The prostate is known to move between daily fractions during the course of radiation therapy using external beams. This
movement causes problem with 3D conformal or intensity-modulated radiation therapy, in which tight margins are used
for treatment planning. To minimize the adverse effect of this motion on dose delivery, daily localization of the prostate
with respect to the planning CT is necessary. Current ultrasound-based localization systems require manual alignment of
ultrasound images with the planning CT. The resulting localization is subjective and has high interobserver variability.
To reduce the alignment uncertainty and increase the setup efficiency, we proposed an automatic prostate alignment
method using a volume subdivision-based elastic image registration algorithm. The algorithm uses normalized mutual
information as the measure of image similarity between the daily 3D ultrasound images and the planning CT. The
prostate contours on the CT are mapped to the ultrasound space by applying the transformation fields from image
registration. The displacement of the center-of-mass of the mapped contours is calculated for automatic patient setup. For
validation purposes, six experts independently and manually aligned the archived CT and 3D ultrasound images using
the SonArray system and reported their readings as shifts along the three principal axes. The mean shift and standard
deviation of the readings along each axis were calculated. We regarded the automatic alignment as being acceptable if
the difference between the mean shift and the automatic shift is within two times the standard deviation. Three out of
five patients were successfully aligned with two failures.
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