Disorders of the Mediastinum

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Chapter 71 Disorders of the Mediastinum

The mediastinum is a central space within the thoracic cavity that is bounded by the sternum anteriorly, the pleura and lungs laterally, the vertebral column posteriorly, the thoracic inlet superiorly, and the diaphragm inferiorly. Although the mediastinum is often arbitrarily divided into compartments, true anatomic planes do not exist, and a mediastinal lesion can occupy more than one compartment. Anatomists and surgeons traditionally divide the mediastinum into four compartments: superior, anterior, middle, and posterior.

The best-known imaging division of the mediastinum is based on the location of a mediastinal mass on the lateral chest radiograph. Because the initial imaging evaluation of mediastinal lesions is typically performed with radiography, localization of a mass in a radiographic mediastinal compartment allows the formulation of a focused differential diagnosis.

This chapter divides the mediastinum into anterior, middle-posterior, and paravertebral compartments (Figure 71-1). The anterior mediastinum is defined by an imaginary line drawn along the anterior trachea and posterior cardiac border on a lateral chest radiograph. The paravertebral compartment is located posterior to an imaginary line drawn to connect the anterior aspects of the thoracic vertebrae. The middle-posterior mediastinum is the compartment located between the anterior and paravertebral compartments. Note that the paravertebral regions do not form part of the mediastinum proper.

Overview of Mediastinal Masses

Mediastinal masses are generally described as being predominantly within the anterior, the middle-posterior, or the paravertebral compartment. Cross-sectional imaging allows more accurate localization of mediastinal abnormalities, characterization of the lesions, and visualization of their relationship with and effects on adjacent normal structures.

In adults, approximately 65% of primary mediastinal lesions are located in the anterior mediastinum, 10% in the middle-posterior mediastinum, and 25% in the paravertebral compartment. In contrast, approximately 38% of childhood mediastinal lesions are located in the anterior, 10% in the middle-posterior, and 52% in the paravertebral compartment (Table 71-1).

Table 71-1 Compartmental Classification of Mediastinal Disorders

Abnormality/Source Disorder/Abnormality
Anterior Mediastinum
Disorders of thymus gland Thymomas: thymic carcinoma, thymic carcinoid, thymolipoma, thymic cyst, thymic hyperplasia
Lymphoma: Hodgkin disease, non-Hodgkin lymphoma
Germ cell neoplasms
Benign: Mature teratoma
Malignant: Seminoma, nonseminomatous germ cell neoplasms
Thyroid Intrathoracic goiter
Parathyroid Parathyroid adenoma
Pericardial cysts  
Miscellaneous Mesenchymal neoplasms (lipoma, liposarcoma, angiosarcoma, leiomyoma)
Cystic hygroma (mediastinal lymphangioma)
Middle-Posterior Mediastinum
Lymph node enlargement Lymphoma
Benign mediastinal lymphadenopathy:
Granulomatous disease, infectious (tuberculosis, fungal infections)
Noninfectious (sarcoidosis, silicosis)
Miscellaneous causes:
Castleman disease
Amyloidosis
Metastatic mediastinal lymphadenopathy
Lung, renal cell, gastrointestinal, and breast carcinoma
Cysts Foregut cysts
Bronchogenic and enteric cysts
Esophageal disorders Achalasia, benign tumors, esophageal carcinoma, esophageal diverticulum
Vascular lesions Aneurysms, hemangioma
Miscellaneous Herniations, pancreatic pseudocyst
Paravertebral Lesions
Neurogenic neoplasms Peripheral nerve neoplasms:
Schwannoma, neurofibroma
Malignant peripheral nerve sheath neoplasm
Sympathetic ganglia neoplasms:
Ganglioneuroma, ganglioneuroblastoma, neuroblastoma
Paraganglionic neoplasms:
Pheochromocytoma, paraganglioma
Spinal Lateral thoracic meningocele
Paraspinal abscess (Pott disease)
Miscellaneous Extramedullary hematopoiesis
Thoracic duct cysts

The most common mediastinal masses are intrathoracic hernia, goiter, neurogenic neoplasms, thymic lesions, lymphomas, cysts, and germ cell neoplasms. Thymic and neurogenic neoplasms and foregut cysts are the most frequent primary lesions in adults, and neurogenic neoplasms, foregut cysts, and lymphoma are the predominant lesions in children. Less frequent mediastinal lesions include lymphangioma, pancreatic pseudocyst, extramedullary hematopoiesis, and meningocele. In addition, lung cancer and mediastinal lymph node metastases may manifest as a mediastinal mass or as extensive mediastinal lymphadenopathy. Non-neoplastic lymphadenopathies, such as sarcoidosis and silicosis, and infectious granulomatous lymphadenopathy may manifest as multiple enlarged mediastinal and hilar lymph nodes and may exhibit calcification.

The nature of mediastinal diseases varies significantly with the patient’s age and the clinical presentation. Overall, approximately one third of mediastinal neoplasms are malignant. The mediastinal neoplasms that affect children (40%-50%) are more likely to be malignant than those affecting adults (25%). Most masses (80%-90%) in asymptomatic individuals are benign, whereas approximately 50% of the lesions that produce symptoms are malignant. Conversely, approximately 75% of patients who have malignant neoplasms also have symptoms, compared with less than 50% of patients with benign lesions. Patients with mediastinal masses may experience constitutional symptoms, paraneoplastic syndromes, and symptoms related to compression or invasion of adjacent mediastinal structures. The latter may herald a large, locally invasive or malignant lesion.

The initial evaluation of patients with mediastinal abnormalities includes a detailed history and physical examination to discover specific symptoms and signs of various mediastinal disorders and associated diseases (Table 71-2). Posteroanterior (PA) and lateral chest radiography, computed tomography (CT), and occasionally magnetic resonance imaging (MRI) and fluorine 18 (18F) fluorodeoxyglucose (FDG) positron emission tomography (PET) with CT are used for lesion detection and characterization, usually followed by an invasive procedure for tissue diagnosis. However, some mediastinal lesions have a characteristic radiologic appearance, and biopsy may be unwarranted or contraindicated, as in congenital cysts and vascular lesions, respectively. In addition, laboratory evaluation to include a complete blood count, electrolytes, renal and liver function tests, and serologic tests for various autoantibodies and tumor markers may be useful in the initial evaluation and in posttherapy follow-up.

Table 71-2 Mediastinal Disorders: Constitutional and Paraneoplastic Findings

Diseases Clinical Findings
Lymphoma Fever, weight loss, night sweats, pruritus, hypercalcemia
Thymoma Myasthenia gravis, hypogammaglobulinemia, pure red cell aplasia
Thymic carcinoid secretion Cushing syndrome, syndrome of inappropriate antidiuretic hormone
Germ cell neoplasm Gynecomastia, Klinefelter syndrome, hematologic neoplasms
Intrathoracic goiter Hyper/hypothyroidism
Pheochromocytoma Hypertension, hypercalcemia, polycythemia, Cushing syndrome
Autonomic ganglia neoplasm Opsomyoclonus, hypertension, watery diarrhea, Horner syndrome
Sarcoidosis Hypercalcemia

Diseases of Anterior Mediastinum

Mediastinal Neoplasms

Thymoma

Thymoma is the most common primary mediastinal neoplasm in adults and the most frequent tumor of the anterior mediastinum. It usually affects adults older than 40 years, with no gender predilection. Approximately 30% of patients with thymoma have thoracic symptoms of cough, dyspnea, and/or chest pain; 40% to 70% have symptoms related to one or more of the parathymic syndromes, typically myasthenia gravis (MG), but hypogammaglobulinemia, pure red cell aplasia, and nonthymic malignancies may also occur. Some affected patients are asymptomatic and are discovered incidentally because of abnormal chest radiographs. Although the association of thymoma and MG is well recognized, approximately 85% of patients with MG have thymic lymphoid hyperplasia, and only 15% are found to have thymoma. In contrast, 30% to 50% of patients with thymoma may develop MG. Hypogammaglobulinemia and pure red cell aplasia occur in 10% and 5% of patients who have thymoma, respectively. Nonthymic malignancies occur in 12% to 20% of patients with thymoma and include thyroid carcinoma, lung cancer, and lymphoma.

Thymomas manifest on radiography as rounded, well-circumscribed, unilateral, anterior mediastinal masses (Figure 71-2). They are typically located anterior to the aortic root but may be found anywhere from the thoracic inlet to the diaphragm and rarely in the neck. On cross-sectional imaging, thymomas are often homogeneous, soft tissue masses but may exhibit heterogeneity because of cystic change, hemorrhage, or necrosis, especially in large neoplasms. Drop metastases to the ipsilateral pleura, pericardium, or upper abdomen through a diaphragmatic hiatus are well documented and represent invasive disease (Figure 71-3). Pleural drop metastases may encase the lung and mimic diffuse malignant mesothelioma, although associated pleural effusion is infrequent. Lymph node and hematogenous metastases are rare. Mediastinal invasion may be detected with CT and may manifest as an irregular tumoral surface, contralateral extension of thymoma across the midline, and obvious invasion of mediastinal fat and structures. Imaging evidence of local invasion must be correlated with microscopic evidence of capsular transgression and tissue invasion, because encapsulated thymomas may produce fibrous adhesions to adjacent structures. MRI is more sensitive than CT in the detection of vascular invasion. When evaluating a thymic mass, PET-CT may be useful for depicting tumoral invasion, differentiating subgroups of thymic epithelial neoplasms, and staging extent of disease. The maximum standard uptake values (SUVs) of thymoma are typically lower than those of thymic carcinoma. Guided needle biopsy, mediastinotomy, mediastinoscopy, and video-assisted thoracoscopy may establish the diagnosis. However, histologic diagnosis before excisional surgery may not be required in classic cases of thymoma.

Thymomas represent neoplastic proliferation of thymic epithelial cells, intermixed with mature lymphocytes. The histologic typing has been complex and challenging. The Revised 2004 World Health Organization (WHO) classification divides thymic epithelial neoplasms into A, B, and C types on the basis of epithelial cell morphology, the ratio of lymphocytes to epithelial cells, and prognosis. Types A and B correlate with the traditional major histologic cell types of thymoma—epithelial, lymphocytic, mixed lymphoepithelial, and spindle cell. Type C refers to thymic carcinoma. The multidisciplinary International Thymic Malignancy Group (ITMG) assesses diagnostic and therapeutic options of thymoma. Anatomic staging is based on the presence or absence of capsular invasion, determined macroscopically during surgery and confirmed microscopically. Thymomas are now considered malignant neoplasms, and all stages have the potential to metastasize. Most thymomas are completely encapsulated; however, approximately 30% are invasive and grow through the capsule into surrounding adipose tissue, pleura, pericardium, great vessels, and/or heart. Encapsulated and invasive thymomas are microscopically identical, and “invasive” is used to denote capsular invasion.

The treatment of choice for encapsulated and invasive thymomas confined to the mediastinum is surgical resection. Postoperative radiation therapy is included in the treatment of invasive thymoma, but the role of preoperative irradiation is controversial. Chemotherapy with cisplatin-based regimens is generally recommended for metastatic or unresectable recurrent disease, with an overall response rate of 50% to 70% in small studies.

The prognosis of patients with thymoma varies with the stage and extent of surgical resection. Patients with completely resected, encapsulated lesions have the best prognosis. Overall 5-year and 10-year survival rates in patients who have encapsulated thymomas are 75% and 63%, respectively, whereas patients with invasive thymoma have survival rates of 50% and 30%, respectively. Delayed recurrence of thymomas may occur, even in patients with completely resected encapsulated lesions, which emphasizes the importance of long-term follow-up.

The role of thymectomy in the treatment of parathymic syndromes is controversial. Thymectomy is more effective in patients with MG in the absence of a thymoma, with a clinical remission rate of approximately 35% and improvement in another 50%. Neurologic improvement is less likely when MG is associated with thymoma. Thymectomy in patients with pure red cell aplasia results in a 40% to 50% remission rate, in contrast to those with hypogammaglobulinemia, who do not benefit from thymic resection.

Hodgkin Disease

Hodgkin disease is most often seen in adults age 20 to 30 and in those older than 50. Even though the nodular sclerosis subtype is more common in women, HD in general does not exhibit a gender predilection in young patients. Patients who have mediastinal lymphoma tend to be younger. The usual presenting finding is cervical and supraclavicular lymphadenopathy. In less than 25% of patients, HD is limited to the thorax. Approximately one third of patients have systemic symptoms. Patients with mediastinal involvement are generally asymptomatic, although bulky lymphadenopathy may induce symptoms related to mediastinal compression.

Radiologically, most patients with HD have bilateral, asymmetric, mediastinal lymphadenopathy, which frequently involves the prevascular and paratracheal lymph nodes and rarely the posterior mediastinal or juxtacardiac lymph nodes. Nodular sclerosis HD can manifest as a discrete, unilateral anterior mass (Figure 71-6) or bilateral mediastinal mass (Figure 71-7) or as large, lobular, coalescent lymphadenopathy in the anterior mediastinum (Figure 71-8). Invasion, compression, and displacement of mediastinal structures, lung, pleura, and chest wall may occur. Affected lymph nodes usually exhibit homogeneous attenuation but may be heterogeneous because of hemorrhage, necrosis, or cystic change. De novo lymph node calcification rarely occurs but may develop 1 to 5 years after radiation therapy. Direct invasion of the lung occurs in 8% to 14% of untreated patients and is usually associated with hilar lymphadenopathy. The diagnosis is established with either core biopsy or surgical excision of affected palpable lymph nodes or masses found on imaging.

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