We are developing a platform for virtual (in silico) clinical trials in x-ray breast imaging and dosimetry fully based on Monte Carlo simulations and which adopts patient-like digital phantoms. We report the initial steps and first results of this project. Our collaboration was established for producing a “user-friendly” computational platform which reproduces both 2D (full-fields digital mammography) and 3D (digital breast tomosynthesis, CT dedicated to the breast) breast imaging examinations for technique optimization and for developing protocols and computed aided diagnostic applications. Dose optimization strategies and technique performance comparisons will be explored. A dataset of anatomically realistic, 2D and 3D digital breast phantoms have been produced by means of voxel classification and digital compression of clinical breast CT scans acquired at UC Davis. Monte Carlo simulations, based on the Geant4 toolkit, have been developed for insilico FFDM, DBT and BCT examinations.