There are relatively few malignant primary mediastinal tumors. However, the ability to distinguish one from another is important for the determination of tumor-specific treatment protocols, as well as tumor-specific outcomes. These tumors commonly occur in younger patients and often cause symptoms of acute hemodynamic embarrassment owing to compression of neighboring structures. A priori strategies and experience are essential in the management of these tumors.
The mediastinum is divided into three compartments, as discussed in Chapter 137. Malignant posterior compartment mediastinal tumors are described in Chapter 138. Primary malignant mediastinal tumors tend to occur in the anterior compartment as compared with the middle compartment, which is occupied primarily by benign cysts. Malignant involvement of the middle compartment of the mediastinum occurs chiefly as a consequence of metastatic involvement of mediastinal nodes from pulmonary or esophageal cancers. Malignant mesenchymal tumors are rare, representing fewer than 5% of all primary malignant mediastinal tumors, and may occur in all compartments.1 Most of these tumors are vascular or lymphatic sarcomas. All benefit from excisional surgery with the possibility of adjuvant therapies.1
There are three primary malignant tumors that occur in the anterior compartment, and these constitute most of the malignant mediastinal tumors (Fig. 139-1). Each tumor type possesses unique biologic properties that separate it from the other anterior mediastinal tumors. In addition, each tumor type may have intratumoral diversity.
The three primary malignant tumors that occur in the anterior compartment are thymoma, seminoma, and lymphoma. Clamshell incisions (A) and median sternotomy (B) provide access for wide surgical margins.
Thymomas often occur in association with parathymic syndromes. Myasthenia gravis is the most common syndrome, occurring in 45% of patients (range 10–67%), followed by pure red blood cell aplasia (2–5%) and hypogammaglobulinemia (2–5%).2 These tumors also may be associated with immune-modulated disorders, affecting either the neuromuscular or hematologic systems, as well as with other autoimmune, endocrine, and miscellaneous disorders.3
Thymic tumors are classified as noninvasive, invasive, thymic carcinoma, and neuroendocrine tumors of the thymus. Thymomas often demonstrate benign features on histologic examination. Invasiveness often is determined by histologic criteria demonstrating extracapsular invasion. This can occur irrespective of size, such that at least 30% of thymomas have invasion of the mediastinal fat.3,4 The bland cytologic nature of these tumors has led some authors to refer to noninvasive thymomas as “benign thymomas.” However, even stage I thymomas (i.e., encapsulated tumors without invasion) may recur and metastasize.5 These recurrences may be indolent; however, this behavior is a malignant characteristic that decries the nomenclature of thymoma.2
Clinical staging is helpful to determine the invasive nature of these tumors. The Masaoka staging system is the one used most widely (Table 139-1). Difficulties with the morphologic separation of these ...