[Four years' experience with radiotherapy planning based on computed tomography (author's transl)].

Since 1976, radiotherapy planning is based on computed tomography in the Radio-Oncologic Department of the hospital of Passau. Every year we have about 1200 new tumor patients, 30 to 40% of them being submitted to an individual CT planning. So we are able to evaluate about 1000 cases of CT-planned radiotherapies. The single steps of CT planning are described. The position of the patient is an important factor for CT planning as well as for the subsequent irradiation. A topogram has been mounted on the tomograph in order to find out the best position and, in order to guarantee a precise reproduction of this position, identical laser systems have been installed on the tomograph and in the irradiation room. Some examples are presented in order to describe the mode of action of these devices. Furthermore, a new control method is presented which allows to check the accuracy of field screening by means of computed tomography.