A strategy to minimize errors from differential intrafraction organ motion using a single configuration for a 'breathing' multileaf collimator

Intensity-modulatedradiation therapy (IMRT) can be delivered by the 'sliding-leaves' dynamic multileaf collimator(DMLC) technique. Intrafraction organ motion can be accommodated by arranging an identical tracking motion for 'breathing leaves'. However, this is only possible for very specific circumstances such as regular, mathematicallyparameterizable, rigid-body, density-conserving, one-dimensional translations. In this paper, we investigate what happens when planes of tissue in the line of sight of the MLC have differential motion with respect to the moving leaves. In this situation, there is no solution to the problem and a perfect tracking motion cannot be arranged. However, an iterative minimization-of-errors 'solution' (or strategy) can be found and the technique is presented for this. From this, under certain mathematically simple differential motions it is possible to obtain some elegant algebraic solutionswhich are presented. In general, however, a lengthy computational minimization is required and results of examples of these are presented.