A computational investigation of microwave scattering in mechanically (or acoustically) excited breast tissue is conducted to explore the feasibility of combining dielectric and elastic properties contrasts to enhance breast cancer detection. The mechanical excitation induces tissue-dependent displacements in the heterogeneous breast interior, which modulate the scattered microwave signals. Sheet boundary conditions are implemented using the finite-difference time-domain (FDTD) method to efficiently compute the Doppler component of the scattered microwave signals. Simulation results for a 2D numerical phantom testbed demonstrate increased microwave scattering contrast between malignant and normal fibroglandular inclusions when elastic properties are exploited.