Computed Tomography and Three-Dimensional Planning in Radiotherapy

Computed tomography provides the radiotherapist and the therapy-physicist with information concerning the respective positions of the tumor, the different organs and the surface of the body. In addition, data about the densities of the structures and tissues within the irradiated volume have been available. Repeated CT-examinations during the course of and after treatment reveal the response of the tumor to the irradiation. All these data which are of vital importance for radiotherapy have often been only unsufficiently known before the introduction of CT-scanning. They allow a truly three-dimensional planning, using non-coplanar fields, to obtain a treatment scheme which guarantees the desired dose within the target volume while protecting radiation-sensitive surrounding tissues as far as possible, taking into account the different attenuation coefficients in tissues of different densities. The therapeutic procedure must be adapted to the response of the tumor during the treatment; for example it might become necessary to diminish the field size to account for a shrinking of the tumor.