# Safety Assessment of Microwave Breast Imaging: Heating Analysis on Digital Breast Phantoms
## 4 Discussion
### Comparison with Existing Studies About the Safety of MI  
This section compares the findings of the current study regarding the safety of the MI system with results from other relevant studies in the field. In [15], a dosimetric analysis on a monostatic radar-based MI system is proposed, employing a geometrical three-layer phantom with inclusions simulating tumors of various sizes and a microstrip patch antenna functioning as both transmitter and receiver. The SAR and the temperature increase induced in the phantom were computed at 4 GHz, 6 GHz, and 8 GHz, taking into account various distances of the antenna from the phantom and different dielectric property values to simulate the influence of the patient's age. In the absence of tumors, the highest SAR is found in the skin layer, while it is found in the tumor if it is present. Temperature analysis reveals a slight increase, with maximum values of 55 mK in healthy tissue and 95 mK in cancerous tissue, with distribution patterns varying for continuous exposure times ranging from 10 to 60 min. The temperature variations are approximately four orders of magnitude higher than those reported in the current research. However, this discrepancy can be attributed to differences in system architecture and to the longer simulated exposure time, which is at least 10 min. Additionally, the model used in this study does not include fibroglandular tissue, which is found to be the primary energy-absorbing tissue in the current research. Simulating a realistic anatomical structure allows for better consideration of the breast's heterogeneities, enhancing the accuracy of the analysis. In [16], a worst-case scenario was analyzed involving the exposure of a 3D anatomical breast model to a plane wave at various frequencies in the 0.4-9 GHz range. The input surface power density of 10 W/m\({}^{2}\), maximum exposure level in the frequency range of interest according to [28], resulted in SAR peak values located in the skin higher than those observed in the current study. Despite differences in the physical nature of the source and input power, the increasing trend of SAR peak values with frequency reported in this study agrees with the trend of temperature rise with frequency calculated in the current work. To produce a quantitative comparison, the averaged incident surface power density was computed on the breast surface from a simulation performed on free space (without the breast tissues), yielding a value of 0.005 W/m\({}^{2}\) at a frequency of 2 GHz. Using this value, the SAR was scaled to an input surface power density of 10 W/m\({}^{2}\), resulting in a peak SAR of 2.97 W/kg in the fibroglandular tissue. This value is lower than the local peak SAR at 2 GHz reported in the other study, which is approximately 8 W/kg. Nevertheless, the order of magnitude of the SAR is consistent between the two studies. Experimental measurements available in the literature [17] show that the SAR value due to a Vivaldi antenna transmitting at 8.5 GHz with an incident power of 1 mW is in the order of 1 mW/kg. As can be seen in Figure 5, the results of this study are in the same order of magnitude, although a horn antenna was used in transmission. Moreover, the computation presented here shows that, despite the adopted conservative model, the thermal effect is small enough to be considered negligible. In [17], a variation of \(\pm\)10% at 10 mW in the experimental SAR with respect to simulated SAR is reported. Since the current study does not provide an experimental setup for measuring SAR, this expected variation in the case of simulated data can be used, asserting that a \(\pm\)10% variation in computed SAR values and, consequently, in temperature increases does not change the biological effect on breast tissues, which remains negligible (conservatively, the maximum temperature increase of 58.5 uK could be corrected with a 10% increase to 64.3 uK).