Abstract—Improved breast cancer screening technology may allow early detection of the disease. Recently, novel multi-modal techniques have been proposed which take advantage of the high spatial resolution of Digital Breast Tomosynthesis (DBT) and the relative high physical contrast of Microwave Radar (MWR). In this work, the slice-based X-ray images generated by DBT are treated as prior knowledge. The relationship between X-ray attenuation and healthy Microwave dielectric properties (- relationship) is computed using MWR measurements in a modified gradient descent algorithm. Using this relationship, a corresponding distribution of dielectric constants for healthy tissue is generated from the DBT image. The expected electric field for the healthy case is then computed using a computational electromagnetic scattering model. By subtracting this presumed healthy background from the measured field, the component due to any anomalous lesion’s dielectric contrast is isolated. The lesion position is then localized using standard phase-based radar imaging. In this paper, the mathematical justification for this approach is derived. Numerical experiments are presented for which the - relationship is successfully computed and the lesion position is retrieved.