The experimental determination of the dose distribution in the body of patients treated with high-energy electrons is very difficult and inaccurate as, among other things, the body contours and the size of the patient's organs are seldom identical with those of a phantom. With the aid of the computer, it is possible to elaborate quickly for each treatment isodose plans sufficiently accurate for practical application. The dose distribution depends on the following factors: number of fields or rotation angle, size of the radiation fields, electron energy, curvature of the body surface and shape and composition of the organs. With the aid of the computer, which is capable of simulating a multiple-field or moving-beam irradiation by the summation of the dose values of two or more single fields, the above-mentioned parameters can be changed as required and thus various types of treatment plans can be prepared. Prior to the treatment, the radiotherapist can select the optimal plan for the case in question from a number of possibilities.