Abstract
The accuracy of the CyberKnife Synchrony Respiratory Tracking System (SRTS) is considered to be patient‐dependent because the SRTS relies on an individual correlation between the internal tumor position (ITP) and the external marker position (EMP), as well as a prediction method to compensate for the delay incurred to adjust the position of the linear accelerator (linac). We aimed to develop a system for obtaining pretreatment statistical measurements of the SRTS tracking error by using beam's eye view (BEV) images, to enable the prediction of the patient‐specific accuracy. The respiratory motion data for the ITP and the EMP were derived from cine MR images obtained from 23 patients. The dynamic motion phantom was used to reproduce both the ITP and EMP motions. The CyberKnife was subsequently operated with the SRTS, with a CCD camera mounted on the head of the linac. BEV images from the CCD camera were recorded during the tracking of a ball target by the linac. The tracking error was measured at 15 Hz using in‐house software. To assess the precision of the position detection using an MR image, the positions of test tubes (determined from MR images) were compared with their actual positions. To assess the precision of the position detection of the ball, ball positions measured from BEV images were compared with values measured using a Vernier caliper. The SRTS accuracy was evaluated by determining the tracking error that could be identified with a probability of more than 95% (Ep95). The detection precision of the tumor position (determined from cine MR images) was <0.2 mm. The detection precision of the tracking error when using the BEV images was <0.2 mm. These two detection precisions were derived from our measurement system and were not obtained from the SRTS. The median of Ep95 was found to be 1.5 (range, 1.0–3.5) mm. The difference between the minimum and maximum Ep95 was 2.5 mm, indicating that this provides a better means of evaluating patient‐specific SRTS accuracy. A suitable margin, based on the predicted patient‐specific SRTS accuracy, can be added to the clinical target volume.PACS number: 87.53.Ly
Highlights
The CyberKnife Robotic Radiosurgery System (Accuray, Inc., Sunnyvale, CA) is a frameless image-guided stereotactic radiotherapy system
The CyberKnife was used to treat brain and head/neck tumors.[1,2] Technological developments allowed the system to be applied to extracranial regions with no respiratory displacement, such as in the treatment of spinal cord tumors.[3,4] Subsequent advances have enabled the treatment of extracranial regions subject to movement as a result of respiratory displacement, such as in the case of the treatment of lung tumors.[5,6]
Patient characteristics We analyzed 23 lung treatment patients for whom it was possible to obtain respiratory motion data from cine magnetic resonance (MR) images and who had been treated with the current version (3.1.1) of the Synchrony Respiratory Tracking System (SRTS) between September 2013 and July 2014
Summary
The CyberKnife Robotic Radiosurgery System (Accuray, Inc., Sunnyvale, CA) is a frameless image-guided stereotactic radiotherapy system. A 6 MV linac is mounted on a high-precision, six-axis robotic manipulator arm. The SRTS is a CyberKnife subsystem, developed to irradiate extracranial tumors that move as a result of respiration. It continuously synchronizes the beam delivery with the moving tumors. With the SRTS, patients can breathe normally during treatment, and the treatment margins are reduced through precise tracking.[7,8,9]
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