AbstractID: 9503 Title: Beam orientation optimization in IMRT using single beam
characteristics and mixed-integer formulations
While the process of IMRT planning involves optimization of the dose distribution, the
process of beam selection continues to be “trial and error”. We have developed a method
to optimize beam orientation using mean organ-at-risk (OAR) dose (MOD) and mixedinteger programming (MIP) optimization. Two test cases were selected in which one and
four OARs were simulated, respectively along with a PTV. Beam orientation space was
discretized in 10º increments. For each beam orientation, a single beam plan was
generated without intensity modulation and normalized to a mean PTV dose of 2 Gy and
MOD was calculated. IMRT plans with beam orientations (5, 6, 7 and 9 beams) resulting
in lower MODs produced better OAR sparing. The degree of OAR sparing was related to
the average OAR MODs resulting from the beam orientations utilized with improvements
of > 15% at some dose levels. OAR DVHs were insensitive to beam number for similar
average MODs. MOD was integrated within a MIP model (with appropriate OAR
weighting) which was solved using the branch-and-cut algorithm. Seven beam
orientations obtained from solving the MIP were applied to the 4-OAR case and the
resulting plan with a dose prescription of 63 Gy was compared with an equi-spaced beam
plan. The MIP selected beams produced 6% and 7% DVH improvement at 25 Gy for
two OARs proximal to the PTV. Further improvement in the DVH can be obtained by
increasing the weights assigned to these OARs but at the expense of the remaining
OARs.