AbstractID: 8762 Title: Three-Dimensional Rigid Body Transformation for IMRT
Prostate Setup Correction
Three-dimensional rigid body transformation is used for accurate IMRT prostate setup correction. The calculated transformation
aligns the bony structures in portal images with the corresponding planning digital reconstructed radiographs (DRRs). The centers of
right and left femurs and inferior pubic symphysis from the planning CT images define a reference set P. The same points on three
oblique portal images at setup determine the patient set P’. A circular fit to the center of each femoral head contour provides accurate
and reproducible points. The difference in the center of mass between the two sets is the translation component of the transformation
that corresponds to lateral, vertical, and longitudinal couch shifts. A robust downhill simplex algorithm is used to calculate the
classical rotation matrix R defining the gantry, collimator, and couch angles in the center of mass coordinate system. Setup errors can
then be corrected for translation and rotation from the relationship 3¶ 5 3 . The theory has been validated for ideal models,
simulated portal images of known transformations, and actual patient images. The results indicate an excellent agreement between the
reference and the transformed DRRs within +1 mm and +0.5 degree in the treatment region. Errors in the user selected points are
minimized by assuring congruence between the reference and transformed sets. Applications of this formalism include accurate
retrospective analysis of the effects of 3D setup errors and on-line setup correction strategies.