  The International Electrotechnical Commission (IEC) has previously defined
standard rotation operators for positive gantry, collimator and couch rotations
for the radiotherapy DICOM coordinate system that is commonly used by treatment
planning systems. Coordinate transformations to the coordinate systems of
commonly used Monte Carlo (MC) codes (BEAMnrc/DOSXYZnrc and VMC++) have been
derived and published in the literature. However, these coordinate
transformations disregard patient orientation during the computed tomography
(CT) scan, and assume the most commonly used 'head first, supine' orientation.
While less common, other patient orientations are used in clinics - Monte Carlo
verification of such treatments can be problematic due to the lack of
appropriate coordinate transformations.
  In this work, a solution has been obtained by correcting the CT-derived
phantom orientation and deriving generalized coordinate transformations for
field angles in the DOSXYZnrc and VMC++ codes. The rotation operator that
includes any possible patient treatment orientation was determined using the
DICOM Image Orientation tag (0020,0037). The derived transformations of the
patient image and beam direction angles were verified by comparison of MC dose
distributions with the Eclipse treatment planning system, calculated for each
of the eight possible patient orientations.
