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We are developing a PET system dedicated to breast cancer imaging based on a pseudo-pinhole PET geometry. The system consists of a small half-ring of high resolution detectors and a large half-ring of medium resolution detectors. Coincidence detection profile between a small LSO crystal and a large LSO crystal reveals that the image resolution of the system is primarily determined by the intrinsic spatial resolution of the small detector, and slightly affected by the resolution of the large detector. Monte Carlo simulation of the system was performed using a modified SimSET package. The system consists of a small half ring (r=153 mm) and a large half ring (R=413 mm) of LSO detectors. The dimension of the crystal is 1.6×1.6×20 mm3 for the small half-ring and 4.3×4.3×25 mm3 for the large half-ring. The detectors in the small half ring are modeled as 2- or 4-layer detectors with DOI capability. 1 mm point source was simulated to measure the image resolution of the system at 0, 2, 6, and 10 cm offset along +X, +Y and -Y directions in the system. Results show an image resolution of 1.7-2.5 mm FWHM for breast tissues and 1.8-3.1 mm FWHM for tissues near the chest wall, including internal and axillary lymph nodes. Sensitivity of the system is from 5.4 to 7.8% and can be significantly increased if two planar detectors are attached to the system along the axial direction.