Though mammography is one of the most sensitive methods to detect breast cancer, the benefit of the mammography screening programmes is still not clearly proven. One of the reasons is the radiation dose delivered by the examinations. Simulations of the radiation transport based on realistic breast phantoms are a useful tool to estimate the dose for the risk relevant parenchymal tissue. Specimens of real breasts have been fixated using a specially designed process while being compressed as in mammography. They have been scanned using the high-resolution mode of a CT. A segmentation has been carried out by assigning the voxels to different tissues. The resulting voxel phantom allows the assessment of tissue doses by Monte-Carlo calculations and can be used to simulate the diagnostic outcome of different imaging procedures. Three different tissues were separated: skin, adipose and 'breast tissue'. This allows reasonable calculations of the average glandular doses in mammography.