Mediastinal Cysts

Mediastinal cysts are rather uncommon and account for 10-15% of radiographically detected masses in this area.

There is a wide variety of tissues in this region because of the embryonic relationships of the branchial pouches, foregut anlage, pleuropericardial folds and pharyngeal pouches. Not surprisingly, many of mediastinal cysts have a congenital rather than acquired etiology.

Bronchogenic Cysts

Bronchogenic cysts are small portions of the tracheobronchial tree that separate from the rest of the airways during development. They tend to present from childhood through middle age. Mediastinal bronchogenic cysts tend to be assymptomatic. This is in contrast to intrapulmonary cysts, which are often connected to large airways and present as an infection.

Seventy-five percent of bronchogenic cysts occur in the mediastinum while 25% are surrounded by lung parenchyma.

(The location depends on how early in development the separation occurred.)

The majority of mediastinal cysts are at the carina or adjacent to the pericardium between the root of the aorta and the superior vena cave although they may be found anywhere.

They do not communicate with the rest of the tracheobronchial tree but are usually connected by a non-patent stalk to one of the airways.

Generally solitary, the cysts are thin walled, unilocular and roughly spherical.

They are filled with either mucoid or serous fluid. Radiographically, they present as solitary clearly defined masses of homogeneous density just inferior to the carina and often protruding slightly to the right, overlapping but not obscuring the hilar shadow.

Calcification of the cyst wall is uncommon. On CT they may be confused with solid masses due to the high attenuation of their contents.

Generally asymptomatic even when very large, some cysts in the carina may produce sufficient mass effect when very small to cause problems even when radiographically occult.

Microscopically they have two key features: respiratory-type pseudostratified columnar epithelium, which usually has cilia, and foci of mature cartilage.

Pericardial Cysts

Pericardial cysts are rare fluid filled congenital lesions.

Typically, they are asymptomatic and found incidentally on chest radiograph in patients of all ages.

Most often they appear as a smooth, round, sharply defined mass at the right costophrenic border, but can be seen at the left costophrenic border, the hilum or even the superior mediastinum.

The cysts may vary slightly in size over time but as most patients remain asymptomatic, they may be managed conservatively.

They can generally be differentiated from solid masses or diaphragmatic or hiatal hernias easily by CT. Additionally CT can help differentiate pericardial from bronchogenic cysts as the essentially water density pericardial cysts have a lower attenuation than the proteinaceous fluid in the bronchogenic cysts.

Macroscopically, they have a thin fibrous wall and contain serous fluid.

The gross pathology is confirmed at the microscopic level, where one sees a thin fibrous wall lined by a monolayer of mesothelium.

Thymic Cysts

The thymus is derived from the third and fourth pharyngeal pouches, and cysts are typically located in the anterosuperior mediastinum or low neck. Thymic cysts account for only 2-3% of all anterior mediastinal tumors. As with most of these lesions, they are generally asymptomatic. The findings on a plain radiograph are nonspecific (solid appearing smoothly marginated mass). Computed tomography and MR however, are able to reveal the cystic nature of the lesion. Even with this however, the lesion needs to be thoroughly sampled as several malignant tumors, notably Hodgkin's lymphoma, thymoma, and seminoma may have prominent cystic components.

Congenital thymic cysts are unilocular and acquired thymic cysts are multilocular. Macroscopically, unilocular cysts have a thin wall and contain serous fluid; whereas multilocular cysts have a thick wall with pericystic fibrous adhesions and have thick, hemorrhagic, "cheesy," or turbid contents. Microscopically, these type types of thymic cysts are also quite different. The unilocular cysts have a thin fibrous wall and are lined by a bland squamous monolayer. Multilocular cysts have proliferative features including multilayered squamous epithelium, which may or may not contain a variety of other epithelial types, and a thick fibrous wall, which may contain cholesterol granulomas, hemorrhage or granulation tissue. Hassal's corpuscles or remnants of thymic tissue may also be present.