163: Malignant Primary Anterior Mediastinal Tumors

Introduction

Among the three mediastinal compartments, the anterior compartment has the greatest potential for developing a primary mediastinal malignancy. Malignant tumors often are symptomatic and have considerable bulk compared with their benign counterparts. The most common primary malignant tumors of the anterior mediastinum include invasive thymomas, seminomas, nonseminomatous germ cell tumors (GCTs), malignant teratomas, and lymphomas. Commonly, at presentation, these masses manifest clinical symptoms as a result of either extrinsic compression of neighboring mediastinal structures or due to paraneoplastic syndromes. It is important to identify the specific pathology of these tumors, as each of these clinical entities requires a unique treatment strategy with different clinical outcome and prognosis.

Anatomy and Pathology

The anatomy of the three mediastinal compartments is outlined and reviewed in Chapters 155 and 161. The anterior compartment differs from the middle and posterior compartments in that it has a higher rate of malignant tumors. In a study of over 400 patients with primary mediastinal tumors, malignancy was identified in 59% of anterior compartment tumors as compared with only 29% for middle compartment and 16% for posterior compartment tumors in adults.^1,2 Malignancy is also dependent on age and the presence of symptoms. Children are more likely to develop lymphoma or malignant neurogenic tumors.³ The presence of symptoms foretells malignant disease in 85% of patients, while only 46% of patients with symptoms have benign disease.^1,4 Posterior compartment mediastinal tumors are described in Chapter 162 and the middle mediastinal compartment is described in Chapter 161.

The three primary malignant mediastinal tumors that commonly occur in the anterior compartment are thymic tumors, GCTs, and lymphomas (Fig. 163-1). Each of these tumors can be further divided into subtypes, each possessing a unique tumor biology which influences treatment and outcomes. Mesenchymal tumors are uncommon, comprising less than 10% of anterior mediastinal tumors, and can be divided into tumors of vascular origin, lymphatic origin, or tumors of connective tissue origin.⁵

Thymoma and Thymic Tumors

Thymic tumors are the most frequently identified primary mediastinal tumor in adults and although they originate in the anterior mediastinum they may invade other neighboring structures or cavities. Thymic tumors are pathologically subclassified as noninvasive (encapsulated) thymomas, invasive thymomas, thymic carcinomas, and neuroendocrine thymic tumors. Of these thymic tumors, noninvasive thymomas are defined by encapsulation, without involvement of other surrounding structures. Invasive thymomas may ostensibly possess benign appearing features on histologic examination but they may also demonstrate malignant properties with invasion past the capsule into contiguous structures.⁶ This extracapsular invasion can occur ...