Improvements in 99Tcm-sestamibi breast lesion visualization using single photon emission tomography (SPET) may help define the clinical role of this technique alongside X-ray mammography in the diagnosis and management of breast cancer. Pinhole SPET offers the advantages of high resolution and sensitivity when compared to conventional parallel-beam collimation for sources located near the pinhole aperture. In this work, the potential of incomplete (180 degrees) circular orbit (ICO) SPET with pinhole collimation is investigated as a means to visualize small (6.4 and 9.6 mm diameter) spherical simulated tumours, at clinical count densities and tumour-to-background ratios, in a breast phantom. ICO pinhole SPET is compared to complete circular orbit (CCO) pinhole SPET for reference, and planar breast imaging (scintimammography) using parallel-beam and pinhole collimators. A prototype box-shaped pinhole collimator with a 4 mm diameter circular aperture was used to acquire projections of an 890 ml breast phantom both in isolation and mounted on a cylinder filled with a mixture of 99Tcm-pertechnetate and water. A heart phantom containing 99Tcm activity in the myocardium was placed in the cylinder. Simulated tumours containing 99Tcm were placed in the breast phantom and scanned at clinically relevant count densities and scan times with tumour-to-normal tissue concentration ratios of 5.0:1 (9.6 mm sphere) and 7.7:1 (6.4 mm sphere). Phantom data were reconstructed using pinhole filtered backprojection (FBP) and maximum likelihood-expectation maximization (ML-EM). The tumours were not visualized with scintimammography, in which lesion contrast and signal-to-noise were estimated from region of interest analysis to be < 2% and 0.01, respectively. Average (over lesion size and scan time) contrast and signal-to-noise in the ICO (CCO) SPET images were 33% and 1.72 (34% and 1.3), respectively. These values indicate that ICO pinhole SPET has the potential to improve visualization of small (< 10 mm) breast tumours when compared with scintimammography, which may be beneficial for the early classification of cancers of the breast.