  Purpose: To assess the fiducial motion in abdominal stereotactic body
radiotherapy (SBRT) using the cone-beam computed tomography (CBCT) projections
acquired for pre-treatment patient set-up. Materials and Methods: Pre-treatment
CBCT projections and anterior-posterior (AP) and lateral (LAT) pair of
fluoroscopic sequences of 7 pancreatic and 6 liver SBRT patients with implanted
fiducials were analyzed for 49 treatment fractions retrospectively. A tracking
algorithm based on template matching and sequential stereo triangulation
algorithms was used to track the fiducials in the CBCT projections and the
fluoro sequence pairs. We predicted the clinical couch adjustment from CBCT
tracking and compared it with the clinical couch decision made during the
patient's treatment. Results: In 3D coordinate, the fiducial motion ranges for
pancreas cases were 9.90+/-3.52 mm, 10.65+/-5.91 mm, and 10.74+/-6.24 mm for
CBCT, AP, and LAT fluoro, respectively, while in the liver, they were
13.93+/-3.39 mm, 11.17+/-3.75 mm, and 11.52+/-4.33 mm, respectively. Prediction
of couch adjustment in LAT, SI, and AP coordinates from CBCT tracking agrees
with the actual clinical couch correction within 0.92+/-0.74 mm, 1.37+/-1.26
mm, and 0.68+/-0.56 mm for pancreas cases and within 1.12+/-0.96 mm,
1.15+/-0.92 mm and 0.90+/-0.86 mm for liver cases, respectively. Conclusion:
Tracking pre-treatment CBCT projections using template matching and sequential
stereo triangulation is suitable for assessing fiducial motion and adjusting
the patient setup for abdominal SBRT. CBCT can be used for motion modeling,
potentially eliminating the need for additional fluoroscopic pair acquisition
and thus reducing the imaging dose to the patient and the total treatment time.
