There are many tissues and organs in the mediastinum, and the sources of embryonic development are complex, so there are many types of primary mediastinal tumors and tumorlike lesions. Generally, mediastinal tumors and tumorlike lesions have specific predilection sites。In the thoracic inlet, there are mostly thyroid mass in adults and lymphangioma in children. In the anterior mediastinum, the most common is thymoma and teratoma, the tumor in the cardiophrenic angle is mostly pericardial cyst and lipoma, especially in the right anterior cardiophrenic angle. In the middle mediastinum, due to abundant lymphatic tissue, the most common is lymphoma and mediastinal lymph node metastasis, followed by bronchogenic cyst. In the posterior mediastinum, due to abundant nerve tissue, neurogenic tumor is common, mainly neurofibroma, schwannoma, and ganglioneuroma, and may be accompanied by abnormal changes in local vertebral bone. Therefore, clarification of the anatomical structure and tissue composition of each area of the mediastinum is helpful for the accurate positioning and qualitative diagnosis of the lesion.
Clinical presentation and pathology
The clinical manifestations of mediastinal tumor and tumorlike lesion are closely related to the size, site, nature, and growth pattern. There are often no apparent symptoms and positive signs in the early stage. In the benign tumor and tumorlike lesion, due to slow growth, corresponding compression symptoms appear only after it develop into a large size. For example, compression of the superior vena cava may cause thickening of the jugular vein, and edema of the head, neck, face, and upper limbs; compression of the trachea may cause irritating dry cough and tachypnea; compression of the phrenic nerve may cause hiccups and diaphragmatic paralysis; compression of the sympathetic nerve may cause Horner syndrome; compression of the vagus nerve may cause bradycardia, nausea, and emesis; invasion of the recurrent laryngeal nerve may cause hoarseness; compression of the esophagus may cause dysphagia. Malignant tumor has rapid progression and high invasiveness, and clinical symptoms may occur when the tumor is small. Some tumors and tumorlike lesions in the mediastinum have characteristic clinical manifestations, about 1/3 of thymoma patients have myasthenia gravis, few patients with retrosternal goiter may have hyperthyroidism; hair and sebum-like substances may be coughed up when dermoid cyst or teratoma ruptures into the bronchus.
Imaging manifestations
Intrathoracic goiter
Intrathoracic goiter is at the entrance of the mediastinum and often extends to unilateral anterosuperior mediastinum. Due to the high iodine content of the tumor, its density is often higher than that of the peripheral soft tissue on CT. MRI often shows slightly long T1 and long T2 signals. Cystic changes or calcification often occur in the tumor, and its density or signal intensity may be inhomogeneous at this time. In addition, it should be noted that intrathoracic goiter can also cause thyroid tumor and adenocarcinoma, and corresponding manifestations appear on CT and MRI.
Thymoma
Thymoma is often in the middle and upper part of the anterior mediastinum and often occurs in individuals over age 30. Histologically, it is divided into different subtypes according to the ratio of lymphocytes and epithelial cells in the tumor. Traditionally, it is divided into invasive and non-invasive thymoma, and 10% - 15% are invasive thymoma. CT shows homogeneous massive opacity of soft tissue density. When cystic changes occur in the tumor, the density is inhomogeneous.
Figure 1 Thymoma
a. Plain CT shows massive opacity of soft tissue density in the anterior mediastinum with smooth edges; b. Contrast-enhanced CT shows slightly inhomogeneous enhancement.
MRI shows inhomogeneous slightly long T1 and long T2 signals. Invasive thymoma has unclear edges and inhomogeneous intensity. Adjacent structures are often involved and may be accompanied by pleural metastasis. Contrast-enhanced MRI shows homogeneous or inhomogeneous enhancement of the tumor.
Teratoma
Teratoma is often in the middle of the anterior mediastinum, including cystic and solid teratoma. Cystic teratoma is also termed dermoid cyst, and contains ectoderm and mesoderm tissues and may show cystic density on CT. Solid teratoma contains tissues from three germ layers.
CT shows mixed density. Characteristic manifestations such as fat-fluid level, bones, and teeth in the mass are helpful for qualitative diagnosis. Due to the complex composition of the mass, MRI often shows mixed signals.
Figure 2 Teratoma
a. Plain CT shows a mass on the right side of the anterior mediastinum with inhomogeneous density and irregular calcification; b, c. Contrast-enhanced coronal and sagittal CT show inapparent enhancement in the mass, nonenhanced low-density fatty area, and compression of adjacent blood vessels.
Lymphoma and lymph node metastasis
Lymphoma is often in the anterior and middle mediastinum. Chest x-ray shows that the mediastinum is widened to both sides with wavy edges. CT shows multiple enlarged lymph nodes of inhomogeneous soft tissue density, which can fuse into mass. MRI mostly shows slightly long T1 and long T2 signals. The mass is moderately enhanced on contrast-enhanced CT and MRI, and the mass is prone to be wrapped by blood vessels. Enlarged lymph nodes in multiple parts of the body help to suggest diagnosis. Lymph node metastasis is often manifested by multiple lymphadenopathies in the mediastinum, mainly level 4 and 7 lymph nodes, with unclear boundaries and fusion. Contrast-enhanced scan shows inhomogeneous enhancement. Tumor often found in the lungs and other parts is helpful for diagnosis.
Figure 3 Lymphoma
a. Plain CT shows multiple, variously sized, fused, enlarged lymph nodes in the anterior and middle mediastinum; b. Contrast-enhanced CT shows homogeneous enhancement of the enlarged lymph nodes.
Neurogenic tumor
Neurogenic tumor is mostly in the posterior mediastinum, mainly neurofibroma, neurolemmoma, and ganglioneuroma. Most masses are subrounded on CT and MRI. If part of the mass is in the spinal canal and part is beside the spine, doughnut pattern can be seen, and the mass is often accompanied by enlargement of the corresponding intervertebral foramen. The mass shows mostly homogeneous opacity of soft tissue density or homogeneous signal, with smooth edges. The mass may be accompanied by calcification and cystic changes. Malignant tumor such as neuroblastoma may be accompanied by massive calcification, vertebral bone destruction, and involvement of adjacent soft tissues.
Figure 4 Neurolemmoma in the posterior mediastinum
a. Coronal MRI shows a subrounded mass (↗) in the paravertebral sulcus on the left side of the posterior mediastinum, and the inner side is connected to the intervertebral foramen and partially extends into the spinal canal; b. Contrast-enhanced T1WI shows inhomogeneous moderately enhancement of the mass (↗).
Cystic mass
Common cystic masses in the mediastinum include cystic lymphangioma, bronchogenic cyst, and pericardial cysts. Cysts are closely related to the organs of origin, and their positioning is usually clear. On CT examination, the lesions are mostly watery in density, and the CT value is usually 0 - 20HU; but when the cyst fluid is rich in protein or there is intracystic hemorrhage, the CT value can be as high as 30 - 40HU, and CT shows opacity of soft tissue density. Different cysts are difficult to distinguish on plain CT, and contrast-enhanced CT or MRI may be required. Solid tumor has enhancement, while high-density cyst has no enhancement. MRI shows predominantly high signal on T2WI, resembling watery signal. Generally, MRI is better than CT in the diagnosis of cystic mass in the mediastinum, and has higher sensitivity for detecting intracystic hemorrhage.
Diagnosis and differential diagnosis
In the imaging diagnosis and differential diagnosis of mediastinal tumor and tumorlike lesion, attention should be paid to the site, density or intensity, and edges and changes of adjacent structures. Different lesions have different predilection sites, and their source can be inferred based on the site of the lesions. Density or intensity can indicate the lesion is solid, cystic, cystic and solid, or even fatty, and its nature can also be inferred based on the density or intensity. Mediastinal lesion is closely related to the heart and large blood vessels, so whether the lesion invades adjacent structures is very helpful in distinguishing benign and malignant lesions.
CT and MRI show tomographic images, there are no limitations of image overlap and blind areas on chest x-ray film, and the resolution of soft tissues is high, so the tissue components in mediastinal tumor can be distinguished, and the spatial relationship between tumor and peripheral structures can be determined. Therefore, they are not only helpful for tumor positioning and qualitative diagnosis, but also can help with the formulation of treatment plan.