Anterior Mediastinal Masses

The four most common anterior mediastinal tumors are thymomas, teratomas, lymphomas, and enlarged or ectopic thyroid tissue. These are often referred to as the "4 T's" (Thymoma, Teratoma, Terrible lymphoma, and Thyroid.)

In an adult, an upper anterior mediastinal mass (i.e. above the clavicles on the PA projection) which causes tracheal deviation is most likely of thyroid origin.

Below the clavicles, thymoma is the most common.

Thymoma

Thymoma are typically located at the junction of the heart and great vessels, although in about half the cases they project over the hilum.

They vary from very small, subtle opacities (17% of cases) to very large masses that project over the heart and are easily mistaken for the heart border.

Their shapes are round to oval with smooth or lobulated borders and are not commonly calcified, (about 7% of cases).

When calcifications are present, they can range from coarse, dense and irregular to ring-like.

Most often, thymomas are found in patients between the ages of 40 and 60 and as often in men as in women.

About 1/3 of patients present with symptoms due to compression or invasion of thoracic structures, usually chest pain, dyspnea or cough.

About 35% present with myasthenia gravis.

Less commonly, hypogammaglobulinemia or anemia due to erythrocyte hypoplasia are seen.

Sometimes, particularly with myasthenia gravis, the tumor is occult with the resected gland appearing normal.

Histologically thymomas are composed of lymphocytes and epithelial cells, and categorized based upon the dominant cell type. It has been found, however, that the patient's prognosis relates more to the extent of invasion than the cell type.

Thus thymomas are classified as encapsulated (benign cytology with a complete fibrous capsule), invasive, (benign cytology but an infiltrating growth pattern), or metastasizing (benign cytology with pleural or parenchymal seeding, but no extension outside the chest).

Computed tomography aids in the evaluation of suspected tumors by revealing the presence, absence or uncertainty of invasion as well as possibility of metastases.

A certain diagnosis of thymoma, however, requires a tissue sample. As the treatment for thymoma is surgical resection, definitive diagnosis and treatment are performed concurrently.

The majority of thymomas are benign, and patients have a survival rate equal to that of the general population.

However, if the tumor is invasive, the prognosis is much worse, with a long time survival of 12.5%.

Teratoma

Germ cell tumors derive from primitive cells which differentiate into a spectrum of benign and malignant neoplasms.

Teratomas make up the large majority of mediastinal germ cell tumors and are composed of cells derived from 2 or 3 of the embryonic layers which have developed abnormally.

Most commonly seen in young adults, teratomas generally cause no symptoms unless they are very large when they may cause mass effect on adjacent structures.

The tumors can be divided into 5 types:

mature solid,

cystic (dermoid cyst),

immature,

malignant (teratocarcinoma)

and mixed.

The mature solid type makes up about 75% of all mediastinal germ cell tumors.

They are made up predominantly of ectodermal components (hair, calcifications, sebaceous material) and have a large cystic component.

The plain film appearance is very similar to that of thymoma. By CT, the cystic component is generally seen as are the regions of calcifications.

Additionally, and most important diagnostically, in up to 50% of cases regions of fat or material which is of mixed low attenuation and has a density closer to that of fat than to water is present.

Fat/fluid levels have also occasionally been reported.

The treatment for teratomas, whether benign or malignant, is resection as the tumor may grow to impinge on adjacent structures.

The prognosis is very good.

Lymphoma

Lymphoma accounts for 10-20% of all anterior mediastinal masses.

Although lymphoma is most common between 30 and 40 years of age, is it the second most common anterior mediastinal mass in children.

In adults, the nodular sclerosing form of Hodgkin's disease accounts for most of the lymphomas seen. When confined to the anterior mediastinum, lymphoma may be asymptomatic or present with cough, chest pain or less commonly with mass effect on, or invasion into, adjacent structures. Radiographically, primary mediastinal lymphoma look much like the other anterior mediastinal masses.

However, secondary signs, such as pleural effusion or erosion of the sternum are common in Hodgkin's disease while rare in the other masses. Calcification is rare. Computed tomography has proven useful in that it often will show that the apparently solid masses seen on plain film is actually composed of enlarged matted lymph nodes separated by well defined interfaces.

Additionally, one study found that 14% of lyrnphomatous masses had associated adenopathy elsewhere in the chest.

Resection is not necessary, however, thoracotomy or mediastinoscopy are usually required to confirm the diagnosis. Chemotherapy or radiation are the common modes of treatment.

Prognosis varies with the histology.

Thyroid [see my pathology and endcrinology page for details]

The majority of mediastinal thyroid tissue is a substernal extension of a cervical goiter. About 75% extend anterior to the trachea, while the rest extend posteriorly behind the trachea, innominate and subclavian vessels. Most patients are women (4:1 ratio to men) and are in their fifth decade. On plain film the mass is well defined and homogeneous, causing smooth displacement and often narrowing of the trachea. Calcification is common. Although 131I studies are usually diagnostic and should be the first study to evaluate the suspected mass, false negative results occur. In the face of a negative radionuclide study but clinical suspicion of an intrathoracic goiter, a CT exam should be performed. By CT, five findings are characteristic of mediastinal thyroid:

continuity with the cervical thyroid.

well defined borders.

coarse, punctate or ring-like calcifications.

inhomogeneity, with discrete non-enhancing low attenuation regions

attenuation values at least 15H greater than adjacent muscles before intravenous contrast administration and 25H after the contrast is given (due to the normally high iodine in the thyroid tissue).

Additionally, as the mass is encapsulated and does not invade, the displaced mediastinal vessels generally are arranged concentrically around the mass. A second, much rarer presentation of intrathoracic goiter is from heterotopic tissue. It is important to make the differentiation between these two types for surgical planning as each has a distinct blood supply. An intrathoracic thyroid is usually asymptomatic, but it is easily diagnosed by an 131I scan, and does not require resection, unless there are symptoms