Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Chiba, 263-8555, Japan.
Research and Development Center, Toshiba Corporation, Kanagawa, 212-4582, Japan.
Med Phys. 2019 Apr;46(4):1561-1574. doi: 10.1002/mp.13403. Epub 2019 Feb 23.
To perform the final quality assurance of our fluoroscopic-based markerless tumor tracking for gated carbon-ion pencil beam scanning (C-PBS) radiotherapy using a rotating gantry system, we evaluated the geometrical accuracy and tumor tracking accuracy using a moving chest phantom with simulated respiration.
The positions of the dynamic flat panel detector (DFPD) and x-ray tube are subject to changes due to gantry sag. To compensate for this, we generated a geometrical calibration table (gantry flex map) in 15° gantry angle steps by the bundle adjustment method. We evaluated five metrics: (a) Geometrical calibration was evaluated by calculating chest phantom positional error using 2D/3D registration software for each 5° step of the gantry angle. (b) Moving phantom displacement accuracy was measured (±10 mm in 1-mm steps) with a laser sensor. (c) Tracking accuracy was evaluated with machine learning (ML) and multi-template matching (MTM) algorithms, which used fluoroscopic images and digitally reconstructed radiographic (DRR) images as training data. The chest phantom was continuously moved ±10 mm in a sinusoidal path with a moving cycle of 4 s and respiration was simulated with ±5 mm expansion/contraction with a cycle of 2 s. This was performed with the gantry angle set at 0°, 45°, 120°, and 240°. (d) Four types of interlock function were evaluated: tumor velocity, DFPD image brightness variation, tracking anomaly detection, and tracking positional inconsistency in between the two corresponding rays. (e) Gate on/off latency, gating control system latency, and beam irradiation latency were measured using a laser sensor and an oscilloscope.
By applying the gantry flex map, phantom positional accuracy was improved from 1.03 mm/0.33° to <0.45 mm/0.27° for all gantry angles. The moving phantom displacement error was 0.1 mm. Due to long computation time, the tracking accuracy achieved with ML was <0.49 mm (=95% confidence interval [CI]) for imaging rates of 15 and 7.5 fps; those at 30 fps were decreased to 1.84 mm (95% CI: 1.79 mm-1.92 mm). The tracking positional accuracy with MTM was <0.52 mm (=95% CI) for all gantry angles and imaging frame rates. The tumor velocity interlock signal delay time was 44.7 ms (=1.3 frame). DFPD image brightness interlock latency was 34 ms (=1.0 frame). The tracking positional error was improved from 2.27 ± 2.67 mm to 0.25 ± 0.24 mm by the tracking anomaly detection interlock function. Tracking positional inconsistency interlock signal was output within 5.0 ms. The gate on/off latency was <82.7 ± 7.6 ms. The gating control system latency was <3.1 ± 1.0 ms. The beam irradiation latency was <8.7 ± 1.2 ms.
Our markerless tracking system is now ready for clinical use. We hope to shorten the computation time needed by the ML algorithm at 30 fps in the future.
为了对使用旋转机架系统的基于透视的无标记肿瘤跟踪进行最终质量保证，我们使用模拟呼吸的移动胸部体模评估了基于透视的无标记肿瘤跟踪的几何精度和肿瘤跟踪精度。
由于机架下垂，动态平板探测器（DFPD）和 X 射线管的位置会发生变化。为此，我们通过束调整方法生成了一个几何校准表（机架弯曲图），在 15°机架角度步长下进行。我们评估了五个指标：（a）使用 2D/3D 配准软件计算每个 5°机架角度的胸部体模位置误差，评估几何校准。（b）使用激光传感器以 1mm 的步长测量±10mm 的移动体模位移精度。（c）使用机器学习（ML）和多模板匹配（MTM）算法评估跟踪精度，该算法使用透视图像和数字重建射线照相（DRR）图像作为训练数据。胸部体模以 4s 的运动周期以正弦路径连续移动±10mm，并以 2s 的周期模拟±5mm 的扩张/收缩。在机架角度设置为 0°、45°、120°和 240°的情况下进行此操作。（d）评估了四种互锁功能：肿瘤速度、DFPD 图像亮度变化、跟踪异常检测和两个对应射线之间的跟踪位置不一致。（e）使用激光传感器和示波器测量门控的开启/关闭延迟、门控控制系统延迟和束照射延迟。
通过应用机架弯曲图，对于所有机架角度，体模位置精度从 1.03mm/0.33°提高到<0.45mm/0.27°。移动体模位移误差为 0.1mm。由于计算时间长，在成像帧率为 15 和 7.5fps 时，ML 实现的跟踪精度<0.49mm（95%置信区间[CI]）；在 30fps 时，跟踪精度降低至 1.84mm（95%CI：1.79mm-1.92mm）。MTM 的跟踪位置精度在所有机架角度和成像帧率下均<0.52mm（95%CI）。肿瘤速度互锁信号延迟时间为 44.7ms（1.3 帧）。DFPD 图像亮度互锁延迟为 34ms（1.0 帧）。通过跟踪异常检测互锁功能，将跟踪位置误差从 2.27±2.67mm 提高到 0.25±0.24mm。跟踪位置不一致互锁信号在 5.0ms 内输出。门控的开启/关闭延迟<82.7±7.6ms。门控控制系统延迟<3.1±1.0ms。束照射延迟<8.7±1.2ms。
我们的无标记跟踪系统现在可以投入临床使用。我们希望在未来缩短 ML 算法在 30fps 时所需的计算时间。
