Optimization of the step-and-shoot leaf sequence for delivery of intensitymodulated radiation therapy using a variable division scheme

To deliver an intensity modulated radiation therapy (IMRT) plan by multilea f collimator (MLC) it is necessary to convert beam profiles, generated from the inverse treatment planning algorithm, into a series of instructions th at the MLC control system can execute. An idealized IMRT beam profile can b e regarded as a continuously varying two-dimensional function and is usuall y represented by an intensity map, i.e., a discretized description in space and in intensity of the beam profile. It is common to assume that the inte nsity map be defined over a regular grid with N steps and equal increments of intensity levels. In reality, this may not be the optimal representation of the beam profile and may introduce unnecessary discrepancies between th e intensity pattern delivered and that ideally required. We have implemente d an algorithm capable of minimizing the difference between the two pattern s on a beam specific basis. In other words, it can produce optimized intens ity maps, individually produced to suit the (continuous function) intensity profile they are intended to approximate. This enhancement in conformation is achieved by allowing variable step size and unconstrained intensity lev els in the final leaf sequence.