Abstract This paper proposes a novel mathematical approach
to the beam selection problem in intensity modulated
radiation therapy (IMRT) planning. The approach allows
more beams to be used over the course of therapy while
limiting the number of beams required in any one session. In
the proposed field rotation method, several sets of beams are
interchanged throughout the treatment to allow a wider
selection of beam angles than would be possible with fixed
beam orientations. The choice of beamlet intensities and the
number of identical fractions for each set are determined by
a mixed integer linear program that controls jointly for the
distribution per fraction and the cumulative dose distribution
delivered to targets and critical structures. Trials showed the
method allowed substantial increases in the dose objective
and/or sparing of normal tissues while maintaining cumulative
and fraction size limits. Trials for a head and neck site
showed gains of 25%–35% in the objective (average tumor
dose) and for a thoracic site gains were 7%–13%, depending
on how strict the fraction size limits were set. The objective
did not rise for a prostate site significantly, but the tolerance
limits on normal tissues could be strengthened with the use
of multiple beam sets.