Classification of mediastinal compartments
The mediastinum is bounded anteriorly by the sternum, posteriorly by the spine, laterally by the lungs, superiorly by the thoracic inlet, and inferiorly by the diaphragm. For diagnostic convenience, it is divided into compartments. The traditional classification distinguished the anterior, middle, and posterior mediastinum. The modern ITMIG radiological classification (based on cross-sectional imaging) defines three spaces:
- Prevascular (prevascular): between the posterior sternum and the anterior pericardial reflection. Contains the thymus, adipose tissue, lymph nodes, and the left brachiocephalic vein.
- Visceral (visceral): from the anterior pericardial reflection to a line 1 cm posterior to the anterior aspect of the vertebral bodies. Includes the trachea, carina, esophagus, thoracic duct, and most vessels (extrapericardial pulmonary vessels belong to the lungs).
- Paravertebral (paravertebral): from the posterior border of the visceral compartment to the lateral edge of the transverse processes. Contains soft tissues and the thoracic spine. Most pathologies here are neurogenic neoplasms.
Diagnostic imaging methods
- Radiography (CXR). Often the first imaging method where pathology is incidentally detected via the blunting, thickening, or displacement of mediastinal lines.
- Computed tomography (CT). The primary method for localizing and assessing the structure of a mass. It identifies areas of varying density (fat, fluid) and calcifications, while contrast enhancement clearly shows the relationship with blood vessels.
- Magnetic resonance imaging (MRI). The method of choice when neurogenic tumors are suspected, as it clearly demonstrates their intraspinal extension. MRI reliably differentiates cysts from solid tumors and detects internal septations. On spin-echo sequences (spin-echo sequences), rapidly flowing blood produces a "signal void" effect, allowing visualization of blood vessels without contrast.
- PET/CT. Used to evaluate lymph nodes in lung cancer and lymphoma. However, thymic hyperplasia and fibrosing mediastinitis can avidly take up FDG, yielding false-positive results.
- Ultrasound. Used for biopsy guidance and for differentiating cardiac and paracardiac masses.
Mass evaluation algorithm
The diagnostic approach consists of three stages:
- Localization within the mediastinum. On a radiograph, a mediastinal mass is distinguished from a pulmonary mass by the absence of the air bronchogram sign and obtuse margins (obtuse margins) at the interface with the lung parenchyma.
- Compartment localization. On a chest X-ray, anterior masses are identified by the hilum overlay sign. On CT, the compartment is determined by the center of mass on the axial slice (where the size is maximal) and by the displacement of structures.
- Tissue characterization. Based on CT or MRI data, masses are categorized by their content type: solid (lymphoma, thymoma), cystic/fluid (cysts), fatty (lipoma, thymolipoma), and vascular (aneurysms).
Key pathologies
Thymomas (thymomas) The most common primary tumors of the anterior (prevascular) mediastinum in adults (mean age 50 years). Up to 50% of patients with thymoma have myasthenia gravis (myasthenia gravis). These tumors originate from thymic epithelium, have a lobulated contour, and may contain necrosis and calcifications. In patients over 40 years of age, they are easily detected on CT against the background of age-related fatty involution of the thymus.
Intrathoracic goiter (intrathoracic goitre) Approximately 3–17% of nodular goiters extend from the neck into the thoracic cavity. On CT, the key feature is continuous tissue continuity between the mass and the cervical thyroid gland. Goiter has a high native attenuation (due to iodine), strongly enhances with contrast, and almost always displaces or compresses the trachea.
Parathyroid masses A rare pathology associated with the migration of the glands into or around the thymus during embryogenesis. It causes hyperparathyroidism. If the ectopic adenoma is not visible on ultrasound, CT combined with 99mTc-sestamibi radionuclide scintigraphy is used.
Briefly: answers to topical questions
Which types of cysts belong to foregut cysts in the mediastinum?
Foregut cysts (foregut cysts) in the mediastinum include several types of congenital bronchopulmonary malformations.
- Bronchogenic cysts (Bronchogenic cysts) — predominantly located in the middle mediastinum.
- Esophageal duplication cysts (Duplication cyst) — also typical of the middle mediastinum.
- Neuroenteric cysts (Neuroenteric cyst) — located in the posterior mediastinum.
What are the histological types of mediastinal germ cell tumors?
The following histological types of germ cell tumors occur in the mediastinum:
- Teratomas (Teratomas) — usually follow a benign course, but may contain malignant elements.
- Seminomas (Seminoma) — the most common malignant germ cell tumor of the mediastinum.
- Non-seminomas — malignant tumors whose primary localization in the mediastinum belongs to the poor-prognosis group.
Specifically, which neurogenic tumors are located in the paravertebral compartment of the mediastinum?
Neurogenic tumors located in the paravertebral compartment of the mediastinum include, in particular:
- Schwannomas (Schwannoma).
- Ganglioneuromas (Ganglioneuromas).
- Ganglioneuroblastomas (Ganglioneuroblastomas).
- Neuroblastomas (Neuroblastomas).
Pheochromocytomas as a variant of paragangliomas may be localized in the posterior mediastinum.
What paraneoplastic syndromes and diseases are associated with thymoma, besides myasthenia gravis?
In addition to myasthenia gravis, thymoma is associated with the development of pure red cell aplasia.
- Pure red cell aplasia (acquired pure red blood cell aplasia) is a condition in which erythroblasts are almost completely absent from the bone marrow while other hematopoietic lineages are preserved. This pathology occurs predominantly in adult patients and is often associated with the presence of a benign thymoma.
According to the international IASLC classification, into which groups (stations) are mediastinal lymph nodes divided?
According to the international IASLC classification, mediastinal lymph nodes are divided into 7 anatomical zones, including specific stations:
- Supraclavicular zone (supraclavicular) — stations 1R and 1L.
- Upper zone (upper) — upper paratracheal (2R, 2L), prevascular (3a), retrotracheal (3p), and lower paratracheal (4R, 4L).
- Aortopulmonary zone (aortopulmonary).
- Subcarinal zone (subcarinal).
- Lower zone (lower).
- Hilar zone (hilar).
- Peripheral zone (peripheral).
Diagnostic algorithm for mediastinal masses: 1. Confirm localization (obtuse margins at the lung interface). 2. Determine the compartment (by center of mass). 3. Evaluate the contents (solid, fluid, fat, vessels).
Class and exam questions
How to distinguish a mediastinal mass from a pulmonary lesion on an X-ray?
A mediastinal mass forms obtuse margins with the lung parenchyma, deforms mediastinal lines, and lacks the air bronchogram sign.
Why is MRI prescribed for mediastinal tumors if CT is already available?
MRI better differentiates solid masses from cystic ones, demonstrates internal septations, and is the method of choice for assessing intraspinal extension of neurogenic tumors.
Why is it difficult to detect a small thymoma in young patients?
In patients younger than 30 years, the thymus has not yet undergone fatty involution, so normal glandular tissue can mask small tumor nodules.
What else to study on this topic
- Detailed analysis of the ITMIG classification (Table 4.2) for each tissue subtype.
- Radiological signs of mediastinal pathology: hilum overlay sign, widening of paratracheal stripes.
- Features of PET/CT diagnostics: causes of false-positive and false-negative results.
- Specifics of the differential diagnosis of invasive and non-invasive thymomas.
- Radionuclide imaging of ectopic parathyroid and thyroid glands.
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