# Safety Assessment of Microwave Breast Imaging: Heating Analysis on Digital Breast Phantoms
## 1 Introduction
### Safety Assessment of Microwave Technology  
The introduction of MI systems in the clinical world is foreseen, considering that the CE mark [10], denoting products compliant with EU safety, health, and environmental standards for commercial purposes, was granted to some of them [11; 12] and that clinical studies are ongoing [13]. Since MI is an evolving field, it is useful to check through scientifically sound dosimetric studies that the interaction between electromagnetic (EM) fields and biological tissues related to this application is safe. Because MI devices are based on non-ionizing radiation, the advantages over X-ray technology are indisputable, particularly as they would allow screening without age limitations. However, quantifyingpotential adverse effects caused by exposure to this type of EM field is essential to assess MI safety. This is important also to understand the room for further development of the technology, in terms of better hardware configurations, to ensure sufficient and good quality data for image reconstruction. In the MI range of frequencies, the only recognized biological effect of EM radiation is heating, which could increase the temperature in a localized manner. The specific absorption rate (SAR), defined as the power absorbed per mass of tissue, with units of watts per kilogram (W\(\cdot\)kg\({}^{-1}\)), is a well-established metric to quantify the thermal effects of EM waves in the tissues. Since there is no specific technical regulation for MI that establishes patient exposure limits, the guidelines for limiting exposure to EM fields from 100 kHz to 300 GHz published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [14] could be a reference starting point. For the general public, in the range from 100 MHz to 6 GHz over a 6-min time interval, SAR should not exceed the level of exposure defined by the guidelines. Above 6 GHz, SAR is replaced by the absorbed power density (W\(\cdot\)m\({}^{-2}\)) as the metric to assess a more superficial effect of EM radiation localized at the skin level. As a matter of fact, the set of metrics established by ICNIRP acts as a surrogate of the temperature increase. For this reason, in this work, the calculation of SAR was performed not as a metric to be tested with respect to safety limits but as an intermediary parameter used to estimate the final temperature increase. Previous works presented studies of the thermal effects consequent to the use of MI in breast cancer detection. They show that the level of exposure is not dangerous for breast tissues. However, some studies were conducted with a non-anatomical model to mimic breast tissue [15] and a plane wave illumination [16], which could not mimic a real transmitted field. Another study used one specific virtual phantom from a repository of 3D biological models for numerical simulation and constructed a simple phantom that was used in a real experimental environment for validating the simulation results [17]. In this case, the exposure to the field generated by a Vivaldi antenna was assessed with an analysis performed by varying the signal power and the distance from the target. The computations were performed only at the frequency of 8.5 GHz, and the temperature increase was evaluated after a very long exposure of two hours. A dosimetric analysis was also performed for a hand-held MI [18], which works in direct contact with the breast and, as such, uses a relatively low transmitted power (0.045 mW). The particular design of this MI device makes it difficult to generalize the results of this analysis to other systems. This paper aims to assess the safety of an MI system in a wide range of frequencies by computing the SAR distribution in different numerical breast models selected from MRI scans of healthy and pathological breasts to cover variability in the patient population and analyze the maximum temperature increase in the different breast tissues.