Impacts of respiratory phase shifts on motion‐tracking accuracy of the CyberKnife Synchrony™ Respiratory Tracking System

Abstract

Purpose The SynchronyTM Respiratory Tracking System (SRTS) component of the CyberKnife® Robotic Radiosurgery System (Accuray, Inc., Sunnyvale CA) enables real‐time tracking of moving targets by modeling the correlation between the targets and external surrogate light‐emitting diode (LED) markers placed on the patient’s chest. Previous studies reported some cases with respiratory phase shifts between lung tumor and chest wall motions. In this study, the impacts of respiratory phase shifts on the motion‐tracking accuracy of the SRTS were investigated. Methods A plastic scintillator was used to detect the position of the x‐ray beams. The scintillation light was recorded using a camera in a dark room. A moving phantom moved a U‐shaped frame on the scintillator with a 4th power of sinusoidal functions. Three metallic markers for motion tracking and four fluorescent tapes were attached to the frame. The fluorescent tapes were used to identify phantom position and respiratory phase for each video frame. The beam positions collected, when considered relative to the phantom motion, represent the degree of tracking error. Beam position was calculated by adding error value to phantom position. Motions with respiratory phase shifts between the target and an extra stage mimicking chest wall motion were also tested for LED markers. Log files of the SRTS were analyzed to evaluate correlation errors. Results When target and LED marker motions were synchronized with a respiratory cycle of 4 s, the maximum tracking errors for 90% and 95% of beam‐on time were 1.0 mm and 1.2 mm, respectively. The frequency of tracking errors increased when LED marker motion phase preceded target motion. Tracking errors that corresponded to 90% beam‐on time were within 2.4 mm for 5–15% of phase shifts. In contrast, the tracking errors were very large when the LED marker delayed to the target motions; the maximum errors of 90% beam‐on time were 3.0, 3.8, and 7.5 mm for 5%, 10%, and 15% of phase shifts, respectively. The patterns of the tracking errors derived from the scintillation light were very similar to those of the correlation data of the SRTS derived from the log files, indicating that the tracking errors caused mainly due to the errors in modeling the correlation data. With long respiratory cycle of 6 s, the tracking errors were significantly decreased; the maximum tracking errors for 95% beam‐on time were 1.6 mm and 2.2 mm for early and delayed LED motion. Conclusion We have investigated the motion‐tracking accuracy of the CyberKnife SRTS for cases with the respiratory phase shift between the target and the LED marker. The maximum tracking errors for 90% probability were within 2.4 mm when the target delays to the LED markers. When LED marker delays, however, very large tracking errors were observed. With a long respiratory cycle, the tracking errors were greatly improved to less than 2.2 mm. Coaching slow breathing will be useful for accurate motion tracking radiotherapy.

Evaluation of radixact motion synchrony for 3D respiratory motion: Modeling accuracy and dosimetric fidelity

“…In addition, motion interplay and dose blurring effects were not observed with Synchrony enabled. The sensitivities of CK Synchrony to surrogate phase shifts reported in the literature 16 were not observed for the Radixact Synchrony system. This work provided evidence that Synchrony reduces effects of intrafraction respiratory motion that are not accounted for using the current, ITV‐based motion management strategy for helical tomotherapy plans.…”

“…For example, for similar tracking experiments on CK Synchrony, Akino et al . reported values of δ 95% to be as large as 2.8 mm for irregular respiratory motion with the surrogate and target in‐phase 16 and Inoue et al reported values of δ 95% ranging from 1.0 to 3.5 mm 25 . Akino et al found extremely large tracking errors when the surrogate and the target were out of phase, such as δ 95% >9 mm for a surrogate phase of +15% and δ 95% >6 mm for a surrogate phase of +10%.…”

“…2. This setup was used to explore the effect of varying the relationship between the surrogate LED motion and the internal target motion, as described by Akino et al 16 . The motion of the surrogate stage replicates motion of the patient’s chest in the IEC‐Z direction, which may be different from the motion of the internal tumor, often majorly in the IEC‐Y direction.…”

“…Chen et al performed acceptance testing of Synchrony deliveries, 15 but these tests were limited to a single clinical delivery quality assurance (DQA) test for a respiratory with fiducial treatment. Akino et al investigated the tracking accuracy of the CK Synchrony system and found that CK may suffer from large tracking errors in the presence of phase shifts between the surrogate and target motions 16 . The effects of separate target and surrogate motion on tracking accuracies and dosimetric fidelity of Radixact Synchrony have not been investigated.…”

AAPM Task Group 264: The safe clinical implementation of MLC tracking in radiotherapy

“…66,67 Across CyberKnife, Vero and MLC tracking lung SBRT treatment planning studies, planning target volume (PTV) reductions of 40-50% and corresponding reductions in mean lung dose have been demonstrated depending on the motion magnitude. 4,66,67,70,71 Corresponding clinical outcomes are yet to be demonstrated, however literature suggests ~2% reduction in toxicity per Gy reduction in mean lung dose. 72,73 MLC tracking may enable SBRT treatment for large lesions in the lung, as a reduction of the PTV with MLC tracking allows larger lesions to possess the same dose footprint as smaller lesions with an ITV expansion.…”