The spectrum of breast lesions in children and adolescents varies markedly from that for adults, with the former lesions being overwhelmingly benign. A breast mass in a young boy or girl may arise from normal and abnormal breast development. Other causes of masses include infection, trauma, and cyst formation. After onset of puberty, most cases of breast enlargement arise from benign fibroadenoma in girls and gynecomastia in boys. These conditions have specific imaging appearances, although juvenile (often giant) fibroadenoma cannot be distinguished from phyllodes tumor, which can be benign or malignant. In children, both conditions usually appear as well-circumscribed, hypoechoic masses at sonography and show diffuse enhancement except for nonenhancing septations at magnetic resonance imaging. A diagnosis of juvenile papillomatosis (a benign lesion) portends later development of breast cancer, and patients with this condition should be closely monitored. Malignant lesions of the breast in children are rare. The most common malignant lesions are metastases and are usually associated with widespread disease. The most common primary breast malignancy is malignant phyllodes tumor. Primary breast carcinoma is exceedingly rare in the pediatric age group, but its imaging appearance in children is the same as seen in adults and is different from that of almost all benign lesions. In girls, diagnostic interventions may injure the developing breast and cause subsequent disfigurement. Given this risk and the low prevalence of malignant disease in this population, a prudent course should be followed in the diagnosis of breast lesions. Imaging findings are very helpful for selecting patients for further diagnostic procedures. Although malignancy is rare, lesions with suspicious imaging findings or progressive growth should be subjected to cytologic or histologic examination.

Malignant primary hepatic tumors in children include lesions unique to the pediatric age group and others that are more common in adults. Important considerations when evaluating a child with a liver tumor are the age of the patient, laboratory findings, and specific imaging features. The most common primary malignant hepatic tumor in the pediatric population, hepatoblastoma occurs almost exclusively in patients younger than 5 years with no history of liver disease. Hepatoblastoma is associated with elevated serum α-fetoprotein (AFP) level and appears predominantly solid. Hepatocellular carcinoma (HCC) is the most common malignant liver tumor in older children, often with a history of liver disease. HCC is associated with elevated serum AFP level and also appears as a solid mass. Fibrolamellar carcinoma occurs in adolescents without elevated AFP level and contains a T2-hypointense fibrous scar that usually does not enhance. Undifferentiated (embryonal) sarcoma occurs in young children, contains cystic and mucoid components, and mimics a cyst at magnetic resonance imaging and computed tomography but appears solid at ultrasonography. Epithelioid hemangioendothelioma is a multifocal vascular tumor in older children with a distinctive imaging appearance of confluent peripheral nodules with adjacent capsular retraction. Angiosarcoma rarely occurs in young children and frequently shows evidence of hemorrhage. Embryonal rhabdomyosarcoma of the biliary tree is unique to children, usually under 5 years of age, and frequently demonstrates an intraductal growth pattern. Knowledge of the pathologic features of these tumors and their imaging appearances can help radiologists offer an appropriate differential diagnosis and management plan.

Many extraocular masses involving the pediatric orbit have an osseous origin. The most common is the dermoid inclusion cyst; these cystic lesions may contain lipid and are most often found near the zygomaticofrontal suture, adjacent to an indolent-appearing erosion of bone. Some primary bone lesions may involve the orbit, producing a lytic or dense lesion with enlargement of the bone; these lesions include fibrous dysplasia, juvenile ossifying fibroma, and osteosarcoma. Fibrous dysplasia tends to produce a mass of ground-glass appearance with longitudinal osseous expansion, whereas juvenile ossifying fibroma is likely to produce a mixed lytic and sclerotic lesion and focal osseous enlargement. Osteosarcoma causes marked bone destruction and variable osteoid production. Langerhans cell histiocytosis, an idiopathic reticuloendothelial proliferative disorder, tends to involve the bones of the skull, especially the lateral orbital roof; it produces lytic destruction of bone with a sclerotic rim and a large intraorbital soft-tissue mass. Granulocytic sarcoma is a solid tumor that may occur in children with myelogenous leukemia. These tumors tend to arise in the subperiosteum of the lateral orbital wall, although they usually do not disrupt the bone. Finally, the orbit is a common site for bone metastases from neuroblastoma, which cause aggressive periosteal reaction in the orbital roof or lateral wall. The last three conditions are often bilateral. At imaging evaluation, osseous lesions may appear similar to each other and to nonosseous masses of the orbit. Knowledge of the pathologic features of these tumors and how these features are reflected in their imaging appearances may help radiologists differentiate them.

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Benign hepatic tumors in children include lesions that are unique to the pediatric age group and others that are more common in adults. Infantile hemangioendothelioma, or infantile hepatic hemangioma, is a benign vascular tumor that may cause serious clinical complications. It is composed of vascular channels lined by endothelial cells. At imaging, large feeding arteries and draining veins and early, intense, peripheral nodular enhancement with centripetal filling on delayed images are characteristic features. Mesenchymal hamartoma of the liver occurs in young children and is characterized pathologically by mesenchymal proliferation with fluid-containing cysts of varying size and number. The mesenchymal component or cystic component may predominate; this predominance determines the imaging appearance of the tumor. Benign epithelial tumors that are common in adults may infrequently occur in childhood. These include focal nodular hyperplasia (FNH), hepatocellular adenoma, and nodular regenerative hyperplasia. All are composed of hyperplastic hepatocytes similar to surrounding liver parenchyma and may be difficult to discern at imaging. Preferential hepatic arterial phase enhancement helps distinguish FNH and hepatic adenoma from uninvolved liver. Hepatic adenoma often has intracellular fat and a propensity for intratumoral hemorrhage, neither of which are seen in FNH. Unlike adenoma, FNH often contains enough Kupffer cells to show uptake at sulfur colloid scintigraphy. Nodular regenerative hyperplasia is often associated with portal hypertension, which may be evident at imaging. Knowledge of how the pathologic features of these tumors affect their imaging appearances helps radiologists offer an appropriate differential diagnosis and management plan.