两个分别使用6 MV和10 MV FFF束流的容积调强弧形治疗（RapidArc）计划，并通过上述步骤创建QA计划并进行输出。分析并比较了在治疗期间和QA计划输出期间获得的TrueBeam的轨迹日志文件。治疗计划和QA计划之间预期轨迹的最大差异在MU方面为0.002 MU以内，在机架旋转运动方面为0.06°，在MLC叶片位置方面为0.006 mm，表明与机器的机械规格相比，精度水平要高得多。为了进一步验证该方法，使用胶片剂量测定法对输出的QA FF束流与治疗FFF束流进行了直接比较，结果表明，在所有四条弧形束流中，在2%/2 mm标准下的γ通过率为99.0% - 10%。该方法可用于FFF束流的容积调强弧形治疗（RapidArc）计划，而无需对IMRT的传统射野剂量测定法进行任何额外的程序或修改，因此是常规临床使用的实用选择。
