  We present a technique for continuous generation of volumetric images during
SBRT using periodic kV imaging and an external respiratory surrogate signal to
drive a patient-specific PCA motion model. Using the on-board imager, kV
radiographs are acquired every 3 seconds and used to fit the parameters of a
motion model so that it matches observed changes in internal patient anatomy. A
multi-dimensional correlation model is established between the motion model
parameters and the external surrogate position and velocity, enabling
volumetric image reconstruction between kV imaging time points. Performance of
the algorithm was evaluated using 10 realistic eXtended CArdiac-Torso (XCAT)
digital phantoms including 3D anatomical respiratory deformation programmed
with 3D tumor positions measured with orthogonal kV imaging of implanted
fiducial gold markers. The clinically measured ground truth 3D tumor positions
provided a dataset with realistic breathing irregularities, and the combination
of periodic on-board kV imaging with recorded external respiratory surrogate
signal was used for correlation modeling to account for any changes in
internal-external correlation. The three-dimensional tumor positions are
reconstructed with an average root mean square error (RMSE) of 1.47 mm, and an
average 95th percentile 3D positional error of 2.80 mm compared with the
clinically measured ground truth 3D tumor positions. This technique enables
continuous 3D anatomical image generation based on periodic kV imaging of
internal anatomy without the additional dose of continuous kV imaging. The 3D
anatomical images produced using this method can be used for treatment
verification and delivered dose computation in the presence of irregular
respiratory motion.
