# Comparative Study of Minimally Invasive Microwave Ablation Applicators
## 4 Discussion
However, there is a notable lack of control over temperature distribution (examine the temperature contours at 38 \({}^{\circ}\)C and 42 \({}^{\circ}\)C), which becomes particularly critical when the tumor is located near sensitive biological structures, such as blood vessels. This issue is partially mitigated by the balun-free, base-fed monopole antenna and fully addressed only by the helical open waveguide structure. As shown in Table 4, the 38 \({}^{\circ}\)C penetration zone exceeds the tumor dimensions by less than 1 cm in both diameter and length only for the helical open waveguide structure. This, combined with the safety margin of approximately 1 cm, makes this applicator the most secure and ideal choice for tumors located near vulnerable biological tissues. The main drawback of this helical open waveguide is its challenging construction and placement. Our group is brainstorming on this issue, where a convenient and practical solution is found that will be presented in a follow-up paper. In any case, other researchers are welcome to propose appropriate mechanical solutions. Additionally, precise power calibration is essential to achieve spherical ablation zones; deviations in power can lead to elliptical ablation zones. Accurate applicator positioning is also critical, as the design directly affects its efficacy in ablating the tumor. The applicator must be inserted into the tumor before the microwave ablation (MWA) process begins, which may risk the dissemination of tumor cells. Positioned at the tumor periphery, the applicator initiates ablation immediately upon insertion, a critical step to prevent metastasis that can occur if cancer cells enter the bloodstream during treatment. The helical open waveguide structure, for instance, could address this concern by initiating MWA immediately upon insertion, thereby reducing the risk of tumor cell dissemination and enhancing treatment outcomes. In the helical open waveguide design, spirals serve as a protective shield, enabling precise control of the electromagnetic field and allowing flexibility in adjusting the shielding based on the clinical context. For example, if the tumor is adjacent to sensitive biological tissues and no safety margin exists, a stronger shield can be applied. Conversely, if a safety margin is present, the shielding can be adjusted to a lower intensity. As MWA is an image-guided treatment modality, the helical open waveguide design further enhances precision by encapsulating the target tumor with minimal operator error when guided by imaging techniques, thus increasing the safety of the procedure. However, given that MWA is a minimally invasive treatment, the size of the helical open waveguide is a primary limitation compared to existing applicators and other designs analyzed here. Additionally, it appears to create a smaller ablation zone than other designs. However, the ablation zone coincides with the tumor volume; thus, different sizes can be utilized in each case so as to exactly fit each tumor. It is important to note that several designs in this analysis--including the capacitive capped monopole antenna, dual-slot antenna (as referenced in [45]), and helical open waveguide--have not yet been physically constructed. While this may limit direct experimental validation, the computational model provides an initial assessment of each design's theoretical performance relative to existing designs. Moreover, it is important to address the management and control of maximum temperature during therapy. The simulator utilizes the point-wise Specific Absorption Rate (SAR) as the thermal source point, which may occasionally lead to an overestimation of maximum temperature levels. Despite this, it is essential to continuously monitor temperature throughout the therapy to ensure it remains within a safe range. Temperature control can be achieved using sensors that provide real-time temperature measurements. Additionally, regulating the microwave source power or employing cooling mechanisms can help maintain safe temperature levels.