A mediastinal tumor is a tumor in the mediastinum, the cavity that separates the lungs from the rest of the chest. It extends from the sternum to the spinal column. It contains the heart, esophagus, trachea, thymus, and aorta.
The mediastinum has three main parts: the anterior mediastinum (front), the middle mediastinum, and the posterior mediastinum (back). Tumors in the anterior portion of the mediastinum can include thymoma, lymphoma, germ cell tumors including teratoma, thyroid tissue, and parathyroid lesions. Masses in this area are more likely to be malignant than those in other compartments. Middle mediastinal tumors are less common and refer to cardiac or neurogenic tumors. Masses in the posterior portion of the mediastinum tend to be neurogenic in origin, and in adults tend to be of neural sheath origin including shwannomas and neurofibromas. The most common mediastinal masses are thymoma (20% of mediastinal tumors), usually found in the anterior mediastinum, followed by neurogenic tumor (15–20%) located in the anterior mediastinum. Adults usually present with cancers of the anterior mediastinum and children usually present with cancers of the posterior mediastinum. Other conditions that can cause mediastinal enlargement includes aortic aneurysms, lymphadenopathy, cysts, inflammation, and abscesses.
Types
Anterior Mediastinal Tumors 50% of mediastinal masses occur in the anterior compartment. The most common anterior masses are commonly known by the '4 T's' which refer to thymomas, teratomas, thyroid tissue and 'terrible' lymphomas. 35% of tumors in this region are thymic malignancies, 25% are lymphomas, 10% are benign teratomas, 15% are endocrine and thyroid tumors and the remaining portion are malignant germ cell tumors and benign thymic lesions.
Thymic carcinoma and thymoma
The thymus is a lymphatic organ in the anterior mediastinum where lymphocytes, immune system cells, are produced and matured. It is most prominent in infants and begins to involute at 1 year of age. Tumors of this organ are the most common primary anterior mediastinal tumor, comprising 20% of all mediastinal cancers. Thymomas and thymic carcinomas most commonly occur between ages 40 and 60 and only rarely occur in children. Patients usually initially present with an incidental finding on imaging, symptoms due to compressive effects or symptoms due to an associated paraneoplastic syndrome such as myasthenia gravis. Half of patients with a thymoma are found to have myasthenia gravis, an autoimmune disorder presenting with diplopia, ptosis, dysphagia, weakness or fatigue. Both thymic tumors originate from thymic epithelial cells. Under microscope, thymomas have lobulations with bands of fibrous stroma, while thymic carcinomas lack lobulated architecture and have more cystic and necrotic changes. Thymomas can develop into carcinomas, although this only occurs after about a decade. Diagnosis is achieved through CT or MRI of the chest, which can differentiate between other mediastinal masses. Treatment may include chemoradiotherapy, immunotherapy, and/or surgical resection.
Lymphoma
Lymphomas comprise 15% of primary mediastinal masses and 45% of anterior mediastinal masses in children. The most common are nodular sclerosing Hodgkin lymphoma, primary mediastinal large B-cell lymphoma and lymphoblastic lymphoma. Most are seen in the anterior compartment and rest are seen in middle compartment. Hodgkin's lymphoma usually present in 40–50's with nodular sclerosing type (7), and non-Hodgkin's appears in all age groups. There is also primary mediastinal B-cell lymphoma with exceptionally good prognosis.
Common symptoms of a mediastinal lymphoma of include fever, weight loss, night sweats, and compressive symptoms such as pain, dyspnea, wheezing, superior vena cava syndrome, and pleural effusions. Diagnosis is usually by CT or MRI showing lobulated mass. Confirmation done by tissue biopsy of accompanying nodes if any, mediastinoscopy, mediastinotomy, or thoracotomy. FNA biopsy is usually not adequate. Treatment of mediastinal Hodgkin's involves chemotherapy and/or radiation. Prognosis is variable and dependent on the lymphoma type and staging. Stage 1 and 2a Hodgkin lymphoma has a good prognosis with 5-year overall survival of 90%.
Germ cell tumors
Germ cell tumors comprise 15% of mediastinal masses and usually affect young adult men between the ages of 18 and 29. These are rare tumors that develop from reproductive cells that grow in the chest instead of the gonads during early development. These tumors resemble testicular germ cell tumors and are classified into three types: teratomas, seminomas, and non-seminomatous tumors such as yolk sac tumors or embryonal carcinomas. Benign teratomas are the most common subtype and may contain tissue such as hair, teeth and bone. Teratomas can rarely be malignant if they are characterized by biopsy as immature with 3 germ layers. Seminomas and nonseminomatous GCTs are rare and malignant. Tumors are initially asymptomatic but may cause chest pain, cough, or shortness of breath when the size causes compression on surrounding structures. The tumor is diagnosed by CT and MRI scans as well as blood markers like AFP, β-hCG and LDH that help classify the type of germ cell tumor. Most teratomas are benign tumors but require close follow-up and treatment. Teratomas are associated with Klinefelter syndrome.Teratomas are usually noncancerous and treated with surgery, while seminomas and non-seminomatous tumors are more aggressive and treated with chemotherapy and surgery. The outlook depends on the tumor classification, with seminomas and teratomas having excellent prognosis and non-seminomatous tumors having a higher chance of metastasis.
Thyroid and parathyroid tumors
While the thyroid and parathyroid are usually located in the neck, they can grow large enough to extend inferiorly and present as mediastinal masses. This is sometimes referred to as a substernal or retrosternal goiter. Alternatively, thyroid tissue may present in the mediastinum from embryonic remnants or ectopic thyroid tissue displaced during embryonic development. Ectopic thyroid tissue can occur anywhere along the embryologic migratory pathway, from the diaphragm to the tongue. This ectopic thyroid tissue represents only 1% of mediastinal masses.
Middle Mediastinal Tumors
Cardiac tumors Cardiac tumors are rare, with estimated prevalence of 0.02%.
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