Focal nodular hyperplasia (FNH) FNH is the second most common benign liver tumor after hepatic hemangioma, with a prevalence of 0.9% in women of childbearing age and a male-to-female ratio of 1:8 to 10. The etiology of FNH is not known, but is thought to be related to the proliferative reaction of hepatocytes induced by congenital vascular malformations, and has also been reported after chemotherapy and hematopoietic stem cell transplantation. The cause of FNH is unknown, and most of them are believed to be related to the proliferative response of hepatocytes caused by congenital vascular malformations, and there are reports that FNH occurs after chemotherapy, radiotherapy and hematopoietic stem cell transplantation.FNH is often a single, non-enveloped, substantial lesion; about 20% to 30% of the patients present with 2-5 lesions, but few patients have more than 5 lesions. On gross pathology, there is often a characteristic central fibrous scar radiating deep into the hepatic parenchyma, and the typical histologic picture is characterized by the irregular arrangement of normal hepatocytes in nodules, fibrous connective tissue and small blood vessel formations, and dysplastic macrovessels, ductal hyperplasia, and inflammatory cells seen in the septa. The majority of FNH is asymptomatic and is detected accidentally during ultrasonography, while a few patients have abdominal symptoms due to the large size of the mass. On ultrasonography, most FNH are isoechoic compared with normal liver tissue, and a few are slightly hypoechoic or hyperechoic. Color Doppler ultrasound showed that more than 90% of FNH had abundant internal blood flow, and 50% to 70% of the lesions showed “star-shaped color flow” in the center of the lesion, and the lesion could often be surrounded by thick arteries supplying blood; during ultrasonography, about 60% of the cases showed “radial” or “stellate” blood flow in the arterial phase. In about 60% of cases, the arterial phase shows “radial” or “stellate” internal vascularization, with contrast filling in centrally from the central feeding artery to the periphery in a centrifugal fashion. It is hyperechoic or isoechoic in the portal phase. The central scar is hypoechoic in both the arterial and portal phases.The CT scan is a hypointense mass, and the lesion rapidly enhances in the arterial phase and begins to diminish in the portal phase of enhanced CT, and is isointense in the delayed phase when compared with normal liver tissue.MRI is an effective method of detecting FNH, with a T1-weighted iso-signal or mildly hyposignal, and a T2-weighted iso-signal or mildly hyper-signal.The sensitivity of MRI for detecting FNH is 70%, and the specificity is 70%, and the sensitivity is 70%, and the specificity is 70%. The sensitivity of MRI for FNH is 70% and the specificity is 90%. For some FNH lesions with a diameter of less than 3 cm, imaging is often difficult, and ultrasound or CT-guided fine-needle aspiration biopsy can be helpful for diagnosis. Studies have shown that FNH is not malignant, and the majority of lesions do not progress over a long period of time, and even a few lesions disappear naturally. Therefore, no treatment is needed for diagnosed and asymptomatic lesions, and an unclear diagnosis and the presence of symptoms are the indications for surgery for FNH. Because FNH is often surrounded by large blood vessels, it should be properly managed intraoperatively.