Introduction: In intensity modulated radiation therapy
(IMRT), the number and arrangement of beams affects the
quality of the dose distribution produced by the inverse
planning process• Beam directions are either selected based
on clinical experience, or plans are created based on the
minimum number of equidistant beams that gives an
acceptable result (typically 5 to 7 beams). Depending on the
target location, an optimised beam arrangement can produce
a better plan with fewer beams than an equidistant
configuration.
Materials and methods: We present a simple method for
determining the optimal beam arrangement for a fixed
number of coplanar beams, and describe how we determine
the number of beams to be used in our clinical practice. The
method involves a preliminary plan with 18 equidistant
coplanar beams, with subsequent plans based on the beams
that contributed the most dose after the first optimisation.
Using this method we can often prepare 4 or 5 field plans
with target coverage, dose homogeneity in the target volume,
and normal tissue sparing, that is not significantly degraded
compared with the initial 18 field plan.
Results: Preliminary 18 beam plans and resulting 4 or 5
beam plans used for treatment are presented and evaluated
using different methods. The preliminary investigations used
to decide on the method are also presented, including the
effect of 20 degrees increments compared with 10 degrees
increments for the initial plan, the effect of a 5 degrees
rotation of the 18 beams in the initial plan, and the
importance of the isocenter and weight point locations.
Conclusions: For some treatment sites, class solutions for
IMRT treatment planning (that include the beam
arrangement) are an effective way to reduce the time needed
to find an optimal plan. For other sites - unusual or paediatric
cases - the method we present is a useful way to reduce the
time required for plan optimisation without significant dose
distribution degradation.