SAR Analysis Using Various Substrates of Microstrip Antenna for Breast Cancer Hyperthermia Treatment

Hyperthermia is an alternative technique used to treat breast cancer using high heat, around 41°C to 45°C, to denature cancer tissue into necrotic tissue. It can work independently or in adjuvant with other conventional methods. The design of an antenna with a suitable substrate is essential for the applicator to transmit maximal heat to the intended area and distribute the heat uniformly on the treated tissues. Therefore, the inset feed microstrip antenna is design with five different substrates, simulated, analyzed, and their performance is compared to get the best substrate. The substrates used include Rogers Duroid RT5880, Rogers RO4003, FR4, Alumina and Rogers RO3010. SEMCAD X 14.8.4 software is used to design, simulate and generate SAR. RT5880 and RO4003 show better SAR distribution and focus position distance (FPD), which can be further examined for future research. 2450 MHz, 915 MHz and 434 MHz frequencies are compared to find the most suitable frequency to apply with the applicator. A 915 MHz frequency shows better performance which can penetrate more toward the cancerous areas. However, there are several deficiencies such as difficulty in controlling FPD, wide unwanted hotspot and massive skin burn problem, that needs to be improved to provide less adverse health effects due to the execution of hyperthermia treatment. The selection of suitable substrates can help reduce the deficiencies and improve the effectiveness of the hyperthermia technique. The estimated period is within 2 hours to 3.6 hours to achieve sufficient heating to destroy the cancerous cells.


Introduction
According to the GLOBACON 2018 statistics, breast cancer is the most common cancer affecting women, and it was reported as the second-highest leading cause of mortality rate worldwide [1].Breast cancer caused the deaths of 627,000 people globally in 2018, and the rate of death increased to 685,000 in 2020 [2].The number of women diagnosed with breast cancer and cancer mortality cases due to breast cancer has increased dramatically every year [3].In Malaysia itself, breast cancer is the first leading cause of mortality rate [2].In conjunction with the increment of breast cancer cases, it has become the main concern among researchers globally to find alternative treatments for cancer, especially breast cancer.Hyperthermia treatment can be seen as one of the promising techniques to be used as an alternative treatment to kill and destroy cancer cells [4].Hyperthermia can work alone or combined with other conventional methods, such as chemotherapy and radiotherapy, to speed up the treatment efficiency [5] [6].Hyperthermia has become the interest of many researchers lately as it has produced and shown many positive results and has fewer side effects than the conventional method.The usage of high heat temperatures from 40 0 C to 45 0 C in hyperthermia aims to destroy or obstruct the malignant tissues [7].
In the hyperthermia treatment procedure, an antenna is a crucial element since it directs the heat from the source to the treated tissues [5].The optimized features of the antenna substrates can help in improving and enhancing the antenna performance [8] [9].Therefore, the proper selection of a suitable antenna substrate is significant in providing heat absorption effectiveness for the hyperthermia applicator, which then improves the hyperthermia execution procedure towards the targeted treated cancer tissue.By proper selection of the antenna's substrate, the FPD on the treated tissue can be improved, as well as the wide area of unwanted tissue.In this paper, five substrates such as Rogers Duroid 5880, Rogers RO4003, FR4, Alumina and Rogers RO3010, are used, investigated, analyzed and compared.The antenna substrate has a permittivity range between 2.2 ≤   ≥ 12 [9].The permittivity of RT5880, RO4003, FR4, Alumina and RO3010 were 2.2, 3.38, 4.0, 9.8 and 10.2, respectively.The thickness of the dielectric substrates retains at 1.59 mm.
A microstrip patch antenna features a lightweight, small size [10] and is suitable to be used in the research.The selection of the substrate material is important to determine the antenna performance [11].The substrate of FR4, as the name implies, is flame retardant, where the dielectric material can cater for high heat.Rogers Duroid has low signal dispersion [12].Alumina can cater for high heat and hardness [13], and it has a low loss signal [14].A suitable antenna substrate can help the applicator to transmit maximal heat to the intended area.The effect of heating depends on the applicator design, temperature set up and duration of the treatments [15].It would be great if the applicator could produce heat which can cover more cancerous areas and distribute the heat evenly on the treated area.The non-invasive hyperthermia treatment procedure applied in the research where the applicator is placed outside of the patient body.SAR is the absorption rate of heat by mass body tissues, and it is expressed in W/Kg or mW/g [16] [17].SAR can be calculated using Equation 1 and Equation 2 [16] [18] [19].

Methodology
A few stages are involved in this research.Data received from the hospital are analyzed using DICOM software.The results are then used to get the penetration depth and focus position distance for the malignant tissues.Then, the process proceeds with the development of the breast phantom and the design of the microstrip antenna using various substrates.Three frequencies of 2450 MHz, 915 MHz and 434 MHz are used in the simulation experiments as recommended by International Telecommunication Union (ITU) application for the non-communication purpose [20].

Breast Phantom and Cancer Development
The research starts with breast cancer image collection received from the referred hospital.A total of 69 breast cancer images have been received and analyzed using DICOM software.The analysis is shown is section 2.2.The average of Medio Lateral Oblique (MLO) and Cranio-Caudal (CC) images are calculated to get the highest occurrence of focus position distance (FPD) and penetration depth (PD).Overall penetration depth from DICOM software analysis is within 0 to 69 mm.The largest FPD size of malignant tissues occurs in the range of 21 mm to 50 mm which is classify under FPD 2. Therefore, this measurement is used further for SAR analysis of different microstrip antenna substrates.FPD is the difference between breast cancer surface depth and inner depth.The surface depth is the measurement of cancer from the tip to the first point of cancer near the skin.Meanwhile, the inner depth is the measurement of cancer from the tip to the last point of cancer inside the breast phantom.Figure 1 shows the measurement of the depth [19].A 100 mm diameter breast phantom and an 18 mm diameter of malignant tissues are developed using SEMCAD X 14.8.4 software simulator.
=   ℎ −   ℎ Measurement point for surface depth and inner depth is as shown in Figure 1.The average of Medio Lateral Oblique (MLO) and Cranio-Caudal (CC) images are calculated to get the highest occurrence of focus position distance (FPD) and penetration depth (PD).Size of malignant tissues is the different of Average inner depth and surface depth.Highest occurrence of FPD in breast phantom is 21 mm -50 mm and maximum FPD range is 61 to 70 mm as shown in figure 2.

Figure 2.
Analysed FPD based on data received from referred Hospital

Antenna Design
The rectangular shape of the antenna with three different frequencies that operate at 434 MHz, 915 MHz and 2450 MHz are developed and used in this study.Five (5) different substrates used, such as RT5880, RO4003, FR4, Alumina, and RO3010, have different electrical permittivity properties.These 5 substrates are selected as its used frequent in antenna design, especially for hyperthermia treatment procedure [8][10] [21].Different frequencies and substrates have resulted in different antenna dimensions and output performance.The sizes and properties of the antenna using three different frequencies and five different substrates are tabulated in Table 2 with a fixed thickness of 1.59 mm.The antenna performance is evaluated where SAR is retained at 1.892.The optimized operating frequency and substrate are then used as a hyperthermia applicator.Size of the antenna increase as the operating frequency decrease.The properties include substrate thickness and permittivity, patch width (Wp), patch length (Lp), substrate width (Wg) and substrate length (Lg).Focus Distance

Result and Discussion
The 2-Dimensional (2D) chart, SAR, and FPD for five different substrates used in the experiment is as shown in Table 3.The simulation experimentation substrates include RT5880, RO4003, FR4, Alumina and RO3010.Each substrate of the antenna is designed and simulated using three different industrial, Scientific and Medical (ISM) frequencies of 2450 MHz, 915 MHz and 434 MHz.From the simulation conducted, RT5880 and RO4003, which operates at 915 MHz, shows the best heating results where heat is focused more toward the treated areas, and it produces fewer areas of unwanted hotspots compared to other substrates.
The deepest penetration depth presented by using an Alumina substrate with a permittivity of 9.8.Using a 434 MHz operating frequency, the heat produced from the antenna can penetrate up to 92 mm, the length of the distance from the front tip of the breast phantom to the cancer cell.From the experiment, Alumina substrates generated at all three operating frequencies show that the heat can reach most of the cancerous areas.Similar to FR4 and RO4003, which operate at 2450 MHz.Even though all substrates mentioned above can heat most of the cancerous cell areas, it also creates a wide area of unwanted hotspots that contribute to massive skin burn problems in the surrounding healthy tissues.
The lowest performance of SAR is presented when using RO3010 substrate with permittivity of 10.2 at 915 MHz and 434 MHz frequency, where heat is produced but cannot reach the cancerous areas.Meanwhile, 2450 MHz of RO3010, a small ratio of heat produced at the front of the cancerous areas.Apart from the aforementioned results, there is no heat produced at 434 MHz frequency for RT5880, RO4003 and FR4 substrates.Therefore, 434 MHz is not suitable to be used as a hyperthermia operating frequency if compared to 915 MHz and 2450 MHz.The period for hyperthermia execution is about 2 hours to 3.6 hours for the hyperthermia treatment planning.Table 3 shows the result of the 2D chart and FPD for each substrate at different operating frequencies.SAR retained at 1.872 Table 3. SAR and FPD performance using five substrates operates at three different frequencies.1.872 = 3.63 ℎ (7) Initial period treatment requires two and a half hours to sufficiently heat to kill cancer cells if the temperature is set at a minimal hyperthermia temperature of 41°C and the Specific Heat Capacity is at 3060J/Kg/K.However, if the temperature is increased to 45 0 C, the time to obstruct cancer cells is about 3.63 hours.Therefore the estimated time for sufficient heating to destroy cancer cells is about 2.14 to 3.63 hours if the SAR value is set at 1.872 mW/g.From table 3, at 915 MHz the focus position distance (FPD) for RO4003 is 21mm to 49mm whereas for RT5880 the FPD is about 12 mm to 62 mm which achieved desired FPD 2. For other substrates operate at 2450 MHz and 915 MHz frequency the antenna generate heat at surrounding healthy tissues and creates more unwanted hotspots such as indicated in table 3. Whereas when antenna operate at 434 MHz, no heat produce except when using alumina and RO3010 as a substrate.
Figure 2 shows penetration depth (PD) for five (5) different substrates at 915 MHz.For Alumina, the heat can penetrate up to 70 mm in the breast phantom, which consists of breast fat and malignant tissues, whereas using RT5880 and RO4003, the penetration depth (PD) can reach up to 62mm and 48mm respectively.Meanwhile, Figure 3 shows PD for five different substrates at 2450 MHz.The substrate which shows good PD performance are RT5880, RO4003, Alumina and FR4.
Based on the rectangular microstrip antenna equation, different antenna substrates have different permittivity that may affect antenna dimensions, whereby may lead to different penetration depths and different focus position distance.Furthermore, in [8] [22], it presented different substrates have different effects on SAR distribution towards cancer tissues.Meanwhile, for different frequencies, it also provides differences in SAR distribution and FPD [19] .

Conclusion
The research is conducted to evaluate the performance of the substrate material for heat transmission in breast cancer treatment.Five different types of substrates are designed with the rectangular shape of the antenna, and their performance is evaluated and compared.The best substrate, RT5880 and RO4003, has shown the best SAR distribution at 915 MHz.Based on the simulation results performed using SEMCAD X 14.8.4 software, RO3010 has shown the most undesired result since the heat that it produced is not delivered to the intended area of the malignant tissues.Alumina can penetrate deeper at 434 MHz, 915 MHz and 2450 MHz.However alumina creates massive skin burn and unwanted hotspots around healthy tissues.434 MHz is not recommended to be used as the signal operating frequency.The hyperthermia treatment execution period has shown a reasonable period, from 2.14 hrs to 3.63 hrs.In conclusion, a suitable substrate can enhance antenna performance in producing and delivering heat to the intended treated areas for breast cancer hyperthermia treatment.A suitable substrate with a suitable frequency can enhance the results.

Table 1 shows
Malignant tissues and breast fat electro-thermal properties used in this research.

Table 1 .
Malignant tissues and breast fat electro-thermal properties.