MEDIASTINAL TUMORS: ANTERIOR MEDIASTINUM
The mediastinum is the central space in the chest cavity that is bounded by the sternum anteriorly, the pleura of the lungs laterally, the vertebrae posteriorly, the thoracic inlet superiorly, and the diaphragm inferiorly. The mediastinum does not have rigid structures that divide it into compartments, but for anatomic and clinical purposes, it is generally divided into anterior, middle- posterior, and paravertebral compartments. It should be noted that the paravertebral regions do not form part of the mediastinum proper. The anterior mediastinum is deﬁned by an imaginary line drawn along the anterior trachea and posterior cardiac border on a lateral chest radiograph. The most common tumors of the anterior mediastinum are thymoma, lymphoma, germ cell tumors, and thyroid (goiter). Cystic hygroma should also be considered, usually in children.
Tumors of the anterior mediastinum account for 50% of all mediastinal tumors. They may be asymptomatic at diagnosis, or patients may complain of cough, dyspnea, or vague chest discomfort. Thymoma is the most common mediastinal tumor in adults. From 30% to 50% of patients with thymoma also have myasthenia gravis. Of patients diagnosed with myasthenia gravis, approximately 15% have a thymoma. Other syndromes associated with thymoma include hypogammaglobulinemia and pure red blood cell aplasia. A signiﬁcant percentage of thymomas are malignant and have spread beyond the capsule of the tumor at the time of diagnosis. Surgical resection is the treatment of choice for localized tumors. Unresectable disease conﬁned to the chest is treated with chemotherapy and thoracic radio-therapy. These tumors are generally moderately sensitive to treatment. Survival with early stage and resectable tumors is excellent, but even unresectable malignant thymomas have a 50% 5-year survival rate with treatment. Speciﬁc treatment and outcomes are stage and histology dependent.
Lymphomas account for 10% to 20% of all mediastinal tumors and occur in both the anterior and middle mediastinum. Hodgkin disease and diffuse large B-cell lymphoma are the most common types in the anterior mediastinum. Patients may present with local symptoms or systemic symptoms of fever, night sweats, and weight loss. Superior vena cava syndrome may be the presenting symptoms or signs in some cases (see Plate 4-54).
Germ cell tumors may present in the anterior mediastinum. Teratomas are the most common germ cell tumor and are usually benign. Teratomas consist of tissues from more than one germ cell layer. They typically occur in children and young adults. Most are asymptomatic but may cause local symptoms. Radiographically, these tumors are lobular and well circumscribed and may contain calciﬁcation or toothlike structures. The computed tomography (CT) scan demonstrates a multiloculated cystic mass that frequently contains fat. Surgical resection is the treatment of choice. Seminomas or nonseminomatous germ cell neoplasms may also occur in the anterior mediastinum. These almost always occur in males, and most are accompanied by elevated blood tumor marker levels of α -fetoprotein (AFP) or β -human chorionic gonadotropin (HCG). These tumors are very responsive to chemotherapy, which is the initial treatment and may be followed by surgical resection of residual disease. The cure rate for seminomas is high, and nonseminomatous germ cell tumors of the mediastinum have an approximate 50% 5-year survival rate.
Intrathoracic goiters are mostly caused by extension from cervical thyroid goiters that can be detected on careful examination of the neck. Patients are usually asymptomatic but may have symptoms related to compression of the trachea or esophagus such as cough, dyspnea, or dysphagia. The vast majority of these tumors are benign. They are located in the anterosuperior mediastinum. The CT demonstrates a lobular, well-deﬁned mass that may have cystic changes or calciﬁcations. Surgical resection is the treatment of choice. Cystic hygromas (lymphangiomas) are an abnormal collection of lymphatic vessels that dilate and collect lymph. They usually occur in the neck in children but rarely are detected in adults. They are uncommonly isolated to the mediastinum alone. CT may demonstrate a solitary or multiple liquid-ﬁlled cysts. If asymptomatic, there is no need to remove them. Other rare tumors of the anterior mediastinum include parathyroid adenomas; pericardial cysts; and mesenchymal neoplasms such as lipomas, liposarcomas, angiosarcomas, and leiomyomas. A foramen of Morgagni hernia of the anterior diaphragm may result in herniation of abdominal contents into the low anterior mediastinum.