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Monte Carlo simulations of X-ray irradiations of breast phantoms of various sizes as PMMA cylinders of different diameters and a hemi-ellipsoidal PMMA phantom were performed in order to evaluate the 2D distribution of primary and scattered photons and Scatter-to-Primary Ratio (SPR) in projection images in cone-beam breast CT. Irradiation geometry and technique factors were set so as to reproduce the experimental conditions used for validation measurements with a prototype CT breast scanner. In the simulations, performed with GEANT4 software, we varied the phantom diameter and shape, the X-ray tube voltage and added filtration. SPR increases from 0.4 (at 8 cm cylinder diameter) up to 1.5 (14 cm cylinder diameter) at the center of the phantom. In the same phantom, SPR has lower values toward the bases of the cylinder than at its center. The scatter fraction increases by adopting a 50 kVp or higher tube voltages, up to 80 kVp, and by increasing the added filtration. Simulated and measured lateral profiles across a 14 cm cylinder diameter in projection images show a relative deviation of 4%. Simulations show a different distribution of scatter in a 14 diameter cylinder and 14 cm hemi-ellipsoidal phantom, so preventing the use of simple cylindrical geometries when simulating the attenuation of the pendant breast for scatter correction procedures.