Where is the anatomic location of the anterior mediastinum?
Can the anterior mediastinum be further divided? If so, what are their contents?
- The anterior mediastinum is the space between the sternum and the pericardium inferiorly and ascending aorta and brachiocephalic vessels superiorly.
- The anterior mediastinum can be thought of as two compartments - the prevascular compartment superiorly and the precardiac compartment inferiorly.
- The contents of the prevascular anterior mediastinum include:
- Lymph nodes.
- Enlarged thyroid gland, if it extends inferiorly into the mediastinum.
- The precardiac anterior mediastinum is a potential space.
What are the anatomic borders of the middle mediastinum?
What are its contents?
- The anterior border of the middle mediastinum is the anterior pericardium and the posterior borders are the posterior pericardium and posterior tracheal wall.
- The contents of the middle mediastinum include:
- Heart and pericardium.
- Ascending aorta and aortic arch.
- Great vessels including SVC, IVC, pulmonary arteries and veins, and brachiocephalic vessels.
- Trachea and bronchi.
- Lymph nodes.
- Phrenic, vagus, and recurrent laryngeal nerves (all of which pass through the AP window).
What are the anatomic boundaries of the posterior mediastinum?
What are its contents?
- The anterior border of the posterior mediastinum is the posterior trachea and posterior pericardium. The posterior border is somewhat loosely defined as the anterior aspect of the vertebral bodies; however, paraspinal masses are generally included in the differential of a posterior mediastinal mass.
- The contents of the posterior mediastinum include:
- Descending thoracic aorta.
- Azygos and hemiazygos veins.
- Thoracic duct.
- Vagus nerves.
- Lymph nodes.
Mediastinum Lines, Stripes, and Interfaces
What are the definitions of each above?
Name all the mediastinal lines and stripes
Which ones can you normally see?
- Interfaces between anatomic structures in the lungs, mediastinum, and pleura may be displaced or thickened in the presence of a mediastinal mass or abnormality. With the exception of the right paratracheal stripe, it is generally uncommon to see these interfaces in the absence of pathology.
- A “line” is a thin interface formed by tissue (typically <1 mm in thickness) with air on both sides.
- A “stripe” is a thicker interface formed between air and adjacent soft tissue.
- An “interface” is formed by the contact of two soft tissue structures of different densities.
Anterior Junction Line
What forms this line?
Where is it located?
Abnormal convexity or displacement of this line suggests what?
- The anterior junction line is formed by four layers of pleura (parietal and visceral pleura of each lung) at the anterior junction of the right and left lungs.
- On a frontal radiograph, the anterior junction line is a vertical line projecting over the superior two-thirds of the sternum.
- Abnormal convexity or displacement of this line suggests an anterior mediastinal mass.
Posterior Junction Line
What forms this line and where?
Where is it located on a frontal radiograph? Contrast this to anterior junction line.
Abnormal convexity of displacement of this line is suggestive of what?
- As with the anterior junction line, the posterior junction line is also formed by four layers of pleura (parietal and visceral pleura of each lung), but at the posterior junction of the right and left lungs.
- On a frontal radiograph, the posterior junction line is a vertical line projecting through the trachea on the frontal view, more superior than the anterior junction line. Unlike the anterior junction line, the posterior junction line is seen above the clavicles because the posterior lungs extend more superiorly than the anterior lungs.
- Abnormal convexity or displacement of this line suggests a posterior mediastinal mass or aortic aneurysm.
Right and Left Paratracheal Stripes
What are these formed by?
Which one is more often seen on normal CXRs?
What is the MCC of right paratracheal stripe thickening? What are other possible causes?
What is the DDx for left paratracheal stripe thickening? How about in the setting of trauma?
- The right and left paratracheal stripes are formed by two layers of pleura where the medial aspect of each lung abuts the lateral wall of the trachea and intervening mediastinal fat.
- The right paratracheal stripe is the most commonly seen of these landmarks, seen in up to 97% of normal PA chest radiographs.
- Thickening of the right paratracheal stripe is most commonly due to pleural thickening, although a paratracheal or tracheal mass (including adenopathy or thyroid or tracheal neoplasm) can also be a cause.
- Thickening of the left paratracheal stripe has a similar differential. In addition, however, a mediastinal hematoma should also be considered, especially in trauma.
What is the only interface seen on the lateral radiograph?
What does it represent?
- The posterior tracheal stripe is the only interface seen on the lateral radiograph, representing the interface of the posterior wall of the trachea with the two pleural layers of the medial right lung.
Right and Left Paraspinal Lines
What forms these?
Contrast this to posterior junction line.
DDx for paraspinal line abnormality?
- The right and left paraspinal lines are actually interfaces but appear as lines due to Mach effect and are formed by 2 layers of pleura abutting the posterior mediastinum.
- In contrast to the posterior junction line, the paraspinal lines are located inferiorly in the thorax, typically from the 8th through 12th ribs.
- A paraspinal line abnormality suggests a posterior mediastinal mass lesion, including hematoma, neurogenic tumor, aortic aneurysm, extramedullary hematopoiesis, esophageal mass, and osteophyte.
What forms this?
Where is it located?
DDx for distortion of the azygoesophageal recess?
- The azygoesophageal recess is an interface formed by the contact of the posteromedial right lower lobe and the retrocardiac mediastinum.
- The azygoesophageal recess extends from the subcarinal region to the diaphragm inferiorly.
- Distortion of the azygoesophageal recess may be due to esophageal mass, hiatal hernia, left atrial enlargement, and adenopathy.
What is this? Where is it located? What is its medial border?
On normal frontal CXR, where is it located?
Abnormal convexity (outward bulging) of the AP window is suggestive of what?
- The aortopulmonary (AP) window is a mediastinal space nestled underneath the aortic arch (which forms the superior, anterior, and posterior boundaries) and the top of the pulmonary artery. The medial border of the AP window is formed by the esophagus, trachea, and left mainstem bronchus.
- On a normal frontal radiograph, the AP window is a shallow concave contour below the aortic knob and above the pulmonary artery.
- Abnormal convexity (outwards bulging) of the AP window suggests a mass arising from or involving structures that normally live within the AP window, including:
- Lymph nodes: Adenopathy is the most common cause of an AP window abnormality.
- Left phrenic nerve: Injury may cause paralysis of the left hemidiaphragm.
- Recurrent laryngeal nerve: The AP window should be carefully evaluated in new-onset hoarseness, especially if associated with diaphragmatic paralysis.
- Left vagus nerve.
- Ligamentum arteriosum.
- Left bronchial arteries.
- A thoracic aortic aneurysm may also cause convexity of the AP window.
Retrosternal Clear Space
What is it? What space does it correlate with on CT?
Obliteration of this space is suggestive of what?
Increase in the retrosternal clear space can be seen in what?
- The retrosternal clear space is a normal area of lucency posterior to the sternum seen on the lateral radiograph only. It correlates to the prevascular space on CT.
- Obliteration of the retrosternal clear space suggests an anterior mediastinal mass, right ventricular dilation, or pulmonary artery enlargement.
- Increase in the retrosternal clear space can be seen in emphysema.
What is the left superior intercostal vein?
When visible, what does it produce?
When may it be dilated?
- The left superior intercostal vein (LSIV) is a normal vein that is not often seen on radiography. When visible, it produces the aortic nipple, appearing as a small round shadow to the left of the aortic knob on the frontal radiograph.
- It may be dilated as a collateral pathway in SVC obstruction.
How does one detect an anterior mediastinal mass?
What is the hilum overlay sign?
What is a direct sign of an anterior mediastinal mass?
- Deformation of the anterior junction line suggests an anterior mediastinal mass. However, since the anterior junction line is not always seen, it is more common to infer the anterior location of a mass by the preservation of the posterior lines in the presence of a mass.
- The hilum overlay sign is present on the frontal view if hilar vessels are visualized through the mass. It indicates that the mass cannot be in the middle mediastinum. The mass may be in the anterior (most likely) or posterior mediastinum.
- Obliteration of the retrosternal clear space on the lateral radiograph is a direct sign of anterior mediastinal location.
How do you detect a middle mediastinal mass?
- Distortion of the paratracheal stripes or convexity of the AP window suggests a middle mediastinal mass.
How does one detect a posterior mediastinal mass?
- Distortion of the azygoesophageal recess, distortion of the posterior junction line, or displacement of the paraspinal lines suggests paravertebral/posterior mediastinal disease.
DDx for prevascular anterior mediastinal masses?
Which one is associated with myasthenia gravis?
What if the patient is a young adult?
What if there is extension of the mass above the thoracic inlet?
Whats the worst case scenario?
- Thymic epithelial neoplasm, such as thymoma if the patient is middle-aged or older, or has a history of myasthenia gravis. Less common would be thymic carcinoma.
- Germ cell tumor, including Teratoma, if the patient is a young adult.
- Thyroid lesion, if there is extension of the mass above the thoracic inlet.
- Lymphoma (Terrible).
- MNEMONIC: The 4 Ts in the anTerior mediastinum
Prevalence relative to other anterior mediastinal masses? Epidemiology?
Most common association? How many people with this association have a thymoma, and visa versa?
Other associations with thymoma?
Pathologic classifications of thymoma?
What percent of thymomas are invasive? What can it invade?
What is suggestive of phrenic nerve invasion?
What is a drop mets for thymoma? Does thymoma metastasize hematogenously?
What are histo subtypes of thymomas and which have worse prognosis?
- Thymoma is the most common primary tumor of the anterior mediastinum and typically occurs in middle-aged or older individuals, between 45 and 60 years.
- Thymoma is associated with myasthenia gravis (MG). Approximately 33% of patients with thymoma have MG, and 10% of patients with MG have a thymoma.
- In addition to MG, thymomas are often associated with other diseases including red cell aplasia, hypogammaglobulinemia, paraneoplastic syndromes, and malignancies such as lymphoma or thyroid cancer.
- Thymoma can be pathologically classified as low-risk or high-risk based on histology, and non-invasive or invasive based on whether the capsule is intact.
- Approximately 30% of thymomas are invasive. If invasive, the tumor may invade adjacent structures including the airways, chest wall, great vessels, and phrenic nerves.
- Elevation of a hemidiaphragm is suggestive of phrenic nerve invasion.
- Invasive thymoma may spread along pleural and pericardial surfaces, called drop metastases. However, hematogenous metastases are exceedingly rare.
- Thymoma is histologically classified by the WHO system into A, AB, B1, B2, B3, and C subtypes, with progressively worse prognosis.
- Type A tumors are relatively uncommon, but are usually encapsulated.
- Type B tumors contain increasing number of epithelial cells, which represent the malignant component.
- Type C represents thymic carcinoma, which may metastasize hematogenously.
Other than thymoma, what are the other less common thymic lesions?
- Thymic carcinoma
- Thymic carcinoid
- Thymic cyst
Compare invasive thymoma with thymic carcinoma.
How can you distinguish the two with imaging?
- Unlike invasive thymoma, thymic carcinoma is histologically malignant, very aggressive, and often metastasizes hematogenously to lungs, liver, brain, and bone. Prognosis is poor.
- The distinction between invasive thymoma and thymic carcinoma is difficult on CT, unless there is evidence of distant metastatic disease.
The cell of origin?
What percent of these are hormonally active? Often secreting what hormone and causing what syndrome?
What syndrome is thymic carcinoid associated with?
How can you distinguish thymic carcinoid from thymoma and thymic carincoma on CT?
If carcinoid is suspected, what can be done to confirm?
- Thymic carcinoid is of neural crest origin. 50% of thymic carcinoids are hormonally active, often secreting ACTH and causing Cushing syndrome.
- Thymic carcinoid is associated with multiple endocrine neoplasia (MEN) I and II.
- On imaging, thymic carcinoid is generally indistinguishable from thymoma and thymic carcinoma on CT.
- If carcinoid is suspected, a preoperative Indium-111 octreotide scan can be performed.
What may this be secondary to?
When multilocular, what would you think of?
When congenital, what embryo tissue is it arising from? Where can they be located and where are they most often located?
- A thymic cyst may be secondary to radiation therapy (e.g., administered to treat Hodgkin disease), may be associated with AIDS (especially when multilocular), or may be congenital.
- When congenital, thymic cysts arise from remnants of the thymopharyngeal duct. A congenital thymic cyst may occur anywhere along the course of thymic descent from the neck, but most commonly in the anterior mediastinum.
- Thymic cyst is typically evident on CT as a simple fluid-attenuation cyst in the anterior mediastinum.
What is a thymolipoma? What does it have a tendency to do?
- Thymolipoma is a benign fat-containing lesion with interspersed soft tissue.
- It may become quite large and drape over the mediastinum.
Germ Cell Tumors
These are usually benign or malignant? Malignant GST occurs more commonly in whom?
What is the most common anterior mediastinal GST? What is it’s imaging appearance? What is a specific finding but not commonly seen? What would make you say this lesion is malignant?
What is the second most common malignant anterior mediastinal mass and who is it almost exclusively seen in?
- Several different types of germ cell tumors may arise in the anterior mediastinum from primitive germ cell elements, most of which are benign. Malignant GCT occurs more commonly in males.
- Teratoma is the most common anterior mediastinal germ cell tumor, usually encapsulated and predominantly cystic in nature, but fat and calcification are common. A fat/fluid level is specific for teratoma, but is not commonly seen. Teratoma can rarely be malignant, especially if large in size and irregular in shape.
- Seminoma is the most common malignant anterior mediastinal germ cell tumor. It occurs almost exclusively in men.
Thyroid Lesion in Anterior Mediastinum
What kinds of thyroid lesions can extend to anterior mediastinum?
What is the key to diagnosis?
What is the cervicothoracic sign? (in diagram)
How might a goiter end up in the posterior mediastinum? (in diagram)
- Benign and malignant thyroid masses may extend into the mediastinum, including goiter, thyroid neoplasm, and an enlarged gland due to thyroiditis.
- The key to diagnosis is to show continuity superiorly with the thyroid.
Anterior Mediastinum Thymoma
What kinds of lymphoma are important DDx considerations for an anterior mediastinal mass?
Which one more commonly involves the anterior mediastinum?
What imaging characteristic is rarely seen in untreated lymphoma?
- Both Hodgkin disease and non-Hodgkin lymphoma are important differential considerations for an anterior mediastinal mass.
- Hodgkin disease commonly involves the thorax, most often the superior mediastinal lymph nodes including prevascular, AP window, and paratracheal nodal stations.
- Non-Hodgkin lymphoma is a diverse group of diseases, which less commonly involve the thorax compared to Hodgkin disease.
- Calcification is rare in untreated lymphoma.
What features are unusual for lymphoma and would prompt consideration of an alternative diagnosis?
What feature is often seen in silicosis? This may be seen more commonly overall in what entity?
Dense calcification within a lymph node can be seen in what disease(s)?
Low attenuation lymph nodes should raise concern for what dz? In what other diseases can it be seen?
Avid lymph node enhancement can be seen in what disease(s)?
Avidly enhancing vascular metastases include?
- Lymph node calcification, low attenuation, and avid enhancement are unusual features for lymphoma. An alternative diagnosis should be considered if these imaging findings are seen.
- Eggshell calcification of lymph nodes is often present in silicosis and coal workers pneumoconiosis, less commonly in sarcoidosis. Given the greater prevalence of sarcoidosis, however, eggshell calcification may be seen more commonly in sarcoid overall.
- Dense calcification within a lymph node can be seen in sarcoidosis or as a sequela of prior granulomatous disease.
- Low attenuation lymph nodes, while nonspecific, should raise concern for active tuberculosis. Low attenuation lymph nodes can also be seen in fungal infection, lymphoma, and metastatic disease.
- Avid lymph node enhancement can be seen in Castleman disease, sarcoidosis, tuberculosis, and vascular metastases.
- Avidly enhancing vascular metastases include:
- Renal cell carcinoma.
- Thyroid carcinoma.
- Lung carcinoma.
What is it?
Localized disease is seen in whom? Treatment?
Multicentric disease manifests in whom? Clinical sequela? Treatment?
Key imaging finding?
- Castleman disease, also known as angiofollicular lymph node hyperplasia, is a cause of highly vascular thoracic lymph node enlargement, of uncertain etiology.
- Localized Castleman disease is seen in children or young adults. Surgical resection is usually curative.
- Multicentric Castleman disease manifests in older patients or in association with AIDS. Multicentric disease often results in systemic illness including fever, anemia, and lymphoma. It is typically treated with chemotherapy.
- The key imaging finding of Castleman disease is avidly enhancing adenopathy.
What is considered to be a precardiac anterior mediastinal mass?
What is the DDx?
Which one may simulate cardiomegaly on a frontal radiograph?
- A precardiac anterior mediastinal mass is in contact with the diaphragm.
- Epicardial fat pad, pericardial cyst, morgagni hernia.
- A prominent epicardial fat pad silhouettes the cardiac border on a frontal radiograph and may simulate cardiomegaly.
What is it? Where are they most commonly located?
Imaging appearance? What does it do on subsequent studies?
- A pericardial cyst is a benign cystic lesion thought to be congenital. Most are located at the right cardiophrenic angle (between the heart and the diaphragm).
- Imaging shows a cystic lesion abutting the pericardium, which may change in shape on subsequent studies.
What is it?
Where does it usually occur?
What is diagnostic of this?
If this diagnostic clue is absent, what other diagnostic key is used?
- Morgagni hernia is a diaphragmatic hernia through the foramen of Morgagni, containing omental fat and often bowel.
- A Morgagni hernia usually occurs on the right.
- An anterior mediastinal mass in contact with the diaphragm containing bowel gas is diagnostic for a Morgagni hernia.
- If bowel gas is absent, a key to diagnosis on CT is the detection of omental vessels in the mass which can be traced into the upper abdomen.
What are the middle mediastinal masses?
- Lymphadenopathy - an important cause of middle mediastinum mass on radiography.
- Ascending aortic or aortic arch aneurysm - may appear as middle mediastinum mass on radiography.
- Enlarged pulmonary artery
- Foregut duplication cyst
Enlarged pulmonary artery simulating middle mediastinum mass:
What sign can help distinguish between an enlarged PA and a mediastinal mass?
What does this sign representing?
- An enlarged pulmonary artery (PA) can simulate a mass on a chest radiograph.
- The hilum convergence sign can help distinguish between an enlarged PA and a mediastinal mass. The hilum convergence sign shows the peripheral pulmonary arteries converging into the “mass” if the mass represents an enlarged pulmonary artery.
Foregut Duplication Cysts
What do these include?
Where can these occur?
- Foregut duplication cysts include:
- Bronchogenic cysts
- Esophageal duplication cysts
- Neurenteric cysts.
- Foregut duplication cysts may occur in the middle or posterior mediastinum.
Posterior Mediastinal Masses
Name them all!
- Descending thoracic aortic aneurysm - may appear as a posterior mediastinal mass on radiography
- Hiatal hernia
- Esophageal neoplasm
- Foregut duplication cyst
- Neurogenic tumor
- Lateral meningocele
- Paraspinal abscess
- Extramedullary hematopoiesis
- Artery, GI stuff, neuro stuff, abscess, that last thing
Where can these arise from? In adults where do they usually arise from? In children?
Overall prevalence of neurogenic tumors relative to other posterior mediastinum masses?
What are the peripheral nerve tumors?
What are the sympathetic ganglion tumors? Describe these.
- A neurogenic tumor may arise from either a peripheral nerve or the sympathetic ganglia. Most adult tumors are peripheral nerve sheath tumors and the vast majority of tumors in children are of sympathetic ganglionic origin. Overall, neurogenic tumors are the most common posterior mediastinal masses.
- Peripheral nerve tumors (more common in adults) include:
- Schwannoma (most common), neurofibroma, and malignant peripheral nerve sheath tumor.
- Sympathetic ganglion tumors (more common in children/young adults) include:
- Ganglioneuroma (most common), a benign tumor of sympathetic ganglion cells.
- Neuroblastoma, a malignant tumor of ganglion cells seen in early childhood.
- Ganglioneuroblastoma, intermediate in histology between ganglioneuroma and neuroblastoma, seen in older children than neuroblastoma.
What is a hiatal hernia?
What is the esophageal hiatus?
- A hiatal hernia is protrusion of a portion of the stomach through the esophageal hiatus.
- The esophageal hiatus is an elliptical opening in the diaphragm just to the left of midline.
- On radiography, air or an air-fluid level is present above the diaphragm.
In what scenarios can this present as posterior mediastinal mass?
Where do these originate from?
- Extramedullary hematopoiesis causes soft tissue paravertebral masses in patients with severe hereditary anemias including thalassemia and sickle cell anemia.
- The masses are of uncertain origin but may represent herniation of vertebral marrow or may represent elements of the reticuloendothelial system.
- On imaging, lobulated soft tissue masses are typically bilateral and inferior to T6.
What is it?
What is it associated with?
- A lateral meningocele is lateral herniation of the spinal meninges through either an intervertebral foramen or a defect in the vertebral body. Lateral meningocele is associated with neurofibromatosis.
How can an esophageal carcinoma present on radiography?
What other benign mesenchymal tumors can present similarly?
- Esophageal carcinoma can present on radiography as abnormal convexity of the azygoesophageal recess, mediastinal widening, or a retrotracheal mass. Benign mesenchymal esophageal tumors, such as leiomyoma, fibroma, or lipoma may appear similar on radiography.
Foregut Duplication Cyst
What is it?
Where do they occur?
- A foregut duplication cyst, a remnant of the fetal foregut, may present as a middle or posterior mediastinal mass.
What is a clue to vertebral body pathology on radiography?
Give an example!
- A clue to vertebral body pathology on radiography may be paraspinal line displacement.
- Paraspinal Abscess!