A simple method for determining the optimal beam arrangement for IMRT

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