Optimum beam configurations in tomographic intensity modulated radiation therapy

We review and extend the theory of tomographic dose reconstruction for inte nsity modulated radiotherapy (IMRT). We derive the basis for a saturation w ith beam number of dose conformation, and provide an analysis which ranks p articular beam orientations in terms of the contribution to the delivered d ose. Preferred beam directions are found which effectively reduce the numbe r of beams necessary to achieve a given level of dose conformation. The ana lysis is a new application of the tomographic projection-slice theorem to t he problem of beam orientation determination. The effects of the beam front filter and the positivity constraint arising from the tomographic approach are analysed, and modifications of the beam front filter for smalt beam nu mbers are suggested. The theory is applied to simple geometric shapes in tw o dimensions. A Gaussian ellipse, where analytical results are obtained, an d simple hard-edged convex prescribed dose shapes are examined to illustrat e beam selection based on the beam overlap metric. More complex concave pre scribed dose shapes which contain a sensitive organ are also analysed and f or low beam numbers are found to have preferred beam directions.