Geometric Characterization of Offline Vs. Online Fiducial Reference Coordinates for Varian Triggered Imaging In Prostate SABR
Abstract
Purpose
To demonstrate an automated workflow for fiducial marker tracking using online daily CBCT data and to characterize the geometric limitations of utilizing offline CT-sim fiducial centroids for the Varian TrueBeam triggered-imaging system.
Methods
A retrospective cohort of 30 prostate SABR patients was analyzed. For each fraction, offline (CT-sim) and online (CBCT) fiducial positions were projected onto the onboard imager plane across the arc. Geometric deviations from expected fiducial positions were used to classify failed projections, defined as a fiducial falling outside a circular gating window on the imager plane centered about the offline fiducial location. The percentage of failed projections was tracked to predict the frequency of treatment interventions. To test the system’s motion sensitivity, intrafraction motion was modeled by perturbing patient position by 1-3 mm. Uncertainty in online fiducial marker centroids was modeled as 2 mm isotropically. Both offline and online methods were evaluated against gating windows from 3-6 mm in diameter.
Results
Quantitative analysis of offline matches revealed mean geometric deviations of -1.9 mm (longitudinal) and 0.4 mm (lateral), limited by detector resolution. Consequently, 43% of offline projections were predicted to require treatment intervention, consistent with a separate clinical audit recording a 41% intervention rate. In contrast, motion simulations for the online workflow predicted zero interventions at the standard 6 mm window. Under 3 mm of simulated motion, the online system demonstrated intervention rates (sensitivities) of 2%, 79%, and 100% for 6, 5, and 3 mm gating windows, respectively. This compares favorably to the offline case, where zero patient motion yielded a 76% failure rate at a 3 mm window.
Conclusion
Reliance on offline fiducial coordinates introduces systematic inter-fraction errors that increase treatment interventions, necessitate manual repositioning, and reduce sensitivity to motion. Transitioning to daily online fiducial coordinates eliminate these issues for future triggered imaging treatments.