Application of trilinear localization method in ablative treatment of hepatocellular carcinoma that cannot be shown by CT plain scan. No matter CT-guided or B-ultrasound-guided ablation therapy, the premise is that the tumor can be clearly displayed. If CT scan cannot be displayed according to the traditional view, then CT-guided puncture and ablation cannot be performed, and many tumors will lose the opportunity of ablation treatment. In 2005, the authors started to use the trilinear localization method to accurately determine the location of tumor through three mutually perpendicular path lines, so as to accurately puncture the tumor that cannot be displayed under CT scan, and introduced the application of trilinear localization method in the ablation treatment of liver cancer that cannot be displayed under CT scan for the first time in the First Shanghai Radiofrequency Ablation Workshop in 2012. 1.Distance from median line to determine left and right: draw a line perpendicular to the examination bed surface at the spinous process, then measure the distance from the center of the lesion to the line, and determine the left and right position of the lesion by this distance. 2.Distance from the anterior edge of the vertebral body to determine the front and rear: draw a line parallel to the examination bed at the anterior edge of the vertebral body, then measure the distance between the center of the lesion and the line, and determine the front and rear position of the lesion by this distance. 3. The distance from the portal vein determines the upper and lower: measure the vertical distance of the center of the lesion from the right or left branch of the portal vein, and use this distance to determine the upper and lower position of the lesion. For postoperative patients, the distance between the center of the lesion and a certain titanium tongs can be taken to determine the up and down position of the lesion. For lesions after TACE, the up and down position of the lesion can also be determined by the distance between the center of the lesion and iodine oil. Some patients can also determine the upper and lower position of the lesion by using the center of the lesion and the top of the diaphragm, the hepatic vein, the gallbladder, and the pericardium as a reference. Let’s see the practical application of the trilinear localization method through a case. Case: Elderly female with small hepatocellular carcinoma of the right liver and complete tumor inactivation after radiofrequency ablation. The patient was an elderly female with a small nodular lesion in the right liver, which could not be shown on CT scan. The distance between the center of the lesion and the median line, the distance between the horizontal line and the anterior edge of the vertebral body and the distance between the center of the lesion and the right branch of the portal vein were measured on CT-enhanced images (Figure A). The distance between the ablation electrode and the median line, the line of the anterior border of the vertebral body and the right branch of the portal vein were measured, and the ablation treatment was performed after confirming that the ablation range of the tumor could cover the lesion (Figure B-C). 【Review】: When using the trilinear localization method for measurement, attention should be paid to whether the patient’s body position corresponds to the preoperative CT film, and appropriate correction should be made if the body position is inconsistent. Secondly, for lesions that cannot be displayed under CT plain scan, a larger ablation range should be tried to avoid the situation that the tumor is not completely ablated due to insufficient ablation range. And for tumors in simpler sites the approximate location of the tumor can be determined by measuring only the distance between the lesion and the median line.