 Introduction/Background Incidental liver masses are commonly discovered on imaging performed for other indications. Because the prevalence of benign focal liver lesions in adults is high, with at least one lesion seen in up to 15% of patients, accurate characterization of incidentally detected lesions is an important objective of diagnostic imaging [1]. Benign lesions are very common in the liver, and even in patients with primary malignancy, benign lesions unrelated to the known malignancy can be found in nearly 30% of patients [2]. Common benign liver masses include cysts, hemangiomas, and focal nodular hyperplasia (FNH). Common malignant tumors include metastases and hepatocellular carcinomas (HCCs). Less common liver masses include hepatocellular adenoma, intrahepatic cholangiocarcinoma, fibrolamellar HCC, biliary cystadenoma and cystadenocarcinoma, lymphoma, stromal tumors, and a variety of sarcomas. On occasion, benign lesions and pseudolesions may mimic liver tumors. These mimics include focal fat deposition or sparing, intrahepatic vascular shunts, transient hepatic attenuation/intensity difference, abscess, hematoma, and peliosis hepatis. Patients with cirrhosis are a special patient population in whom certain benign (regenerating nodules), premalignant (dysplastic nodules), malignant (HCC), and nontumorous (confluent hepatic fibrosis) masses as well as pseudolesions (vascular shunts) are more prevalent than in the general population [3]. For each of the variants in this document, it is assumed that an imaging study has identified a lesion that was not fully characterized by the study that detected it. Prior imaging studies may include ultrasonography (US) with color- flow evaluation, noncontrast or contrast-enhanced multidetector helical CT, or noncontrast or contrast-enhanced MRI.
Liver Lesion Initial Characterization.  Introduction/Background Incidental liver masses are commonly discovered on imaging performed for other indications. Because the prevalence of benign focal liver lesions in adults is high, with at least one lesion seen in up to 15% of patients, accurate characterization of incidentally detected lesions is an important objective of diagnostic imaging [1]. Benign lesions are very common in the liver, and even in patients with primary malignancy, benign lesions unrelated to the known malignancy can be found in nearly 30% of patients [2]. Common benign liver masses include cysts, hemangiomas, and focal nodular hyperplasia (FNH). Common malignant tumors include metastases and hepatocellular carcinomas (HCCs). Less common liver masses include hepatocellular adenoma, intrahepatic cholangiocarcinoma, fibrolamellar HCC, biliary cystadenoma and cystadenocarcinoma, lymphoma, stromal tumors, and a variety of sarcomas. On occasion, benign lesions and pseudolesions may mimic liver tumors. These mimics include focal fat deposition or sparing, intrahepatic vascular shunts, transient hepatic attenuation/intensity difference, abscess, hematoma, and peliosis hepatis. Patients with cirrhosis are a special patient population in whom certain benign (regenerating nodules), premalignant (dysplastic nodules), malignant (HCC), and nontumorous (confluent hepatic fibrosis) masses as well as pseudolesions (vascular shunts) are more prevalent than in the general population [3]. For each of the variants in this document, it is assumed that an imaging study has identified a lesion that was not fully characterized by the study that detected it. Prior imaging studies may include ultrasonography (US) with color- flow evaluation, noncontrast or contrast-enhanced multidetector helical CT, or noncontrast or contrast-enhanced MRI.