## 2 Minimally Invasive Therapies for HCC and Their Tissular Effects
### Thermal Ablative Therapies
#### 2.1.2 Microwave Ablation  
Microwave ablation (MWA) was applied in HCC patients with tumors smaller than 5 cm, obtaining complete nodule ablation in around 90% of cases with a 3-year recurrence-free period in around 30% of cases [34; 35; 36]; the method was also applied in patients with tumors larger than 5 cm, with good response rates [36; 37]. MWA uses electromagnetic energy transmitting microwaves into the tissues at frequencies in the range of 300 MHz to 10 GHz, commonly at 915 MHz and 2.45 GHz [22; 23]. No current is transmitted through the patient as the energy is focused through dedicated antennas. The passing of the microwaves causes friction of water and nearby molecules, generating heat in the process. Particular to this method is that the heat is instantaneous and homogeneous and is produced in the entire area where microwaves are conveyed (\"near field\") [23; 38]. Therefore, local factors are less likely to influence the area and effectiveness of the MWA.  
Studies on animal models and human liver explants have shown that MWA induces histologic changes consisting of three concentric areas: central necrosis surrounded by a thin brown capsule of cells with damaged membranes and a peripheric hemorrhagic  
Figure 2: MRI examination with hepatospecific intravenous contrast of a patient with HCC in liver segment VIII, 5 months after RFA. Axial plane images: (**a**)—T1 weighted image with fat suppression; (**b**)—subtraction image (obtained by subtracting a T1 fat saturated image without contrast enhancement from a T1 fat saturated image with contrast enhancement in the arterial phase; (**c**)—T1 WI with fat saturation in the hepatobiliary phase (obtained 20 min after contrast injection); (**d**)—T2 weighted image with fat saturation; (**e**)—diffusion weighted image; (**f**)—apparent diffusion coefficient map). There is an area of coagulative necrosis hyperintense on T1 fat-saturated images (arrow, (**a**,**b**)), hypointense on T2 fat sat (**d**), without restricted diffusion on DWI/ADC (**e**,**f**), non-enhancing in the arterial phase ((**b**), subtraction) or hepatobiliary phase (**c**). To better depict contrast enhancement in high-intensity T1 fat-saturated lesions, subtraction is a necessary tool. Also, there is capsular retraction on the needle path (chevron in (**a**)).  
rim [39; 40]. Within the central area of necrosis, gas bubbles and cavitation may be observed due to vaporization [41]. As also observed in RFA, blood clots and endothelial injuries may be seen in small and large vessels in the ablated area [40; 42]. In the following hours, the ablation area appears to expand and reaches a maximum diameter at 12 h after the procedure, a consequence of the cellular and DNA damage in the margins of the treated zone [40; 43]. A fibrous capsule develops around one week after MWA and resorption in the necrosis area causes the lesion to significantly decrease in size after several weeks [43] (Figure 3).  
Technological advances in the devices and antennas allowed for improved success rates in treating HCC nodules; however, since vessels with diameters up to 6-7 mm are coagulated, vessel proximity remains a challenge in applying this therapeutic method [40; 44].