The investigators intend to perform RFA therapy on HCC less than 4 cm in size using octopus electrode, double alternating unipolar high-frequency transmission mode and gradual high-frequency energy loading mode, and to find out the therapeutic results. The primary evaluation variable is the ideal technical success rate for securing a safety margin of 5 mm or more around the tumor on the CT obtained immediately after the procedure, and the secondary evaluation variable is the volume of the cauterization lesion per unit time measured on the CT obtained immediately after the procedure, The local tumor recurrence rate, survival rate and disease-free survival rate of 12 months after the procedure, the presence or absence of multiple recurrences within the same segment, the actual procedure time, and the incidence of complications associated with the procedure are examined. The performance of RFA therapy of HCC using the Octopus electrode and gradual high-frequency energy transfer mode is compared with that of the existing Octopus electrode and RFA therapy using the basic maximum high-frequency energy transfer mode using historic cohort.

1. Pre-treatment planning:

   The detailed plan of radio frequency ablation (RFA) will be implemented in accordance with the routine procedure of image evaluation prior to high frequency implementation. In other words, the location of the tumor on the Multiphasic CT or MRI and the RFA planning program at the 3D workstation (Philips, Intraportal) before RFA procedure, evaluate the tumor volume and the relationship with adjacent blood vessels, and insert the electrode for RFA After planning the number of ablation and the like, the location of the tumor in the ultrasound image was found in the pre-surgery image by fusion of the pre-surgery image and the ultrasound image using US-CT-MR fusion and contrast-enhanced ultrasonography, which are recently used. It will evaluate whether the tumors match, and also evaluate whether the distribution of the echo bubble and the tumor position coincide during the procedure in real time.

   In addition, the degree of stiffness of the parenchyma will be measured by using an ultrasound elastic technique. At this time, the ultrasonic fusion device to be used will be one of Phillips, GE, or one of the navigation systems of Siemens, Canon, and Samsung, available here. The fused image guides the location of the electrodes to be installed in the tumor, the number of electrodes, and a safe access route. If the extent of the tumor is not clearly visible, or the image before the procedure is too old (> 6 weeks), low kVp or DECT will be conducted to evaluate the tumor's extent more accurately.

2. RFA procedure As the equipment of RFA, the multi-VIVA generator and octopus electrode used herein will be used. In the method of treatment, three high-frequency electrode needles were placed on the tumor according to the clinical need under the guidance of fusion ultrasound, and then the electrodes were cooled with saline, and the two electrodes were simultaneously applied with high-frequency waves for approximately 6-30 minutes depending on the size of the tumor. The temperature is maintained at 90-100 degrees Celsius. At this time, the high-frequency energy is increased by 10W at 30W intervals at 60Watt for the first 3 minutes, and then gradually increased by 10W per minute after 100W.

The location of the tumor on the ultrasound image by fusion of the ultrasound image with the pre-procedure using the US-CT-MR fusion tool (Navigator- GE, PercuNav-Phillips, Canon, S-fusion: Samsung), which is frequently used in RFA procedures. It will evaluate whether the tumors found in the images before the procedure match, and also evaluate whether the distribution of the echo bubble and the tumor location coincide during the procedure in real time. At this time, the navigation systems used by Samsung, Phillips, GE, and Canon will be used as the ultrasonic fusion device to be used. In the Fusion US image, the Octopus electrode is safely placed in the tumor through the access path of the electrode already planned. Thereafter, while transmitting high-frequency energy, air bubbles are generated in the tumor on the ultrasound image, and an echogenic cloud is formed while the procedure is stopped when the echogenic cloud becomes larger than 5 mm than the tumor.

Inclusion Criteria:

• agree to the protocol's requirements and submit a consent form
• 20 years old-85 years old
• Child-Pugh Class A
• among patients with chronic hepatitis or cirrhosis, MDCT or MRI hepatocellular carcinoma of less than 4 cm in size was suspected, and was referred to the radiology department considering clinical RFA therapy as a clinical judgment.

Exclusion Criteria:

• when the number of malignant HCC is 3 or more
• if the tumor has a maximum size of more than 4 cm
• diffuse infiltrative HCC
• Child-Pugh class B or C
• If there is an invasion of liver vessels due to malignant HCC
• severe coagulopathy
• multiple distant metastasis
• situations where it is very unlikely to obtain appropriate data for research purposes