Octavius 1600SRS三维剂量验证系统临床可行性测试

目的:通过模体实验对PTW Octavius 1600SRS三维剂量验证系统进行临床应用前性能测试，评估其对立体定向放射治疗（SBRT）计划进行剂量验证的可行性。方法:选用PTW Octavius 1600SRS体模的CT图像，模拟勾画7个球形靶区，中心靶区（PTV0）直径大小为3 cm，其余各靶区直径大小分别为1.0 cm（2个）、1.5 cm（2个）和2.0 cm（2个），各靶区中心点距PTV0中心点距离为3~6 cm。设置PTV0的中心点为计划中心，在治疗计划系统中制定SBRT计划（Plan0），处方剂量为8 Gy×3 F。实验分别对Plan0引入临床常见偏差，包括叶片MLC位置偏差（1、2、3 mm）、计划中心点（ISO）位置偏差（1、2、3 mm）和机架位置偏差（0.5°、1.0°、2.0°），并生成相应的偏差计划。使用1600SRS验证系统分别对原计划和偏差计划进行测量，比较两者γ通过率和靶区覆盖率的差别，以评估系统对放疗剂量偏差的敏感性。另外，对6例临床SBRT计划进行治疗前剂量验证，并与EPID验证结果进行比较，以评估其临床计划验证性能。结果:1600SRS验证系统对MLC偏差检测非常敏感，当MLC出现1 mm偏差时，其γ通过率与各靶区覆盖率均出现显著下降，且随着MLC偏差变大，其下降越明显。当MLC出现3 mm偏差时，（3 mm/3%）和（2 mm/3%）的γ通过率分别从99.6%和98.0%下降至92.8%和81.7%，7个靶区体积的覆盖率（V98%）平均下降（58.8±6.8）%。1600SRS对机架旋转偏差和ISO平移误差检测亦敏感，在机架旋转出现2°偏差或ISO出现3 mm偏差时，其（2 mm/2%）的γ通过率分别从95.1%下降至89.5%或86.2%。另外，6例临床SBRT放疗计划的（2 mm/3%）γ通过率平均为（95.5±1.5）%。结论:Octavius 1600SRS能敏感地检测出SBRT计划中MLC到位偏差、机架旋转角度偏差与ISO偏差，能较好地应用于SBRT计划的治疗前剂量验证。

Feasibility of three-dimensional dose verification using Octavius 1600SRS

Abstract: Objective To carry out performance test on PTW Octavius 1600SRS before clinical application through phantom experiment, and to evaluate the feasibility of dose verification for stereotactic body radiation therapy (SBRT) plan using PTW Octavius 1600SRS. Methods Seven spherical target areas, including a central target area (PTV0) with a diameter of 3.0 cm, 2 target areas with a diameter of 1.0 cm, 2 target areas with a diameter of 1.5 cm and 2 target areas with a diameter of 2.0 cm, were delineated in the CT image of the phantom in PTW Octavius 1600SRS. The distance from the center point of each target area to the center point of PTV0 was from 3 to 6 cm. With the central point of PTV0 as the planning center, the SBRT plan (Plan0) was designed in the treatment planning system, and its prescribed dose was 8 Gy×3 F. The experiment simulated and set the MLC position deviations (1, 2, 3 mm), the plan isocenter point (ISO) position deviations (1, 2, 3 mm) and the gantry rotation deviations (0.5°, 1.0°, 2.0°) for Plan0, and the corresponding deviation plans were generated. The 1600SRS verification system was used to measure the original plan and the deviation plans, and compare the difference of Gamma passing rate and target area coverage between the planned dose and the measured dose for evaluating the sensitivity of the system to radiotherapy dose deviation. In addition, pre-treatment dose verification using PTW Octavius 1600SRS was carried out in 6 cases of clinical SBRT plans, and then the results were compared with EPID verification results for evaluating its performance in clinical plan verification. Results The 1600SRS verification system was very sensitive to MLC position deviation. When a 1 mm deviation occurred in MLC, the gamma passing rate and the coverage rate of each target area were decreased significantly, and with the increase of MLC position deviation, the decrease was more obvious. When a 3 mm deviation occurred in MLC, the gamma passing rate decreased from 99.6% to 92.8% (3 mm/3%), and from 98.0% to 81.7% (2 mm/3%) and the coverage rates of 7 target areas (V98%) was decreased by (58.8±6.8)% on average. The 1600SRS was also sensitive to gantry rotation deviation and ISO translation error. When there was a 2° deviation in gantry rotation or a 3 mm deviation in ISO, the gamma passing rate (2 mm/2%) was decreased from 95.1% to 89.5% or 86.2%, respectively. The average gamma passing rate (2 mm/3%) in 6 cases of clinical SBRT plans was (95.5±1.5)%. Conclusion Octavius 1600SRS can detect MLC position deviation, gantry rotation deviation and ISO position deviation in SBRT plan sensitively, and can be used in pre-treatment dose verification for SBRT plan.