# Safety Assessment of Microwave Breast Imaging: Heating Analysis on Digital Breast Phantoms
## 1 Introduction
### Current Breast Cancer Screening Technologies  
To date, X-ray mammography is the gold standard technology for breast cancer screening. Mammography sensitivity can be positively or negatively affected by variables like a woman's age, hormonal status, and breast density (amount of fibroglandular tissue in a breast relative to fat), as well as tumors' size and stage. From a case-control study [2], screening performance was found to be superior in the fat breast group compared to the dense breast group (overall sensitivity 75.7% vs. 57.8%). In addition to the low sensitivity in extremely dense breasts, other drawbacks of X-ray mammography are the painful physical compression of the breast and the risk related to the use of ionizing radiation. According to the WHO, women aged 50 and over should have a screening every two years. The use of X-rays does not allow for a higher examination frequency because of the potentially harmful consequences, such as the onset of cancer. For the same reason, young women are not eligible for screening, although, worldwide, almost 30% of breast cancer cases occur in women younger than 50 years [1]. In this scenario, different research groups are working to develop new imaging devices based on microwaves [3]. The latter have several advantages, including low cost, ease of use, and, mainly, the use of non-ionizing radiation. Moreover, despite low resolution (in the order of some millimeters [3]), the quantitative nature of the technique allows microwave imaging (MI) to differentiate between malignant and benign tumors. Magnetic resonance imaging (MRI) and ultrasound are other relevant breast screening techniques in the field of non-ionizing radiation. However, MRI requires a substantial financial commitment [3] for both the initial acquisition and subsequent costs, including maintenance, infrastructure, safety measures, and specialized staff. The limit of ultrasound is its low sensitivity in distinguishing benign and malignant tumors [3]. Indeed, the extensive investigation into the use of microwaves in breast cancer detection is motivated by the promise of better sensitivity due to the possibility of distinguishing some physiopathological statuses based on tissue electrical properties (EPs). The EPs in the range of microwaves (from 0.5 GHz to 8 GHz) differ greatly depending on the tissues. The electrical permittivity value of a benign tumor and cancer is about 3 and 10 times larger than that of the corresponding healthy tissue, respectively. Similar proportions are found for the conductivity [4]. This strong contrast is expected to become visible in the MI results. An MI system is usually composed of antennas illuminating the patient's breast and collecting the reflected signals. Different systems presented in the literature use horn antennas [5], Vivaldi antennas [6], or patch antennas [7]. They usually use a compact vector network analyzer (VNA) as the signal generator and recording device, which works in the range from 0.5 GHz to 9 GHz. The output power is set on a scale from 0.05 mW to 32 mW, depending on the device. The VNA collects S-parameters, which can be elaborated by software to produce an image of the target object. Different architectures are also used in some radar-based systems, such as antenna arrangements in prosthetic bras [8] and hand-held devices [9]. Advanced MI systems produce qualitative images in which the contrast map can highlight the presence of an abnormal mass.