In recent years, in addition to primary hepatocellular carcinoma, cholangiocarcinoma, and common benign and malignant occupancies such as hepatic cysts and hepatic cavernous hemangiomas, some other primary benign and malignant tumors of the liver with lower incidence have been increasingly reported, which are clinically known as rare liver tumors. Because rare liver tumors are relatively rare, clinicians pay far less attention to them in clinical practice. Their pathological tissue types are complex, clinical symptoms are atypical, and laboratories usually lack specific indices, which makes preoperative diagnosis and differential diagnosis difficult even with the significant advances in diagnostic imaging technology today. For rare tumors of the liver, how to clearly diagnose and select appropriate treatment modalities is a topic that clinical hepatobiliary surgeons need to take seriously. This article outlines several types of rare liver tumors that are relatively common in clinical practice and discusses the key points of diagnosis and treatment according to their characteristics. The primary rare benign tumors of liver can be classified into three categories according to the embryonic origin of liver tissue: (1) epithelial tumors: hepatocellular adenoma, cholangiocellular adenoma (including cystic adenoma), mixed adenoma; (2) mesenchymal tumors: vascular smooth muscle lipoma, lymphadenoma, fibroma, lipoma, smooth muscle tumor; (3) others: inflammatory pseudotumor of liver, focal nodular hyperplasia, teratoma, malignant tumor, etc. Rare malignant tumors of the liver are divided into the following types: (1) hepatocellular tumors and adenomas of the liver: multicystic cavernous hepatocellular carcinoma, hepatoblastoma, undifferentiated hepatocellular carcinoma, endocrine hepatocellular carcinoma, laminar fibrous hepatocellular carcinoma, cystic adenocarcinoma of the liver, primary adenosquamous carcinoma of the liver; (2) angiosarcoma and myxolipoma: hepatic angioendothelial sarcoma, hepatic angiomyolipoma; (3) lymphoid tissue and plasma cell tumors: liver primary lymphoma, hepatic plasmacytoma; (4) myxosarcoma and osteosarcoma: smooth muscle sarcoma of the liver, primary rhabdomyosarcoma of the liver, osteosarcoma of the liver; (5) fibrous, lipid and neurological tumors: malignant fibrous histiocytoma of the liver, primary mucinous liposarcoma of the liver, medullary lipoma of the liver, malignant nerve sheath tumor of the liver; (6) tumors associated with the female reproductive system: primary malignant trophoblastic tumor of the liver, primary yolk (7) other tumors: primary multiple carcinoid tumors of the liver, primary squamous cell carcinoma of the liver, primary melanoma of the liver, undifferentiated hepatic sarcoma, primary APUD tumor of the liver, and hepatic sarcoma. The diagnosis of rare tumors of the liver and the choice of treatment modalities 1. adenoma of liver is an extremely rare benign tumor, which can be classified as hepatocellular, cholangiocellular, and mixed according to the cellular origin. (1) Cholangioadenoma can be subdivided into two types: ductal adenoma and cystic adenoma. The latter is rare and often multi-housed, and the adenoma may be single or multiple, some of which are accompanied by a mesenchymal stroma similar to that of the ovary. It is important to differentiate them from cystic adenomas. Because of the fear of tumor dissemination, puncture for diagnostic purposes is contraindicated. In addition, even in cystic adenomas, CA19-9 values are elevated, which should be noted when differentiating from adenocarcinoma. The differentiation from cystadenocarcinoma is difficult when the wall of the cyst is partially hypertrophied and papillary bulge is present. Cystadenoma itself has the potential to become cancerous, and in principle, liver resection including cyst should be performed. (2) Hepatocellular adenoma (hepatocellular adenoma) is a rare benign tumor, mostly seen in middle-aged women, with an age of onset between 15-45 years, mostly between 20-39 years. The pathogenesis is unknown and there is a history of long-term oral contraceptive use. However, experimental evidence suggests that sex hormones play only a promotional role. Most hepatocellular adenomas are isolated nodules with a spherical shape that expands toward the liver surface. The tumor is often 5-375px in diameter at the time of detection. There is often no envelope, often with areas of necrosis and hemorrhage, or scarring after necrosis. Hepatocellular adenoma is an incidental finding in 5-10% of patients, 20%-25% have chronic or mild episodes of abdominal pain, and 30%-40% have acute abdominal pain, of which 30% are intra-tumor bleeding and 70% are intra-abdominal bleeding. Intra-abdominal bleeding is the most serious complication and requires emergency surgical management. In general, hepatocellular adenoma is rarely malignant, but preoperative diagnosis is difficult, especially the differential diagnosis with highly differentiated hepatocellular carcinoma. However, CT examination can reveal a low-density ring around the tumor, which is more clear on enhanced scans, and this can be a relatively specific sign. Since hepatic adenomas are at risk of rupture and bleeding, and some of them may become cancerous, they should be actively resected after biopsy to confirm the diagnosis. Focal nodular hyperplasia (FNH) is a disease that occurs in normal liver tissue, as is hepatocellular adenoma. The pathogenesis is unclear, but has been suggested to be related to oral contraceptive use and hormones. Most patients with FNH have no significant clinical symptoms or background of hepatitis. The preoperative diagnosis of FNH is difficult, especially when differentiated from hepatocellular adenoma, hepatic hemangioma, and highly differentiated hepatocellular carcinoma, etc. Most patients have a characteristic coarse central vasculature with fast flow velocity and low resistance coefficient as seen on ultrasound. MRI has higher sensitivity and specificity compared to ultrasound and CT, with typical FNH showing equal or low signal in T1WI and high or slightly high signal in T2WI, and the detection rate of central scar in lesions over 3 cm is 50%-70%. FNH differs from hepatocellular adenoma in that complications such as hemorrhage are rare, asymptomatic cases are common, and there is no possibility of malignancy. If the imaging diagnosis is clear, combined with no history of hepatitis and normal tumor markers AFP, CEA, CA19-9, and no obvious symptoms and complications, the disease can be observed. However, if the lesion is too small to be distinguished from malignant disease, resection should be considered, which can remove the lesion and clarify the diagnosis. Hepatic angiomyolipoma HAML is a benign mesenchymal tumor occurring in normal tissues. Histologically, it is composed of three components, including proliferating blood vessels, smooth muscle cells and adipocytes, in different proportions. The proportion of tumor components varies greatly, and it is difficult to distinguish it from benign and malignant tumors such as hepatocellular carcinoma, hepatic hemangioma, hepatic lipoma, hepatic sarcoma or hepatic adenoma before surgery. HAML is classified into 4 types according to the ratio of the 3 components: mixed type, hemangioma type, smooth muscle type, and lipoma type. The differentiation from hepatic lipoma is that CT scans suggest that hepatic lipoma is entirely fatty in density and no vascular shadow is seen on enhanced sweep, whereas HAML is only partially fatty tissue and malformed vessels are seen on enhancement. Most investigators advocate early surgery for HAML, with local excision of the entire tumor and the surrounding liver to avoid residual tumor tissue and recurrence. The operation should be performed gently to prevent fat embolism caused by excessive compression of the tumor into the bloodstream. It is generally believed that HAML is a benign tumor and can be cured by surgical resection, but there are reports of tumor recurrence, invasion and metastasis at home and abroad. Therefore, we should pay attention to the follow-up after surgery to determine the possibility of recurrence and malignant transformation. 4.epithelioid hemangiomaendothelioma is a slow-growing, low-grade malignant tumor between hemangioma and angiosarcoma. The age of onset of the disease ranges from 12 to 86 years, with an average of about 50 years, and 2/3 of patients are female. The clinical manifestations of the disease are mainly poor nasal function, abdominal pain, fever, weight loss, a few with jaundice, and rarely complicated by Budd-Chiari syndrome in the advanced stage, with multiple hypodense areas with moderate peripheral vascular enhancement and multiple hypovascular spots in the tumor. Angiography shows multiple oligovascular masses in the liver. The prognosis of this disease is currently uncertain, but better than that of primary liver cancer, as some patients survive for a long time without treatment, while others have rapidly developing tumors and die within a few months. Most of the tumors in the liver are multifocal, so there is little chance of surgical resection. One patient has survived 15 years after routine liver transplantation, so liver transplantation is considered a good indication for this disease. 5.Hepatic malformation tumor A hepatic malformation tumor (hamartoma) is a tumor-like malformation caused by the developmental misalignment of normal tissues in an organ of the body. Depending on the microscopic origin of the tumor tissue, there are four major types of malformations: endodermal, mesodermal, endo- and meso-epodermal, and mixed malformations. The most common types of malformations are mixed malformations and endodermal malformations. (1) Mixed malformation tumors are mainly mesenchymal hamartoma of liver (MHL). Mesenchymal hamartoma is seen in childhood and accounts for 1/4 of the tumors in this age group. The tumors are usually large, non-invasive in the liver, and can be observed if diagnosed, and surgical resection can be considered when symptoms are present. (2) Substantial endodermal malformation tumors are divided into hepatocyte-based malformation tumors and bile duct-based malformation tumors, with the latter being the most common. Biliary ductal malformation tumors are granular nodules with a diameter of 1-5 mm, mostly found in the adult liver, and can be disseminated or multiple, with polycystic liver and kidney. In principle, biliary ductal malformations are benign tumors, but there are cases in China where untreated hepatic mesenchymal malformations have been reported as undifferentiated embryonal angiosarcoma. It is necessary to differentiate it from metastatic hepatocellular carcinoma, hepatocellular carcinoma and cystic adenocarcinoma and follow up carefully. 6. hepatic lipoma Hepatic lipoma is a rare benign mesenchymal tumor of the liver. 11 cases of surgically resected lipoma were reported in our hospital, among which 2 cases were combined with HBV positivity, tumor markers AFP, CEA and CA19-9 were negative, and only 3 cases were diagnosed preoperatively. The rest were diagnosed as primary hepatocellular carcinoma, hepatic hemangioma, metastatic hepatocellular carcinoma and focal fatty infiltration. Hepatic lipomas are rare, and lack of recognition can lead to preoperative misdiagnosis, and should be differentiated from tumors containing fatty lesions and those without. On imaging, ultrasound shows strong echogenicity with clear margins, regular morphology and homogeneous or heterogeneous internal echogenicity, while CT shows clear margins, round or ovoid hypodense areas with homogeneous or heterogeneous density, and CT values of -90 to +24. MRI shows clear margins, high signal areas in T1WI and T2WI, and slightly lower signal than normal fatty tissue in T1WI. Treatment is mainly surgical local excision. Smaller diagnosed lipomas can be temporarily observed and then surgically treated if there is significant increase in size. 7.hepatic sarcoma Primary hepatic sarcoma (hepatosarcoma) is rare and occurs in men, mostly in children and the elderly. The onset in children may be congenital or related to abnormal development of embryonic connective tissue; 1/3 of adult patients may be complicated by cirrhosis. Fibrosarcoma is the most common pathological type, while other types include smooth muscle sarcoma and lymphosarcoma. The disease can be distinguished from serum fetoprotein-negative primary hepatocellular carcinoma only by histological examination. The clinical symptoms include fever and abdominal mass, and the course of the disease develops sinisterly and rapidly, and most of them die less than a year after diagnosis. 8.Hepatic hemangioendothelioma (hemangiomaendothelioma of the liver), also known as angiosarcoma, is a primary malignant tumor of the liver caused by the heterogeneous proliferation of hepatic sinusoidal cells, which is less common and highly malignant. It is mostly congenital, commonly seen in infants and occasionally in the elderly. The disease has been reported to be associated with thorium dioxide, arsenic or vinyl chloride. The patient’s liver may rapidly enlarge and become cystic in nature with abdominal distention, abdominal pain, anorexia and anemia. Rupture of the tumor results in bloody ascites causing signs and symptoms of acute abdomen. Hepatic hemangiosarcoma is highly malignant and has a short course with a survival period of only a few months, so it should be surgically removed as much as possible after diagnosis to improve the survival rate. Chemotherapy may prolong the survival of patients. In conclusion, with the increasing clinical experience and the improvement of imaging technology, most liver tumors can be diagnosed more clearly before surgery. However, rare liver tumors are still difficult to be diagnosed preoperatively because of their rarity, mainly because of the lack of awareness and the lack of specific changes in clinical manifestations, laboratory tests, and imaging examinations of tumors. Different diseases can have similar CT and MRI manifestations, and there are many factors affecting the diagnosis. Therefore, to improve the diagnosis and treatment of rare liver tumors, firstly, we need to raise awareness and think of the possibility of rare liver tumors when we encounter difficult cases; secondly, we need to accumulate more cases and pay attention to the summary. Avoiding overtreatment of benign lesions and, at the same time, developing targeted treatment strategies for malignant tumors will ultimately improve patient prognosis.