Computed Tomography

CT is the most useful investigation for localising, characterising and demonstrating the extent of a mediastinal mass and its relationships. CT is also useful in delineating calcification, which can help generate a more refined differential diagnosis. Multidetector CT (MDCT) following intravenous contrast medium with multiplanar reformats provides an excellent assessment of mediastinal structures, including vessels in the coronal and sagittal planes (Fig. 11-1). CT may guide biopsy, plan resection and follow response to therapy.

Radionuclide examinations have a limited role in assessing mediastinal masses. Positron emission tomography (PET) and PET–CT using [F-18]2-deoxy-d-glucose (¹⁸F-FDG) has proven useful in evaluating mediastinal lymph node involvement in lung cancer and lymphoma.^5,6 Radionuclide examinations may also be useful in imaging thyroid masses, neuroendocrine tumours and pheochromocytomas.

The information obtained from the relationship of the normal anatomical structures representing the ‘mediastinal lines and stripes’ on chest radiography aid in localising the lesion within the mediastinum to the anterior, middle or posterior mediastinum and generating a differential diagnosis before proceeding to a chest CT examination.⁷

Anterior mediastinal masses can be identified when both the hilum overlay sign and preservation of the posterior mediastinal lines are present. Widening of the right paratracheal stripe and convexity relative to the AP window reflection both indicate abnormality in the middle mediastinum. Disruption of the azygo-oesophageal recess can be caused by disease in either the middle or posterior mediastinum. Paravertebral masses disrupt the paraspinal lines, and the region of masses above the level clavicles can be inferred by their lateral margins: Posterior masses have sharp margins caused by their interface with lung, whereas anterior masses do not.⁸

Although there is no tissue plane separating the divisions of the mediastinum, attempting to localise an abnormality within the mediastinum helps in narrowing the differential diagnosis and determining appropriate further imaging.

Characterise on CT or MR

Mediastinal masses can be further characterised by CT or MR, depending on whether they contain fat, fluid or are vascular (Table 11-1).

Thymomas

Thymomas are the most common tumour of the thymus in adults, and the most common primary tumour of the anterior mediastinum in adults.¹⁷ Thymomas are usually low-grade malignant tumours of thymic epithelium. Thymomas may be encapsulated (non-invasive thymomas) or may extend beyond the capsule (invasive type). The average age at diagnosis is approximately 50 years, earlier in those who present with myasthenia gravis. Thymomas are extremely unusual below the age of 15 and rare under 20. Up to 50% of patients with thymoma have myasthenia gravis, and approximately 10–20% of patients with myasthenia gravis have a thymoma. A variety of other syndromes are seen in patients with thymoma, including hypogammaglobulinaemia and pure red cell aplasia.^18,19

Most thymomas (90%) arise in the upper anterior mediastinum. The mass is usually anterior to the ascending aorta, lying above the right ventricular outflow tract and pulmonary artery. A few are situated more inferiorly, projecting from the left or right heart border, or lying close to the cardiophrenic angles (Fig. 11-4). They are usually spherical or oval in shape and may show lobulated borders. Thymomas may contain one or more cysts and a few are predominantly cystic. Calcification, punctate or curvilinear, may be seen. All of these features are best demonstrated using CT,^20,21 which is the most sensitive technique for detecting thymoma in patients with myasthenia gravis.²² Thymomas as small as 1.5–2.0 cm in diameter are readily identified in men and women over the age of 40, largely because the rest of the thymus is atropic. Diagnosing a small thymoma in those patients younger than age 40, and particularly younger than 30, can be difficult, because the normal gland is variable in size and in myasthenia gravis the associated hyperplasia may cause a bulky gland. Fortunately, thymoma is so infrequent in children that the potentially difficult problem of finding a thymoma in a child with myasthenia gravis rarely arises.

Thymomas usually show homogeneous density and uniform enhancement after contrast media injection and may occasionally be cystic. Invasion of the mediastinal fat and adjacent pleura may be identified with invasive thymomas, and while CT shows such invasion to advantage, it cannot reliably diagnose invasive thymoma if the tumour is still confined to the thymus.²³ Pleural metastases resulting from transpleural spread are a feature of invasive thymomas, and therefore the whole of the pleural cavity should be carefully examined.²³ On MR, the normal thymus in children and young adults characteristically demonstrates homogeneous and intermediate T1- and T2-weighted signal intensity, less intense than mediastinal fat but greater than muscle. In older patients after puberty, the T1 and T2 signals increase with age because the thymus begins to involute and is replaced by fat.^24,25 Thymomas typically demonstrate low T1 signal intensity similar to muscle and relatively high T2 signal intensity. MR can be useful for showing mediastinal spread when there is doubt about the CT. T2-weighted images occasionally show lobulated internal architecture and scattered areas of high intensity that correspond to fibrous septa and cystic areas²⁶ (Fig. 11-4). Heterogeneity of signal intensity caused by septation, cystic change and haemorrhage is common.

Invasive thymomas invade beyond the capsule into the mediastinum and can involve the pleura and pericardium. Complete obliteration of the adjacent fat planes suggests mediastinal invasion (Fig. 11-5). On the contrary, complete preservation of the adjacent fat planes excludes extensive invasive disease but does not rule out minimal capsular invasion.²² MR is superior to CT for defining the invasion of contiguous structures such as the pleura and pericardium in patients with invasive thymoma, but is not used in most cases.²⁷

Thymic carcinoma is a thymic epithelial tumour with a high degree of anaplasia, cell atypia and increased proliferation²⁸ and predominantly occurs in adults. Thymic carcinomas have a poor prognosis despite treatment with surgery and radiotherapy. Thymic carcinomas are aggressive, locally invasive malignancies that have frequently metastasised to regional lymph nodes and distant sites at presentation. They are typically large, heterogeneous masses, containing areas of necrosis and calcification, often demonstrating evidence of invasion of adjacent structures, in particular the mediastinum, pericardium and pleura^29,30 (Fig. 11-6). On MR, thymic carcinoma demonstrates intermediate signal intensity slightly higher than muscle on T1-weighted sequence and high signal intensity on T2-weighted sequence.³¹ Thymic carcinomas are more commonly associated with mediastinal node and extrathoracic metastasis but less commonly associated with pleural implants compared with invasive thymomas.³⁰

Primary neuroendocrine tumours (carcinoids) of the thymus are rare and account for less than 5% of anterior mediastinal tumours. Unlike pulmonary carcinoids, these tumours are aggressive and at least 20% of patients have distant metastasis at presentation to the liver, lung, bone, pleura and pancreas.³² Approximately 40% of patients have Cushing’s syndrome because of adrenocorticotrophic hormone secretion by the tumour and up to 20% have multiple endocrine neoplasia (MEN) syndromes I and II.

Thymic carcinoid is histologically distinct from thymoma, although their imaging features overlap. On CT or MR, the tumours appear as a lobulated thymic mass with heterogeneous enhancement (Fig. 11-7) and central areas of low attenuation secondary to necrosis or haemorrhage and may show local invasion. Bone metastasis is typically osteoblastic.

Thymolipomas are rare tumours composed of a mixture of mature fat and normal-looking or involuted thymic tissue. The reported age range is 3–60 years. The average age of the patient is 22–26 years and most patients are asymptomatic.³³ These tumours occur low in the anterior mediastinum, often in the cardiophrenic angle. Individual cases have been reported in association with a variety of conditions, including myasthenia gravis, aplastic anaemia, Graves’ disease and hypogammaglobulinaemia. Thymolipomas can grow to a very large size before discovery and, being soft, mould themselves to the adjacent mediastinum and diaphragm, and may mimic cardiomegaly or lobar collapse.³⁴ CT shows the fatty nature of the mass, with islands of thymus and fibrous septa running through the lesion.^33,34 On MR, fat within the tumour appears as high signal intensity and soft tissue appears as low signal intensity bands coursing through the mass.³⁵

Teratomas

Teratomas contain elements of all three germinal layers: ectoderm (skin, teeth, hair), mesoderm (bone, cartilage, muscle) and endoderm (bronchial or GI epithelium). Teratomas usually have a benign course and surgical resection is the treatment of choice on the small chance that few malignant elements are present. Malignant teratomas have a poor prognosis. Teratomas are the most common mediastinal germ-cell tumour; most are cystic. Teratomas are found at all ages, particularly in adolescents and young adults, with women slightly outnumbering men.^42,43 They are usually asymptomatic and diagnosed incidentally on chest radiography or CT, but may give rise to cough, dyspnoea or chest pain if they compress the bronchial tree or superior vena cava (SVC), or if they rupture into the mediastinum or lung. Teratomas are usually stable, but haemorrhage or infection may lead to a rapid increase in size.

On chest radiograph or CT most teratomas present as a well-defined, rounded or lobulated mass, localised to the anterior mediastinum. Fat and calcification may rarely be identified on chest radiograph. CT appearances are variable. Combinations of fat, fluid, soft-tissue components and calcification may be seen⁴² (Fig. 11-10).

MR is useful in differentiating teratoma from thymoma and lymphoma. Soft-tissue elements in the teratoma are isointense to muscle, cystic elements demonstrate low T1 intensity and high T2 intensity, and fat appears as high T1 intensity with signal loss on fat saturation sequence. Fat is virtually diagnostic of teratoma.⁴⁴

Seminoma

Seminoma is the most common malignant mediastinal germ-cell tumour.⁴⁵ Seminomas occur almost exclusively in men during the second through fourth decades. They are usually well-defined solid masses with possible small foci of degenerative changes representing haemorrhage and necrosis⁴⁶ (Fig. 11-11). Symptoms are usually caused by mass effect on adjacent structures. On CT and MR seminomas have homogeneous attenuation and signal intensity and may have areas of haemorrhage and necrosis.

The plain radiographic findings are similar, except that the malignant tumours are more often lobular in outline. Fat density and visible calcifications are rare. Because these are malignant tumours, they grow rapidly and metastasise readily to the lungs, bones or, less often, pleura (Fig. 11-12). CT often shows a lobular, asymmetric mass. The adjacent mediastinal fat planes may be obliterated, and the tumours either are of homogeneous soft-tissue density or show multiple areas of contrast media enhancement interspersed with rounded areas of decreased attenuation caused by necrosis and haemorrhage.^47,48 On MR, these tumours may demonstrate heterogeneous intensities with areas of high T2 signal intensity corresponding to degenerative cystic changes.

Extensive lymph node calcification is common following tuberculosis and fungal infection, and is occasionally seen with other infections (Fig. 11-14). It may also be encountered in a variety of other conditions, notably sarcoidosis, silicosis and amyloidosis. Although it may be seen in lymph node metastases from calcifying primary malignancies, such as osteosarcoma, chondrosarcoma and mucinous colorectal and ovarian tumours, lymph node calcification is rare in metastatic neoplasm. It is virtually unknown in untreated lymphoma though it is occasionally seen in nodes involved by Hodgkin’s disease following therapy.

CT is the most sensitive technique for the detection of lymph node calcification. Two common patterns of calcification are coarse, irregularly distributed clumps within the node and homogeneous calcification of the whole node. A strikingly foamy appearance is rarely seen with Pneumocystis jiroveci (previously Pneumocystis carinii) infection in AIDS patients⁴⁹ and in some cases of metastatic mucinous neoplasms. Sometimes there is a ring of calcification at the periphery of the node—so-called ‘eggshell calcification’, which is a particular feature of sarcoidosis and of prolonged dust exposure (e.g. mining).

Low-Attenuation Nodes

On CT, areas of low attenuation within enlarged nodes, corresponding to necrosis, may be seen in a variety of conditions, particularly tuberculosis⁵⁰ and occasionally in fungal disease,⁵¹ infections in immunocompromised patients, metastatic neoplasm (notably from testicular tumours⁵²) and lymphoma⁵³ (Fig. 11-15). Necrotic lymph nodes are common in patients with active tuberculosis. They demonstrate central areas of low attenuation with peripheral enhancement when intravenous contrast medium has been administered. Attenuation values below that of water are seen in fatty replacement of inflammatory nodes and have been described in Whipple’s disease.⁵⁴

Castleman’s disease is a rare cause of strikingly uniform enhancing lymph nodes. Castleman’s disease (also referred to as angiofollicular lymph node hyperplasia) is a specific type of lymph node hyperplasia of uncertain aetiology which can cause substantial lymph node enlargement in many sites in the body. The lymph node mass is often localised to one area, can be huge and may be very vascular. The nodes may calcify and may show striking contrast media enhancement on both CT and MRI^55,56 (Fig. 11-16). Histologically, there are two types: the hyaline vascular type and the plasma cell type.

In addition to Castleman’s disease, marked lymph node enhancement may occur in hypervascular metastases from melanoma, renal cell carcinoma, carcinoid tumours, papillary thyroid cancer and Kaposi’s sarcoma.⁵⁷

Contrast media enhancement of enlarged nodes, when moderate in degree, is non-specific, being seen with inflammatory disorders, particularly tuberculosis, fungal disease, sarcoidosis and neoplasm. A low-density centre with rim enhancement of the enlarged node is a useful pointer towards the diagnosis of tuberculous or other granulomatous infections.

Lymph Node Enlargement

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