Prospective validation of treatment accuracy using implanted fiducial markers for spinal stereotactic body radiation therapy.

In order to accurately assess positioning errors in spinal SBRT, many institutions employ bony-fusion based imaging techniques, such as the ExacTrac™ (Brain Lab) system, in conjunction with 3D verification (performed via CT-on-rails in our practice). We hypothesized that the use of implanted gold fiducial markers could improve the accuracy of patient positioning over bony fusion alone. We addressed this question prospectively, enrolling patients on an IRB-approved protocol. Gold seeds were implanted in the vertebral pedicles flanking the target level. At treatment, setup error was calculated using two methods-standard kV image fusion, and geometric fiducial-based projection, with independent CT-on-rails verification. Analyses of residual set-up error showed that fiducial-based setup agreed with fusion-based determination, but did not significantly reduce error. Offline 6D fusion of the treatment and planning CT illustrated residual rotational error using standard or fiducial based setup. We conclude that the ExacTrac and CT-on-rails platform yields highly accurate results for spinal SBRT setup, with reduced residual error than previously reported. While the addition of fiducials did not further reduce error, the bony fusion approach is now prospectively validated in comparison to implanted fiducials. Both bony fusion and fiducial marker methods are associated with residual rotational error, thus 3D verification remains an important component of spinal SBRT treatment.