@article{oai:repo.qst.go.jp:00075673,
 author = {森, 慎一郎 and 坂田, 幸辰 and 平井, 隆介 and 古市, 渉 and Shimabukuro, Kazuki and 河野, 良介 and Woong Seop, Keum and 井関, 康 and Kasai, Shigeru and Okaya, Keiko and Mori, Shinichiro and Sakata, Yukinobu and Hirai, Ryusuke and Furuichi, Wataru and Kohno, Ryosuke and Keum, WoongSeop and Iseki, Yasushi},
 issue = {4},
 journal = {Medical Physics},
 month = {Apr},
 note = {Purpose: 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.
Methods: 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: (i) Geometrical calibration was evaluated by calculating chest phantom positional error using 2D/3D registration software for each 5° step of the gantry angle. (ii) Moving phantom displacement accuracy was measured (± 10 mm in 1-mm steps) with a laser sensor. (iii) 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°. (iv) 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. (v) Gate on/off latency, gating control system latency, and beam irradiation latency were measured using a laser sensor and an oscilloscope.
Results: 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 fps 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.
Conclusions: 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.},
 pages = {1561--1574},
 title = {Commissioning of a fluoroscopic-based real-time markerless tumor tracking system in a superconducting rotating gantry for carbon-ion pencil beam scanning treatment},
 volume = {46},
 year = {2019}
}