Hu W, Zhao J, Ye J, Peng J, Zhang Z
Fudan University Shanghai Cancer Center, Shanghai, China.
Swedish Medical Center-Tumor Institute, Seattle, WA.
Med Phys. 2012 Jun;39(6Part12):3738. doi: 10.1118/1.4735212.
The patient specific quality assurance (QA) measurements for fixed-gantry and rotational intensity modulated radiation therapy (IMRT and VMAT/RapidArc) are usually performed on a homogeneous phantom prior to the treatment. The purpose of this study is to develop an online method to verify the delivered dose to the patient on the treatment day.
An anthropomorphic (Rando) head phantom was immobilized in treatment position with a thermoplastic mask to simulate a real patient. A sheet of gafchromic film (EBT2) was sandwiched between a 1-cm-thick solid water slab, which was fixed to the Type-S extension board, and the patient's head hold (a pillow used here). The CT images of the Rando phantom were acquired and exported to the treatment planning systems. One step-and-shot fixed-gantry IMRT plan and one RapidArc plan were generated and the dose distributions on the film plane were calculated. The two plans were delivered to the patient (Rando phantom in this study) in the treatment position on a Varian Trilogy linear accelerator with two new films. The films were scanned, and the measurements were compared with the planned doses.
The composite dose distributions measured on the film plane were the actual delivered dose for the treatment. The comparison between the measurement and planned dose profiles shows an agreement within 3% because of the good reproducibility of phantom positioning. Gamma pass rates (using 3mm and 3% criteria) for the IMRT and RapidArc plan were found to be 95% and 94%, respectively.
The phantom study has demonstrated the feasibility of using gafchromic film for online dose verification. This simple method takes into account the patient heterogeneity and the treatment associated uncertainties such as setup error, intrafraction motions and machine related variations. It can be implemented as an online physics and/or clinical QA tool without taking additional machine time.
固定机架和旋转调强放射治疗（IMRT和VMAT/容积弧形调强放疗）的患者特异性质量保证（QA）测量通常在治疗前在均匀体模上进行。本研究的目的是开发一种在线方法，以在治疗当天验证给予患者的剂量。
使用热塑性面罩将拟人化（兰多）头部体模固定在治疗位置，以模拟真实患者。将一片放射变色薄膜（EBT2）夹在固定在S型延长板上的1厘米厚固体水板和患者头部固定装置（此处使用的枕头）之间。获取兰多体模的CT图像并导出到治疗计划系统。生成一个单步和单次固定机架IMRT计划和一个容积弧形调强放疗计划，并计算薄膜平面上的剂量分布。在瓦里安Trilogy直线加速器上，使用两张新薄膜将这两个计划在治疗位置给予患者（本研究中的兰多体模）。扫描薄膜，并将测量结果与计划剂量进行比较。
在薄膜平面上测量的复合剂量分布是治疗的实际给予剂量。测量剂量分布与计划剂量分布之间的比较显示，由于体模定位的良好再现性，二者在3%以内相符。发现IMRT计划和容积弧形调强放疗计划的伽马通过率（使用3毫米和3%标准）分别为95%和94%。
体模研究证明了使用放射变色薄膜进行在线剂量验证的可行性。这种简单方法考虑了患者的异质性以及与治疗相关的不确定性，如摆位误差、分次内运动和机器相关变化。它可以作为一种在线物理和/或临床QA工具实施，而无需额外占用机器时间。
