Evaluation of CyberKnife® Fiducial Tracking Limitations to Assist Targeting Accuracy: A Phantom Study with Fiducial Displacement

Abstract

Introduction The underlying assumptions of the CyberKnife® (Accuray, Sunnyvale, CA, US) fiducial tracking system are: i) fiducial positions are accurately detected; ii) inter-fiducial geometry remains consistent (rigid); iii) inter-fiducial geometric array changes are detected and either accommodated with corrections or treatment is interrupted. However: i) soft-tissue targets are deformable & fiducial migration is possible; ii) the accuracy of the tracking system has not previously been examined with fiducial displacement; iii) treatment interruptions may occur due to inter-fiducial geometric changes, but there is little information available to assist subsequent troubleshooting. The purpose of this study was to emulate a clinical target defined with a two, three, or four-fiducial array where one fiducial is displaced to mimic a target deformation or fiducial migration scenario. The objectives: evaluate the fiducial positioning accuracy, array interpretation, & corresponding corrections of the CyberKnife system, with the aim of assisting troubleshooting following fiducial displacement.MethodsA novel solid-water phantom was constructed with three fixed fiducials (F1,F2,F3) & one moveable fiducial (F4), arranged as if placed to track an imaginary clinical target. Using either two fiducials (F1,F4), different combinations of three fiducials (F1,F2,F4; F1,F3,F4; F2,F3,F4) or four fiducials (F1,F2,F3,F4), repeat experiments were conducted where F4 was displaced inferiorly at 2-mm intervals from 0-16 mm. Data were acquired at each position of F4, including rigid body errors (RBE), fiducial x, y, & z coordinate displacements, six degrees of freedom (DOF) corrections, & robot center-of-mass (COM) translation corrections.ResultsMaximum positioning difference (mean±SD) between the reference and live x, y, & z coordinates for the three fixed fiducials was 0.08±0.30 mm, confirming good accuracy for fixed fiducial registration. For two fiducials (F1,F4), F4 registration was accurate to 14-mm displacement and the F4 x-axis coordinate change was 2.0±0.12 mm with each 2 mm inferior displacement validating the phantom for tracking evaluation. RBE was >5 mm (system threshold) at 6-14 mm F4 displacement: however, F1 was misidentified as the RBE main contributor. Further, F1/F4 false-lock occurred at 16 mm F4 displacement with corresponding RBE <3 mm & COM corrections >13 mm. For combinations of three fiducials, F4 registration was accurate to 10-mm displacement. RBE was >5 mm at 6-16 mm F4 displacement: however, F4 false-lock occurred at 12-16 mm with RBE 5-6 mm. For four fiducials, F4 registration was accurate to 4 mm displacement: however, F4 false-lock occurred at 6-16 mm displacement with concerning RBE <2 & <5 at 6 & 8-mm F4 displacement, respectively. False-locks were easily identified in the phantom but frequently uncorrectable.ConclusionsResults indicate fiducial positioning accuracy and system output following fiducial displacement depends on the number of fiducials correlated, displacement distance, and clinical threshold...

Effect of a hydrogel spacer on the intrafractional prostate motion during CyberKnife treatment for prostate cancer

“…During the CyberKnife treatment of prostate cancer, the fiducial tracking system monitors fiducial markers near the treatment site. This system calculates the offset values and corrects it when the marker array differs between live images and digitally reconstructed radiographs (DRR) images in the TLS 14 . For the fiducial tracking system, the offset values (mm) of the three axes (X‐axis; superior [+] to inferior [−], Y‐axis; right [+] to left [−], and Z‐axis; posterior [+] to anterior [−]) can be obtained by using one or two fiducial markers, although the correction for the other axis (pitch, roll, and yaw [degrees]) needs a minimum of three fiducial markers.…”

“…Among the several radiotherapy modalities, CyberKnife (Accuray, Sunnyvale, CA) can be used to acquire 2D images per 5-150 s by using 2-kV x-ray devices (target locating system [TLS]), resulting in high-precision image-guided radiotherapy. 14 Prostate SBRT using CyberKnife showed fewer grade 2 or worse genitourinary toxicities than other SBRT delivery methods. 15 However, larger intrafractional motion was observed in CyberKnife treatment than in conventional Linac treatment, because the irradiation time for CyberKnife treatment was likely to be long.…”

“…This system calculates the offset values and corrects it when the marker array differs between live images and digitally reconstructed radiographs (DRR) images in the TLS. 14…”

“…Moreover, for high‐precision EBRT, it is important to control inter‐ and intrafractional prostate motion. Among the several radiotherapy modalities, CyberKnife (Accuray, Sunnyvale, CA) can be used to acquire 2D images per 5–150 s by using 2‐kV x‐ray devices (target locating system [TLS]), resulting in high‐precision image‐guided radiotherapy 14 . Prostate SBRT using CyberKnife showed fewer grade 2 or worse genitourinary toxicities than other SBRT delivery methods 15 .…”

Technical challenges of linac-based stereotactic ablative body radiotherapy: short review for non-radiation oncologists

“… 3D and 4D imaging (cone beam CT-CBCT).  Newer technologies including imaging and monitoring of electromagnetic transponders (55)(56)(57) or small fiducial markers inserted near the tumor and visible on X-rays or CBCT (58)(59)(60)(61)(62).  Lately surface guidance is used for verification and monitoring of the patient set-up (63,64).…”