Breast cancer (BC) is the most frequently diagnosed cancer in women worldwide, with nearly 1.7 million new cases diagnosed in 2012, and more than half of BC cases and deaths occurring in economically developing countries.1–3 Asian countries, which represent 59% of the global population, have the largest burden of BC, with 39% of new cases, followed by Europe at 28%.3,4 In 2012, deaths from BC in the USA accounted for 783,000 years of potential life lost and an average of 19 years of life lost per death.5 Early detection has been shown to be associated with reduced BC morbidity and mortality6,7 and the goal of BC screening programs is to reduce both. Most BCs are detected due to clinical symptoms or by screening mammography (MMG). The standard way to assess suspicious lesions is with the so-called triple assessment: clinical examination, imaging by MMG and ultrasound (US), and image-guided needle biopsy. Magnetic resonance imaging is currently used for initial cancer detection in women at high risk of developing BC but is a complex investigation with high direct and indirect costs.8–11 MMG is one of the most effective detection techniques, but suffers from relatively low sensitivity, entails exposure to ionizing radiation and also involves uncomfortable compression of the breast. MMG also performs less well in younger, more dense breasts, which is pertinent as breast density is now established as an independent risk factor for developing BC irrespective of other known risk factors.12–16 This coupled with the increased risk from ionizing radiation in younger women, restricts the lower age for use based on risk/benefit ratio. Limitations of MMG have resulted in research into alternative methods for imaging of breasts with microwave detection of breast tumors being a potential nonionizing alternative.17 Initial results of microwave radar-based imaging have been presented17–23 and approaches rely on a difference in the dielectric properties (Dk) of normal and malignant breast tissues.24–31 The breast as an organ is unique in the human body in that basic structure consists of glandular tissue (high dielectric constant, high conductivity, and radioopaque) in a fat (low dielectric constant, low conductivity, and relatively radiolucent)-based matrix. Inclusions, such as a tumor, are also of high permittivity, enhanced by the angiogenic increase in vascularity, and cysts contain fluid, which also have very high permittivity.