| | Imaging of paediatric mediastinal abnormalitiesAbstract The mediastinum in children can be a difficult area to assess on the chest radiograph: even the normal thymus can give the impression of a mediastinal mass lesion. When there is suspicion of a mediastinal mass, its location within the mediastinum helps to limit the differential diagnosis. Further imaging with ultrasound, computed tomography and magnetic resonance imaging helps to characterise the lesion, define its extent and detect complications. Abnormal mediastinal contours may also be caused by congenital anomalies of the mediastinal vessels, which can be demonstrated non-invasively using magnetic resonance imaging. The main purpose of this article is to review the anatomy of the mediastinum and imaging of mediastinal mass lesions in children together with some vascular anomalies that can simulate a mediastinal mass.
Keywords:
anterior, middle and posterior mediastinum,
lymphoma,
thymus,
germ cell tumour,
mesenchymal tumour,
hiatus hernia,
achalasia,
duplication cyst,
right aortic arch,
double aortic arch,
persistent left superior vena cava,
neurogenic tumour,
diaphragmatic hernia,
pneumomediastinum
INTRODUCTION  The chest radiograph (CXR) is the most commonly requested paediatric imaging procedure. Evaluation of the mediastinum on a child’s CXR may, however, be difficult as the proportionally large thymus can give the impression of cardiomegaly or a mediastinal mass. Abnormal mediastinal contours may raise the suspicion of underlying congenital heart disease or a mediastinal mass lesion. NORMAL MEDIASTINAL ANATOMY AND APPEARANCE The mediastinum extends from the thoracic inlet superiorly to the diaphragm inferiorly and is bounded anteriorly by the sternum, posteriorly by the spine and laterally by the parietal pleura. The anatomical location of a mass within the mediastinum is important in limiting the differential diagnosis so it is useful to divide the mediastinum into compartments, each containing specific structures. Radiologically, the mediastinum can be divided into anterior, middle and posterior compartments each extending from the clavicles to the diaphragm. The mediastinum can also be divided into superior and inferior compartments. The superior mediastinum is located above an arbitrary line extending from the sternal angle anteriorly to the T4/5 interspace posteriorly, with the inferior mediastinum below this. A lateral CXR will demonstrate the boundaries of the anterior, middle and posterior mediastinum (Fig. 1). The anterior (pre-vascular) mediastinum contains the thymus and anterior lymph nodes. The middle mediastinum contains the heart and great vessels, lymph nodes, tracheobronchial tree and oesophagus, whereas the posterior mediastinum contains the descending aorta, sympathetic nerve chain, azygos and hemiazygos veins, intercostal nerves and posterior lymph node groups. In addition to these structures, each compartment contains various mesenchymal tissues, nerves and angiomatous structures from which mediastinal masses may arise.1., 2. The size and shape of the normal thymus in childhood is directly related to age. Although it is unusual to visualise the thymus on a CXR in children over the age of 6 years, the thymus is detected on computed tomography (CT) and magnetic resonance imaging (MRI) at all ages throughout childhood. The thymus is most prominent in infancy when it is at its largest size relative to the remainder of the mediastinum. Thymic weight is greatest at puberty, after which normal involution occurs, fatty infiltration of the gland occurring in the late teenage years and early twenties. Under the age of 5 years, the thymus is quadrilateral, with smooth biconvex lateral margins and a wide retrosternal component. The lateral border may be ‘sail’ shaped or have a wavy outline as a result of indentation of the soft gland by the anterior ribs (Fig. 2). With increasing age, there is craniocaudal growth (with little change in transverse diameter) and a decrease in thickness of the gland. The thymus usually becomes more triangular in shape, with straightening or concavity of the lateral borders, although a quadrilateral shape can occasionally persist. The gland may diminish in size with disease and stress (because of the effect of endogenous corticosteroids) and enlarge to an even greater size upon recovery, known as the rebound phenomenon. The normal thymus is homogeneous on ultrasound (US), CT and MRI throughout childhood and does not normally infiltrate or exert a mass effect.1., 2. MEDIASTINAL MASSES The mediastinum is the most common location of thoracic masses in children,2 these often being asymptomatic and detected incidentally. Patients may present with secondary compressive effects of the mass on adjacent structures within the mediastinum or may have systemic signs but no focal chest signs on examination. The CXR is usually the primary investigation and once a mediastinal abnormality has been detected or suspected, further imaging such as a chest US, CT or MRI scan, may be undertaken. This confirms the location and extent of the lesion and shows internal characteristics of the mass, such as fat content, calcification and a cystic or solid nature, which are useful in determining the correct diagnosis. The choice of further imaging will depend largely on the location of the mediastinal abnormality, on the likely diagnosis and, to a certain extent, on availability and expertise. ANTERIOR MEDIASTINAL MASSES Lymphoma Lymphomas are the most common cause of a mediastinal mass in the paediatric age group, accounting for 46–56% of all mediastinal masses.3 Non-Hodgkin’s lymphoma and Hodgkin’s disease are the two major types of lymphoma. The incidence of non-Hodgkin’s lymphoma increases greatly over the first 5 years of life and then continues to rise steadily throughout life. Hodgkin’s disease is rare in children less than 5 years old but its incidence increases in later childhood and in the teenage years. Paediatric non-Hodgkin’s lymphoma usually presents as abdominal, thoracic or head and neck masses. Paediatric Hodgkin’s disease is more likely to present as asymptomatic cervical or supraclavicular lymphadenopathy, systemic symptoms being more common with Hodgkin’s disease. Between 50% and 70% of children with lymphoblastic lymphoma present with an anterior mediastinal or intrathoracic mass. Over one third of non-Hodgkin’s lymphomas have their primary site in the mediastinum and overall two thirds of patients with Hodgkin’s disease will have mediastinal lymphadenopathy.1., 3. The initial CXR is frequently diagnostic in malignant lymphoma and usually shows a bulky anterior mediastinal mass that may extend into the middle or posterior mediastinum (Fig. 3). Calcification can be seen at presentation but is usually associated with previous radiotherapy or chemotherapy. Contrast-enhanced CT is the next investigation of choice which will define the extent of the disease. The scan should extend from the skull base to include the neck. Lymphoma cells infiltrate the thymus, causing enlargement and lobular lateral borders. The involved thymus is frequently heterogeneous and there may be necrosis manifest as areas of fluid attenuation within the gland.4 Thymic involvement is almost always accompanied by involvement of the mediastinal lymph nodes. CT scanning will also detect lung parenchymal involvement, pleural effusions, chest wall disease and complications caused by impingement on vital structures, most commonly the airway (Fig. 4). Symptoms of airway compromise range from mild cough to life-threatening tracheal compression. Superior vena cava obstruction can also be a presenting feature with a large mediastinal mass.1., 3. CT scanning is used to monitor treatment and detect disease progression following the diagnosis of lymphoma. MRI is complementary but not routinely used at diagnosis because of problems with breathing and cardiac motion artefact; MRI is also not able to assess the pulmonary parenchyma. With successful treatment, the mediastinum usually returns to normal size, but there can be residual thymic or lymph node enlargement following treatment. Signal characteristics on MRI may help to differentiate active disease from fibrotic tissue but cannot be completely accurate.5 Rebound enlargement of the thymus often occurs following treatment. In this phenomenon, the thymus is usually homogeneously enlarged without associated lymphadenopathy. Thymic enlargement and thymic tumours Normal thymic tissue is occasionally found ectopically in the neck or away from its usual anterior mediastinal position in the middle or posterior mediastinum. This aberrant thymic tissue is usually (but not always) in continuity with the normally positioned thymus. It has normal internal characteristics and shows the same uniform mild contrast enhancement as the normally positioned thymus. Thymic hyperplasia can occur with myasthenia gravis, Graves’ disease and other auto-immune diseases but is frequently idiopathic. The gland increases in size and weight but remains architecturally normal. Thymic cysts are rare and usually asymptomatic but can be complicated by haemorrhage causing sudden enlargement, which may compress the airway, leading to acute respiratory distress. Imaging studies show thymic cysts to be uni- or multilocular and contain fluid.6 Although thymomas account for 10% of anterior mediastinal masses in adults, they are rare in children. Thymomas in childhood are also associated with auto-immune disease, and approximately one quarter are malignant. Radiologically, thymomas may contain focal dense, irregular or coarse calcification.4 Increasing size, invasion of adjacent mediastinal structures and associated pleural effusions are indicators of malignancy but a differentiation between benign and malignant lesions cannot be made with certainty on imaging.1 Germ cell tumours of the mediastinum Germ cell tumours occur in various sites within the body, usually in the midline or in para-axial locations. They are believed to derive from multipotential cells that arise from an early event in embryogenesis. Because of their multipotential nature, different tissues may be found within the mass, and these may be well differentiated or immature. Germ cell tumours represent the third most common mediastinal mass after lymphoma and neurogenic tumours, accounting for 6–18% of paediatric mediastinal tumours.6 The majority arise in the anterior mediastinum, either within or near the thymus gland and are derived from cells originating in the third pharyngeal pouch that have descended into the mediastinum with the thymus. A small number arise from the ascending aorta and are situated within the pericardium and germ cell tumours can rarely be found in the posterior mediastinum. The group includes benign teratomas and malignant tumours, such as seminomas, embryonal carcinomas, teratocarcinomas and endodermal sinus (yolk sac) tumours. A teratoma contains tissues derived from all three germ cell layers: endoderm, mesoderm and ectoderm. Most anterior mediastinal germ cell tumours are benign.1 Germ cell tumours can occur at any age but there are two peaks of incidence: at 2 years of age and in adolescence. Mediastinal germ cell tumours may present with symptoms related to mass effects, such as wheezing caused by airway compression, superior vena cava obstruction or chest pain. Those containing functioning ectopic tissues can secrete hormones and may give rise to symptoms, such as precocious puberty in boys as a result of the secretion of beta-human chorionic gonadotrophin. Some tumours can rupture into the pleural space or airways. Most germ cell tumours are fairly large at presentation, the diagnosis usually being established from the presence of a large mass, usually in the anterior and superior mediastinum on the CXR and often containing calcification. CT or MRI scanning will define the internal characteristics of the mass, dependent on the tissue types present and their degree of differentiation. The finding of a complex mass with cystic and solid areas, containing fat and irregular calcifications, is highly suggestive of a germ cell tumour. The cystic areas usually have a relatively thick wall, unlike the wall of thymic or pericardial cysts and cystic hygromas (Fig. 5). The most important role of cross-sectional imaging, whether CT or MRI, is to help to distinguish between benign and malignant lesions by showing any local invasion of adjacent mediastinal structures. The multiplanar capacity of MRI is particularly useful in this respect, although the benign or malignant nature cannot be determined with certainty from imaging findings. The presence of raised serum tumour markers, such as alpha-fetoprotein and beta-human chorionic gonadotrophin and histology, confirm the diagnosis.6 Treatment is surgery, plus chemotherapy for malignant lesions. Mesenchymal tumours Mesenchyme is the embryonic connective tissue derived from the mesodermal layer of the trilaminar embryo. From it, supporting structures, such as fibrous tissue, fat, muscle, cartilage, bone, lymphatic tissue and blood vessels are formed. Lymphatic and vascular malformations are an important subgroup of mesenchymal tumours that are frequently congenital. Haemangiomas are benign tumours of endothelial cells common in infancy, 60% being located in the head and neck, 25% in the trunk and 15% in the extremities. They usually appear in the first year of life and show rapid growth over the first 3–9 months, followed by slow involution over between 18 months and 10 years. During the proliferative phase, US of these lesions demonstrates a variable-echogenicity mass with increased colour flow. During involution, the lesion is smaller, with a reduced number of vessels. On CT scanning, proliferative haemangiomas are homogeneous masses with intense, persistent enhancement, usually organised in a lobular pattern. Involuting lesions show less intense enhancement and have fibrofatty change within them. On MRI, haemangiomas are typically of intermediate signal intensity on T1-weighted sequences, and high signal intensity on T2-weighted sequences, with vascular flow voids in and around the lesion. There may be areas of increased signal intensity on both T1- and T2-weighted sequences as a result of haemorrhage or fatty deposition.7., 8. Vascular malformations are not synonymous with haemangiomas and may be composed of venous, arterial or lymphatic components or a combination of these. The mediastinum is an uncommon site for vascular malformations, but by far the majority are lymphatic in origin.9 Cystic lymphatic malformations can be subdivided into macrocystic, microcystic and mixed types and are most often located in the head and neck, although other locations include the axilla, superior mediastinum, mesentery, retroperitoneum and lower limbs. Fewer than 1% of all lymphatic malformations are purely mediastinal, 75% being located in the neck, 10% of which will extend into the mediastinum. Axillary and chest wall lesions can also extend into the mediastinum.9 Lymphatic malformations are associated with chromosomal abnormalities, such as Turner’s syndrome and trisomies. They appear as smooth soft tissue masses, often in the first 2 years of life. Spontaneous shrinkage can occur and sudden enlargement is an indication of bleeding or inflammation. On US, macrocystic lesions appear as a multi-loculated or septated cystic mass, sometimes with fluid–fluid levels. Using Doppler US, flow can be demonstrated only within the septa. Microcystic lesions are hyperechoic (“bright”) without any lesional flow on Doppler. CT scanning demonstrates a low-attenuation (“dark” or fluid attenuation) mass with walls that enhance with contrast. On MRI, lymphatic malformations are septated masses with a low signal intensity on T1-weighted, and a high signal intensity on T2-weighted, sequences. The presence of proteinaceous fluid or haemorrhage within the lesion can cause variable signal intensity on both T1- and T2-weighted sequences (Fig. 6).7., 8. Mesenchymal tumours of fat cell origin include lipomas, lipoblastomas and liposarcomas. Lipomas can occur at any age and are soft, slow-growing tumours, often asymptomatic. If symptoms occur, they are usually caused by the compression of adjacent structures. They occur within the anterior and superior mediastinum, and also in the posterior mediastinum within the wall of the oesophagus. Some lipomas originate within the thymus and are known as thymolipomas. Lipomas do not calcify; they have a uniform low density on CT scanning, a uniform high signal intensity on T1-weighted MRI sequences and suppress on fat-suppressed sequences. Lipoblastomas are tumours of infancy, most cases occurring in the first year of life and rarely after 3 years of age. Histologically, they are composed of embryonal fat and are vascular tumours that can be locally invasive but do not metastasise. Lipoblastomas are of low density with septa of soft tissue attenuation on CT images and do not enhance. Unlike lipomas, lipoblastomas are often heterogeneous on MRI, and on fat-suppressed sequences areas of high signal can suggest the diagnosis. Liposarcomas are rare in childhood although most common in infancy and adolescence. Eleven per cent of tumours occur in the mediastinum, and they may arise within the thymus gland. Liposarcomas are of low density on CT scanning but may show areas of necrosis or calcification.6., 8. Mesenchymal tumours of muscle cell origin include leiomyomas, rhabdomyosarcomas, extra-osseous Ewing’s sarcomas and epithelioid and undifferentiated sarcomas. These tumours may arise in any compartment of the mediastinum. Mediastinal sarcomas occur at all ages and are large soft tissue masses on cross-sectional imaging. Most patients are symptomatic at presentation, often with pain or respiratory difficulties. Epithelioid sarcomas may show cartilage or bone formation. Extraskeletal chondrosarcomas have been reported in the posterior mediastinum in children in the second decade of life. These tumours can be confused with neuroblastomas and on CT images are soft tissue masses containing punctate calcification.6., 8. MIDDLE MEDIASTINAL MASSES Approximately 20% of all paediatric mediastinal masses are found in the middle mediastinum.2 Lymph node masses account for the majority, lymphoma being the most common. The bulk of disease is usually located in the anterior mediastinum, with extension into the middle mediastinum. Metastatic mediastinal lymphadenopathy is less common. Primary tumours that commonly metastasise to mediastinal lymph nodes are neuroblastomas, Ewing’s sarcomas, Wilms’ tumours and osteogenic sarcomas. Inflammatory causes of middle mediastinal lymph node enlargement are even less common, mostly resulting from granulomatous disease, including tuberculosis and sarcoidosis.1 Lymphangiomas and other mesenchymal lesions are also found in the middle mediastinum and have been discussed in the section on the anterior mediastinum. Foregut duplication cysts Developmental malformations of the embryonic foregut (duplication cysts or bronchopulmonary foregut malformations) represent 11% of mediastinal masses in children. Located in the middle or middle-to-posterior mediastinum, they are congenital in origin. Bronchogenic cysts are the most common type and arise in the middle mediastinum at or near the carina. They can migrate during development and may be found in the neck or pericardium, or at vertebral or subpleural sites. Bronchogenic cysts are formed as a result of abnormal budding from the ventral segment of the primitive foregut, which eventually becomes the tracheobronchial tree. They are thin-walled cysts filled with mucoid material that rarely communicate with the tracheobronchial tree. The wall is composed of structures that form the walls of the airways, including cartilage, smooth muscle and mucous glands. CXRs usually demonstrate a discrete rounded mass near to the carina (often subcarinal) or in the paratracheal region.1 Bronchogenic cysts cause symptoms as a result of compressive effects on the airways, but many are incidental findings (Fig. 7). Oesophageal duplication cysts are believed to arise from abnormal vacuolisation of the oesophageal lumen when, in embryonic life, the gut changes from a solid organ to a hollow tube. They are usually larger than bronchogenic cysts and often extend into the posterior mediastinum. They are commonly right sided and frequently occur near the distal oesophagus, attached to or embedded in the wall. They can compress the adjacent oesophagus or airway (Fig. 8). If the cyst contains ectopic gastric mucosa, symptoms caused by ulceration or bleeding can occur. These lesions rarely communicate with the oesophagus and can become infected. Communication with the upper gastrointestinal tract can be demonstrated with a barium swallow. Early in development, the primitive foregut is closely associated with the notochord. Neurenteric cysts occur when there is a failure of separation of the foregut and notochord. This may result in a portion of foregut becoming sequestered in the developing spinal canal or in the formation of a cyst located in the middle or posterior mediastinum. The mediastinal cysts usually contain some neural tissue as well as foregut tissues. Neurenteric cysts are frequently right sided and are often associated with vertebral body anomalies, such as hemivertebra, butterfly vertebra or anterior spina bifida usually superior to the cyst itself (Fig. 9). In most cases, there is a fibrous tract or fistula connecting the mediastinal cyst through the abnormal vertebra with the spinal canal. Some have a patent communication with the gastrointestinal tract and may contain an air–fluid level. More than 50% of children with a neurenteric cyst have abnormal neurology.1 On CT imaging these various foregut duplication lesions usually appear to be cystic in nature (low attenuation with fluid attenuation values), but they can mimic neoplasms as a result of haemorrhage or the presence of proteinaceous material within them.2 MRI scanning can resolve this and easily discerns the cystic nature of these abnormalities as a result of their T1 and T2 values. Fluid-filled cysts are of low signal intensity on T1-weighted, and high signal intensity on T2-weighted, sequences. Haemorrhage or proteinaceous fluid within the cysts can increase the signal intensity on T1-weighted sequences. The presence of fluid–fluid levels is also characteristic of cysts.5 Other foregut abnormalities A large hiatus hernia can also give the impression of a mediastinal mass, particularly if it is incarcerated and contains an air–fluid level. Oesophageal dilatation in association with achalasia may cause widening of the mediastinum on a CXR, and an air–fluid level may be seen (Fig. 10a). Achalasia is caused by abnormal oesophageal motility and failure of the lower oesophageal sphincter to relax. It is uncommon in children, usually occurring in middle age. Symptoms of dysphagia are common, and there may be a history of recurrent chest infections caused by regurgitation and aspiration. Diagnosis in the early stages is made using oesophageal manometry but in long-standing achalasia, a barium swallow is usually diagnostic (Fig. 10b). CONGENITAL MEDIASTINAL VASCULAR ABNORMALITIES Anomalies of the major vessels, including the aorta and its branches, the superior vena cava and tributaries and the pulmonary arteries and veins, are an occasional cause of abnormal mediastinal contour or mediastinal widening on CXRs. MRI is, because of its non-invasive nature, the imaging modality of choice for further evaluation. Aortic arch anomalies A right aortic arch is identified radiographically by a transverse aortic arch situated to the right of the trachea. It is usually slightly higher in position than the usual left-sided arch, and there may be a subtle deviation of the superior vena cava to the right (Fig. 11). There are two main types of right aortic arch. The first is associated with an aberrant left subclavian artery and is found as a normal variant in 0.1% of the population. The presence of an aberrant left subclavian artery is suggested by anterior bowing of the trachea on a lateral CXR, and on barium swallow the aberrant artery causes a posterior impression on the oesophagus as it passes behind it. The anatomy may be delineated using MRI. Most patients are asymptomatic as this vascular anomaly does not usually result in any significant compression of the trachea or oesophagus unless there is a ligamentum arteriosum in a retro-oesophageal position, which completes a vascular ring. There is a very low incidence of associated congenital heart disease with this type of arch. The other type, right aortic arch with mirror image branching, is much rarer although more commonly associated with congenital heart disease. The branching pattern of the arch vessels is innominate artery (left sided), right carotid artery and right subclavian artery. There is no structure posterior to the trachea or oesophagus and hence no actual vascular ring. The double aortic arch consists of two arches arising from a single ascending aorta. The two arches surround the trachea and oesophagus, forming a true vascular ring that can compress both structures and give rise to symptoms, such as stridor and dysphagia. The right-sided arch is usually the larger of the two, and the distal left arch is sometimes atretic. Each arch gives rise to a subclavian and carotid artery before joining to form a single descending aorta, usually on the left side. On a barium swallow, a double aortic arch will cause indentation of both sides of the barium-filled oesophagus on the frontal projection (Fig. 12a), and a posterior impression on the lateral view. Again, MRI is confirmatory (Fig. 12b).5., 10. Venous anomalies A persistent left superior vena cava is a relatively common anomaly, occurring in approximately 0.3% of the normal population, which may mimic a mass on CXR. There is a higher incidence in patients with congenital heart disease. Persistent left superior vena cava occurs because of a failure of the left common and anterior cardinal veins to regress. The left superior vena cava descends lateral to the aortic arch, parallel to the normal right vessel, and drains into a dilated coronary sinus. Other venous causes of mediastinal widening include partial anomalous pulmonary venous drainage into a left brachiocephalic or azygos vein and any cause of dilatation of the intrathoracic veins.5 POSTERIOR MEDIASTINAL MASSES Neurogenic tumours are the most common cause of a posterior mediastinal mass in children, comprising up to 35% of all mediastinal tumours.11 Neurogenic structures in the posterior mediastinum are the spinal nerves exiting through each intervertebral foramen, and the thoracic sympathetic trunk. The thoracic trunk consists of several ganglia that lie just anterior to the origins of the thoracic ribs. Neoplasms arising from these structures can be grouped into nerve sheath tumours, tumours of autonomic ganglia, paraganglionic tumours and peripheral neuroectodermal tumours (PNETs), each of which can have benign and malignant variants (Table 1). MRI is the imaging modality of choice in the evaluation of a posterior mediastinal mass detected on CXR because of its ability to determine intraspinal extension. It is important to document this accurately because the presence of spinal disease will affect the operative treatment, necessitating a combined thoracic and neurosurgical approach. Neurofibromas and schwannomas (neurilemomas) are benign tumours of nerve sheath origin. Together, they are the most common benign mediastinal neurogenic tumours. Schwannomas arise from the peripheral sheath of the nerve, possess a smooth, relatively thick capsule and compress the nerve fibre. Neurofibromas are the result of a disorganised proliferation of all the cellular components within the nerve, including the Schwann cells, myelinated and unmyelinated nerve fibres and fibroblasts, and are non-encapsulated. Neurofibromas often present as a manifestation of neurofibromatosis type 1, these patients being at risk of the malignant transformation of a neurofibroma. Both types of benign nerve sheath tumour can be discovered incidentally but can present with pain or abnormal neurology as a result of peripheral nerve root compression or extradural spinal cord compression. Both have a similar radiographic appearance, being sharply marginated, spherical or lobulated paraspinous masses. Schwannomas may contain calcification. The tumours can erode or destroy the ribs and widen the nerve root foramina. CT appearances vary from homogeneous to heterogeneous, and on MRI these tumours have a low-to-intermediate signal intensity on T1-weighted sequences and may have areas of high signal intensity on T2-weighted sequences.11., 12. Malignant tumours of nerve sheath origin are very rare in children but can occur in adolescents. These are often high-grade malignancies, many occurring in patients with neurofibromatosis type 1. They can also be sporadic or induced by prior radiation therapy. The imaging characteristics of these tumours are similar to those of their benign counterparts but they are aggressive in nature.11 Sympathetic chain ganglion tumours are derived from the autonomic nerve cells themselves rather than their coverings. Neuroblastoma is the most immature and malignant of the ganglion tumours, and the mediastinum is the second most common site of occurrence, these tumours being most frequently found in the abdomen. They are fast-growing tumours, over 50% exhibiting malignant behaviour. The tumour may manifest as a result of neural compression, thoracic scoliosis or symptoms related to the production of catecholamines. Neuroblastomas are non-encapsulated and can be locally aggressive and metastasise. Ganglioneuromas are benign sympathetic neural tumours, representing 40–60% of this group. Only half of all patients with ganglioneuromas are symptomatic, usually because of the mass effects of the tumour. Ganglioneuroblastomas are uncommon, comprising fewer than 15% of this group, and have histological features of both neuroblastoma and ganglioneuroma. Their malignant potential is variable, which is reflected in their prognosis.1., 11. Conventional radiographic findings of sympathetic ganglion tumours are those of a paravertebral soft tissue mass (Fig. 13a), usually vertically elongated with tapered superior and inferior paravertebral margins. The lateral borders are typically smooth and convex. Calcification can be seen in 30%, and there may be thinning and spreading of the ribs and enlargement of the neural foramina. Further imaging is used primarily to determine the extent of disease, CT and MRI being used. On both CT and MRI images, the mass has soft tissue characteristics with contrast enhancement similar to that of skeletal muscle. On MRI, the mass is of intermediate signal intensity on T1-weighted and high signal (bright) on T2-weighted, sequences (Fig. 13b). Although CT can better detect calcification and bony involvement, including metastatic disease, MRI is superior in determining chest wall invasion and intraspinal extent. Radio-isotope bone scintigraphy is an essential component of the imaging of a patient with a suspected or known ganglion cell tumour.  -metaiodobenzylguanidine (MIBG) is taken up by autonomic ganglion cell tumours and paragangliomas, and is particularly valuable in the detection of metastatic disease and in monitoring the response to treatment (Fig. 13c). Approximately 10% of phaeochromocytomas are found in extra-adrenal locations, these being known as paraganglionic tumours. Paragangliomas are rare tumours in children but should be considered in the differential of a posterior mediastinal mass lesion, where they develop in association with the sympathetic chain. They may be incidental findings but if the tumours secrete catecholamines, symptoms, such as hypertension, flushing and watery diarrhoea may be present.  -MIBG scanning is also useful in the detection of paragangliomas. Using CT, the tumours have characteristics not dissimilar to those of other neurogenic tumours, and on T1-weighted MRI sequences the tumour has a signal lower than that of liver. On T2-weighted sequences, the lesions are of extremely high signal (bright).11 PNET of childhood is the generic term for small, round tumours that are probably neural in origin. They are found in many different locations and are highly malignant. A thoracopulmonary PNET is otherwise known as an Askin tumour, named after the person who first described it. This tumour, seen in older children and adolescents, often presents as a soft tissue thoracic or chest wall mass.12 MISCELLANEOUS MEDIASTINAL ABNORMALITIES A congenital diaphragmatic hernia is an uncommon cause of a mediastinal mass in children and even adults. The most common type is a hernia of the peritoneal contents through the foramen of Bochdalek; this is more frequent on the left side (75%). Herniation through the foramen of Morgagni is less common. These are anterior hernias and more commonly right sided. If the defect is large, patients will present with respiratory distress early in life, but small hernias may go undetected for some time. The CXR classically shows air-filled bowel loops in the affected hemithorax, the anterior or posterior location of which is confirmed on a lateral view. Diaphragmatic hernias can sometimes be confused with congenital cystic lung lesions. Air can enter the mediastinum secondary to perforation of an air-containing viscus, such as the oesophagus or a major airway. This is commonly traumatic, for example, after a road traffic accident, or occurs during endoscopic procedures. Other common causes of pneumomediastinum in children are asthma and foreign body aspiration.8 Pneumomediastinum can also be a complication of positive-pressure ventilation. Mediastinal air is seen as a thin, lucent stripe of air outlining the contours of the mediastinal structures, particularly the large vessels. The appearance seen when mediastinal air elevates and outlines the lobes of the thymus in younger children is often likened to “angel wings” (Fig. 14). Mediastinal venous or arterial injury can cause mediastinal enlargement as a result of mediastinal haematoma or false aneurysm formation. This finding on CXR, or a high clinical suspicion of it, should prompt further imaging with contrast-enhanced CT or catheter angiography to detect potentially life-threatening vascular injury. Mediastinal haematoma can also be seen after cardiac surgery. Pleuropulmonary pseudotumour is the name given to atypical left lower lobe pneumonia with collapse or consolidation that mimics a paraspinal mass density. This is thought to occur because of a lax or absent attachment of the pulmonary ligament, which allows the affected segment to collapse towards the mediastinum. Clinical signs and symptoms of infection in this situation are reassuring, and an irregular border to the ‘mass’ on CXR and CT is helpful in differentiating between pseudotumours and actual posterior mediastinal tumours.2 CONCLUSION Localising a mediastinal abnormality to a specific compartment can be useful in limiting the differential diagnosis, although some thoracic tumours can occur in more than one compartment. Malignant lymphoma, benign thymic enlargement, teratomas, duplication cysts and neurogenic tumours are the most common mediastinal masses in children. A recognition of the imaging characteristics of the more common paediatric mediastinal masses allows a more specific diagnosis to be made. After the initial CXR, CT scanning is the primary cross-sectional imaging procedure for the evaluation of most paediatric mediastinal abnormalities. The exception is children with posterior mediastinal masses or suspected vascular lesions or anomalies, in which MRI is preferred. US may be diagnostic in duplication cysts and benign thymic enlargement. PRACTICE POINTS
•The differential diagnosis of a mediastinal abnormality is reduced by determining its location within the mediastinum.
•The internal characteristics of a mediastinal mass on imaging are useful in establishing the diagnosis.
•Imaging is used to confirm the location of a mediastinal abnormality, define its extent and detect complications.
•Magnetic resonance imaging is a useful, non-invasive alternative to catheter angiography in the investigation of mediastinal vascular anomalies.
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Radiology Department, Birmingham Children’s Hospital, Steelhouse Lane, Birmingham, UK  Correspondence to: H. J. Williams. Tel.: +44-(0)-121-333-9733; Fax: +44-(0)-121-333-9726
PII: S1526-0542(02)00310-X doi:10.1016/S1526-0542(02)00310-X © 2003 Elsevier Science Ltd. All rights reserved. | |
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