To evaluate the diagnostic value of Ultrasound Computer Tomography (USCT), the imaging results have to be correlated with conventional breast imaging techniques. This is challenging due to different patient positioning in the modalities with nonlinear deformations of the breast tissue. We have developed a patient-specific image registration method, which simulates different breast positionings in both X-ray mammography and Magnetic Resonance Imaging (MRI) through biomechanical modelling. An average registration error below 5 and 17 mm for MRI to USCT and USCT to mammography registration, respectively, allowed us to evaluate the diagnostic performance of USCT. It was shown that regions of high sound speed corresponded well with the tumour position indicated from the MRI contrast kinetic map. Moreover, the quantitative analysis of sound speed and attenuation values with respect to the segmented mammograms revealed that sound speed gives a better distinction between breast tissue, whereas their combined information further improves the classification. Although the results are based on a preliminary study, the promising outcome points that the registration could assist radiologists in comparing the USCT with both MRI and X-ray mammography.