Bruce Jason Leavitt Scope of Problem
The mediastinum is an anatomically distinct area of the body located within the center of the chest. The boundaries are the sternum anteriorly and the spine posteriorly. The thoracic inlet is the superior boundary and the diaphragm the inferior one. The lateral pericardium forms the lateral borders. Quite a wide variety of tumors exist in the mediastinum.
Approximately two thirds of all mediastinal tumors are benign and the rest are malignant. Mediastinal tumors are most common in young and middle-aged adults and occur with less frequency in the older population. However, a higher incidence of malignancy is noted in older patients with primary lesions of the mediastinum. The majority of tumors are discovered by radiography, most commonly a plain chest x-ray. Most lesions are benign and have no symptoms. Malignant lesions often produce symptoms such as substernal pain or cough. A clinician can determine the location and type of tumor by an accurate history and physical examination, chest x-ray, CT scan of the chest, and simple tumor marker studies. Once identified, proper treatment can be instituted.
Most mediastinal tumors do not have known risk factors. Patients with Von Recklinghausen's disease have a high incidence of neurogenic tumors. For the most part, these tumors are neurofibromas of the posterior compartment. Tumors of the thymus gland have many associated syndromes, the most common of which is myasthenia gravis. Myasthenia gravis is present in 30-50% of patients with thymoma. Of all patients with myasthenia gravis, only 15% have a thymoma. Other conditions associated with thymoma include pure red cell aplasia, hypogam-maglobulinemia, myotonic dystrophy, and systemic lupus.
Screening for mediastinal tumors has not been a generally accepted practice. Patients with atypical chest pain should be evaluated with a PA and lateral chest x-ray. A thoracic CT scan with intravenous contrast should follow any masses or abnormal mediastinal shadows. Patients with myasthenia gravis and a normal chest x-ray still should have a CT scan of the chest to evaluate for the possibility of a thymoma. Tumor markers, human chorionic gonadotropin (HCG) and alpha feto-protein (AFP) should be drawn on all patients with anterior mediastinal masses.
Surgical Oncology, edited by David N. Krag. ©2000 Landes Bioscience.
Methods of diagnosis depend on the size and location of the tumor. Tumors can be grouped according to an anatomic location within the mediastinum. The mediastinum is separated into three compartments: anterior; middle; and posterior
The anterior compartment lies between the sternum and the anterior pericardium. The most cephalad aspect extends to the thoracic inlet and neck (Fig. 13.1). Tumors of this region include thymic tumors, lymphomas, germ cell tumors, and thyroid masses (both benign and malignant).
Thymomas and thymic carcinomas can often be identified on a CXR (Fig. 13.2), but can always be detected on a CT scan (Fig. 13.3). CT scan with contrast combined with transthoracic needle biopsy (FNA) can diagnose most of the above conditions. If the thymic tumor has been diagnosed by FNA, surgical excision is warranted. If the FNA is not suggestive of a thymic mass, and further tissue is needed to make the diagnosis, an anterolateral thoracotomy with biopsy (Chamberlain procedure) should be considered. Recently, thoracoscopy with biopsy of masses in the anterior mediastinum has been utilized as a diagnostic technique.
Germ cell tumors of the mediastinum are uncommon. There are both benign (teratoma, teratodermoid) and malignant germ cell tumors. Almost all occur in the anterior mediastinum. Most malignant germ cell tumors occur in adult males under the age of 35. The types of germ cell tumors are similar to those found in the testicle. Pure seminoma is the most common, followed by embryonal carcinoma, teratocar-cinoma, endodermal sinus tumor, and choriocarcinoma. Germ cell tumors most commonly present with localized chest pain or pressure, although asymptomatic tumors often occur and are diagnosed via CXR. These seminomatous germ cell tumors are seen as smoothly marginated anterior mediastinal masses on CXR (Fig. 13.4) and CT scan (Fig. 13.5). Nonseminomatous germ cell tumors tend to be more lobular and extensive than pure seminomatous tumors (Figs. 13.6, 13.7). Well-formed teeth have rarely been identified within a teratoma by CXR. CT scans often show a fatty mass with varying amounts of denser tissue such as bone, teeth, or nonspecific calcifications. Beta HCG and AFP levels should always be measured if a mediastinal germ cell tumor is suspected. FNA is the initial diagnostic approach while video-assisted thoracic surgery and/or thoracotomy can be performed if the FNA is nondiagnostic. A full metastatic work-up should be performed with any patient who is suspected of having a germ cell tumor. Nonseminomatous germ cell tumors are very malignant and most always present with metastases.
Mediastinal thyroid masses are most often goiters that arise in the neck and descend into the anterior mediastinum through the thoracic inlet. Thyroid carcinomas that arise within the mediastinum are quite rare. Substernal extension of thyroid goiters is often seen as dense masses on CXR that are located in the most superior aspect of the chest (Fig. 13.8). CT imaging of these lesions often show an enhancing lesion arising in the neck and extending down the right side of the anterior mediastinum (Fig. 13.9). If the goiter is metabolically active, a radioactive iodine scan can be diagnostic. Metabolically inactive substernal goiters will not enhance with the administration of radioactive iodine.
Table 13.1. Mediastinal compartments
Tumors and Cysts
Germ cell tumors
Substernal thyroid goiters
Cysts (pericardial, bronchial, enteric)
Neurogenic tumors (neurofibroma, neurolemmoma, neurosarcoma)
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