The objective was to characterize the changes seen from incident Monte Carlo-based scatter distributions in dedicated three-dimensional (3-D) breast single-photon emission computed tomography, with emphasis on the impact of scatter correction using the dual-energy window (DEW) method. Changes in scatter distributions with 3-D detector position were investigated for prone breast imaging with an ideal detector. Energy spectra within a high-energy scatter window measured from simulations were linearly fit, and the slope was used to characterize scatter distributions. The impact of detector position on the measured scatter fraction within various photopeak windows and the value (ratio of scatter within the photopeak and scatter energy windows) useful for scatter correction was determined. Results indicate that application of a single value with the DEW method in the presence of anisotropic object scatter distribution is not appropriate for trajectories including the heart and liver. The scatter spectra’s slope demonstrates a strong correlation to measured values. Reconstructions of fixed-tilt 3-D acquisition trajectories with a single value show quantification errors up to 5% compared to primary-only reconstructions. However, a variable-tilt trajectory provides improved sampling and minimizes quantification errors, and thus allows for a single value to be used with the DEW method leading to more accurate quantification.