Northern Ireland Cancer Center, Belfast City Hospital, 51 Lisburn Road, Belfast, BT9 7AB, UK.
Centre for Cancer Research and Cell Biology, Queen's University, 79 Lisburn Road, Belfast, BT9 7AE, UK.
Med Phys. 2018 Apr;45(4):1738-1747. doi: 10.1002/mp.12805. Epub 2018 Mar 11.
Stereotactic ablative body radiotherapy (SABR) for lung patients can be performed with volumetric-modulated arc therapy (VMAT) plans using off-axis target geometry to allow treatment in their CBCT verified position. For patient-specific quality assurance measurements using the PTW Octavius 4D phantom (PTW, Freiburg, Germany) (OCT4D) in conjunction with an Octavius 1000SRS array (OCT1000) (PTW, Freiburg, Germany), repositioning the phantom off-axis is required to ensure the measurement area coincides with the tumor. The aim of this work is to quantify delivery errors using an array repositioned off-axis and evaluate new software which incorporates corrections for off-axis phantom measurements.
Dynamic conformal arcs and 25 lung SABR plans were created with the isocenter at the patient midline and the target volume off-axis. Measurements were acquired with an OCT4D phantom in conjunction with a 729 array (PTW, Freiburg, Germany) (OCT729) placed at isocenter. These plans were recalculated and delivered to both the OCT729 and OCT1000 arrays repositioned so that the high-dose region was at the center of the phantom. Comparisons were made using VeriSoft v7.0 (PTW, Freiburg, Germany) and the newly implemented version 7.1 with 2%/2 mm gamma criterion (10% threshold) and results correlated with off-axis distance to the tumor.
Average pass rates for VeriSoft v7.0 significantly reduced from 92.7 ± 2.4% to 84.9 ± 4.1% when the phantom was repositioned compared to the isocenter setup for the OCT729. The gamma pass rates significantly decreased the further the phantom was moved off-axis. Significantly higher pass rates were observed for the OCT1000 of 95.7 ± 3.6% and a significant decrease in gamma pass rate with off-axis phantom distance was again observed. In contrast, even with phantom repositioning, the pass rates for analysis with VeriSoft v7.1 were 93.7 ± 2.1% and 99.4 ± 1.1% for OCT729 and OCT1000, respectively. No significant difference in gamma pass rate was observed with off-axis phantom position irrespective of array type with the new software.
The errors in QA phantom measurements due to dose reconstruction at off-axis target geometry have been demonstrated for conformal arcs and clinical VMAT SABR plans. A novel software solution implemented by the vendor to allow accurate pass rates has been tested. This solution enables high-resolution arrays with small active detection areas to be used for quality assurance of SABR treatment plans in the off-axis treatment position.
对于肺部患者，立体定向消融体放射治疗（SABR）可以使用容积调制弧形治疗（VMAT）计划来完成，该计划采用离轴目标几何形状，以便在 CBCT 验证的位置进行治疗。为了使用配备 Octavius 1000SRS 阵列（OCT1000）（PTW，弗莱堡，德国）的 PTW Octavius 4D 体模（PTW，弗莱堡，德国）（OCT4D）进行特定于患者的质量保证测量，需要重新定位体模离轴，以确保测量区域与肿瘤重合。本研究的目的是量化使用离轴重新定位的阵列进行治疗的误差，并评估新软件中包含的离轴体模测量校正。
在患者中线的等中心点创建了 25 个动态适形弧形和 25 个肺部 SABR 计划，目标体积离轴。使用 OCT4D 体模和放置在等中心的 729 个阵列（PTW，弗莱堡，德国）（OCT729）进行测量。将这些计划重新计算并输送到 OCT729 和 OCT1000 阵列，以使高剂量区域位于体模中心。使用 VeriSoft v7.0（PTW，弗莱堡，德国）和新实施的版本 7.1 进行比较，后者使用 2%/2mm 伽玛标准（10%阈值），结果与肿瘤的离轴距离相关。
与 OCT729 等中心设置相比，当体模重新定位时，VeriSoft v7.0 的平均通过率从 92.7±2.4%显著降低至 84.9±4.1%。离轴距离越远，伽玛通过率越低。OCT1000 的通过率明显更高，为 95.7±3.6%，并且再次观察到离轴体模距离的伽玛通过率显著降低。相比之下，即使重新定位体模，使用 VeriSoft v7.1 进行分析的通过率对于 OCT729 和 OCT1000 分别为 93.7±2.1%和 99.4±1.1%。无论使用哪种类型的阵列，新软件都没有观察到离轴体模位置的伽玛通过率差异。
已经证明，对于适形弧形和临床 VMAT SABR 计划，由于离轴目标几何形状的剂量重建，QA 体模测量会出现误差。已经测试了供应商实施的一种新的软件解决方案，该解决方案允许实现准确的通过率。该解决方案能够使用具有小活动检测区域的高分辨率阵列，在离轴治疗位置为 SABR 治疗计划的质量保证提供支持。
