Abstract: The co-registration of a volumetric ultrasound acquisition of the volume of interest with a magnetic resonance image allows the former to providing the basis for calculation of the dose distribution and the latter to provide the basis for target delineation and isocenter placement. The present invention therefore provides an apparatus for planning the radiotherapeutic treatment of a volume of tissue, comprising a sonographic apparatus for acquiring acoustic data relating to the volume, a means for reconstruction of an internal structure of the volume on the basis of the acoustic data, and a means for classification of the material type within that internal structure to one or more tissue types. A three-dimensional motion-tracking device for the sonographic device will assist. The sonographic device will typically be an ultrasound probe. The tissue type data can be passed to a dosage calculation means for determination of radiation dosage.