In this paper, interactive multiobjective optimization for radiotherapy treatmentplanning is studied. The aim of radiotherapy is to destroy a tumor without caus-ing damage to healthy tissue and treatment planning is used to achieve an optimaldose distribution. In intensity modulated radiotherapy (IMRT), the intensity of theincoming radiation flux can be modulated using some aperture such as a multi-leaf collimator. Radiotherapy goals are conflicting and it is impossible to satisfyall the targets simultaneously. Therefore, an interactive multiobjective optimizationmethod for IMRT is used. With this method, the best compromise can be foundfor all the conflicting targets. Results of numerical studies indicate that in this waya radiotherapy expert, called as a decision maker, can conveniently utilize one’sknowledge and expertize and direct the solution process interactively and itera-tively in order to find the best feasible radiotherapy treatment plan. This approachdecreases the number of uninteresting solutions computed and the best solution canbe selected in a case specific way by using the decision maker’s expertize.