Inverse radiotherapy planning for intensity-modulated photon fields

Substantial improvement in conformal radiotherapy is possible using modulated irradiation fields. Such modulated fields may be generated even with conventional accelerators by means of individual metal compensators or with the recently available dynamic multileaf collimators (MLC). For treatment planning a new kind of planning program is required that can calculate the 2 D-intensity matrices for each photon field. At the German Cancer Research Center (Deutsches Krebsforschungszentrum) such a program has been developed under the name of "KonRad" (Conformal Radiotherapy). Although it is an independent application, it is proposed for clinical usage to supplement a planning system that is already present. So-called inverse planning differs from conventional 3 D planning, as the trial-and-error approach for finding good field parameters is nearly completely avoided. Instead, the radio-oncologist is given the chance directly to specify medically oriented criteria like the prescription dose in the target volume, maximal tolerance dose values for each organ at risk and their weighting factors. In addition, the so-called DVH optimization allows aimed, partial overdosage, especially in parallelly structured organs in order to obtain better overall planning results. Because of very fast dose calculation in connection with rapidly converging gradient optimization and an intuitive user interface, the planning is done in a comfortable and interactive manner. Using a workstation or a PC, a typical plan can be created within a few minutes.