# Safety Assessment of Microwave Breast Imaging: Heating Analysis on Digital Breast Phantoms
## 4 Discussion
In models #2 and #3, the tumor tissue experiences the smallest temperature increase. This could be due to the fact that tumors receive a rather attenuated field, being located in a more internal area of the breast. In addition, it was mostly found that the benign tumor has a higher absorption than the malignant tumor despite the lower conductivity. This may depend on the fact that the tumor in model #3 is deeper than the one in model #2. Larger temperature increases could be observed in more superficial tumors that were not included in the investigation. However, the order of magnitude of the obtained results reassures us of the absence of thermal hazards even in such a situation. In order to better understand the variability of the presented results, the investigation took into account some parameters that could affect the temperature estimation, including breast density and the presence of a receiving antenna. The influence of the former parameter was studied in models #4 and #5. The temperature peak is located in the skin for model #4 at 6 GHz, 8 GHz, and 2 GHz with the horn antenna in position 4, whereas for model #5, it is in the fibroglandular tissue for all the considered frequencies except 8 GHz, excluding the horn antenna in position 5. When breast density is low, as in model #4 (9.3% of fibroglandular tissue), the energy of the radiation tends to be primarily dissipated in the outermost tissue, whereas in the case of high breast density, as in model #5 (25% of fibroglandular tissue), the energy of the radiation is dissipated in a relevant amount also in the most superficial areas of the glandular tissue. The considered MI system has an antenna configuration in which the transmitting and the receiving antennas operate in distinct moveable positions. When the receiving Vivaldi antenna is placed between the transmitting horn antenna and the breast model, the emitted signal can be distorted. Indeed, the results collected in Table 7 show that when the transmitter and the receiver are aligned, a reduction in the temperature peak is found in fat (\(-\)69.7%), fibroglandular tissue (\(-\)79.7%), and skin (\(-\)39.2%). In this case, the receiver, acting as a scatterer, tends to spread the transmitted EM field outward, and the signal received by the breast gets lower, with a reduced temperature increase compared to the case without the receiving antenna. When the transmitter and the receiver are at 30\({}^{\circ}\) from each other, an increase in the peak temperature was found in fat (\(+\)11.8%), fibroglandular tissue (\(+\)53.1%), where the strongest was observed, and skin (\(+\)5.4%). Finally, when the transmitter and the receiver are at 60\({}^{\circ}\) from each other, the temperature peak increases in the fat (\(+\)13.3%) and in the fibroglandular tissue (\(+\)14.9%) but decreases in the skin (6.2%). These results show that if the Vivaldi antenna is shifted with respect to the horn antenna, it can focus the EM waves toward the breast, causing a higher temperature increase than in the case without the receiving antenna. Other positions for the Vivaldi antenna were not studied because if the angle between the two antennas is greater than 90\({}^{\circ}\), the impact was assumed to be negligible. Considering the small temperature elevation that occurs even in the worst case,the presence of the receiving antenna acting as a scatterer does not compromise the safety of the system. Varying the electrical and thermal properties of breast tissues of \(\pm\)10% showed a temperature increase consistent with that obtained with nominal values, with a variation related to the applied changes of at most 15%. A thorough analysis of the influence of tissue property variability would require a stochastic dosimetry approach [27], but considering the extremely low levels of thermal effects, a deterministic approach has been chosen. The outcome builds upon the previous findings and supports the conclusion that the MI system can be considered safe.