Thymus pathologies

Epithelial tumors: thymoma and carcinoma

Thymoma (Thymoma) is the most frequent thymic tumor in adults (mean age of detection is around 50 years). As a rule, these are low-grade epithelial formations. Clinically, thymomas are closely associated with autoimmune syndromes: in addition to the frequent Myasthenia gravis, hypogammaglobulinemia and pure red cell aplasia may be observed.

A key stage in diagnostic imaging is the assessment of invasiveness (absence of a capsule or breakthrough into it). Reliable CT signs of invasion include:

• Encasement of vessels and other mediastinal structures.
• Infiltration of surrounding fat pads (however, their preservation rules out only extensive invasion, not microscopic capsule involvement).
• Irregular contour with the lung parenchyma, pleural nodularity, and the presence of pleural effusion.

In complex cases, as well as for the precise assessment of myocardial or pericardial invasion, MRI is used. Thymomas often have a mixed structure with large cystic components and mural nodules that definitively lack adipose tissue (confirmed on T1-weighted fat-suppressed sequences).

Thymic carcinoma (Thymic Carcinoma) accounts for about 20% of all epithelial tumors of the gland. It is characterized by a high grade of anaplastic features and an extremely aggressive course. Unlike most thymomas, carcinoma lacks a well-defined capsule. The tumor appears as a large heterogeneous mass with areas of necrosis and calcification. On MRI (on T1-weighted images), it demonstrates intermediate signal intensity slightly exceeding that of muscle. The tumor metastasizes early: more than half of patients have regional lymph node involvement and extrathoracic metastases at the time of initial examination.

Neuroendocrine tumors and thymolipomas

Thymic neuroendocrine tumor (carcinoid / Thymic Carcinoid) is a rare (less than 5% of anterior mediastinal masses) but aggressive pathology. About 20% of patients have distant metastases (liver, bones, lungs) at diagnosis. Bone metastases are usually osteoblastic in nature. Carcinoid frequently causes paraneoplastic syndromes:

1. Cushing syndrome (Cushing syndrome) occurs in ~40% of patients due to ectopic ACTH secretion.
2. Multiple endocrine neoplasia (MEN types I and II) syndromes occur in 20% of patients.

Thymolipomas (Thymolipomas) are benign, well-encapsulated masses consisting of a mixture of normal thymic tissue and mature fat. They are localized in the lower parts of the anterior mediastinum. Due to their soft consistency, thymolipomas literally "contour" the diaphragm and heart. They can reach giant proportions and mimic cardiomegaly on chest radiographs. On MRI, the fatty component yields a high hyperintense signal, whereas the soft-tissue bands within the tumor are hypointense.

Thymic hyperplasia and cysts

Non-neoplastic enlargement of the thymus is classified into two main types:

• Lymphofollicular hyperplasia: closely associated with autoimmune diseases such as myasthenia gravis, Graves' disease (thyrotoxicosis), or systemic lupus erythematosus.
• Rebound hyperplasia (Rebound thymic hyperplasia): a physiological process of glandular recovery after stress, steroid administration, or chemotherapy. The gland diffusely enlarges while preserving its normal shape and structure.

To reliably differentiate hyperplasia from true neoplasia, chemical-shift MRI is used (in patients over 15 years of age). Microscopic intracellular fat is present in normal and hyperplastic tissue. On out-of-phase MRI images, this leads to a characteristic signal drop. In thymic tumors, the signal does not decrease.

Thymic cysts (Thymic Cyst) account for only 1% of mediastinal mass lesions. Congenital cysts (from the thymopharyngeal duct) are usually unilocular and have thin, invisible walls. Acquired cysts are multilocular, forming in the setting of tumors or after radiation therapy. On MRI, they demonstrate typical fluid parameters: low signal on T1WI and high signal on T2WI (in the absence of hemorrhage or infection).

Mediastinal germ cell tumors

The mediastinum is the most frequent location for extragonadal germ cell tumors (Germ-Cell Tumours), originating from primitive cells that migrated during embryogenesis. Almost all malignant variants occur exclusively in males in their 2nd to 4th decades of life.

• Teratomas (Teratomas): The most common type. They contain elements of all three germ layers (fat, hair, bone, epithelium). Most of them are mature cystic masses of a benign nature. The presence of adipose tissue on CT or signal loss on fat-suppression MRI is a diagnostic key.
• Seminoma (Seminoma): The most frequent malignant form. This is a well-circumscribed solid mass with a homogeneous structure without prominent calcifications.
• Non-seminomatous tumors: (embryonal carcinoma, choriocarcinoma, etc.). These are lobulated, asymmetric masses with a heterogeneous structure, extensive areas of necrosis, and hemorrhage. They tend to invade adjacent organs and frequently secrete specific markers—hCG and alpha-fetoprotein, which is critical for diagnosis.

Mediastinal lymphadenopathy

Thymic pathologies often require differential diagnosis with malignant lymphomas (Hodgkin lymphoma and non-Hodgkin lymphomas). Lymphoma involves multiple groups of lymph nodes. The most typical localizations are prevascular, para-aortic, and paratracheal lymph nodes. Involvement is usually bilateral but asymmetric. Isolated enlargement of the hila without mediastinal involvement is atypical for lymphomas.

Class and exam questions

What else to study on this topic

• Features of thymoma MRI semiotics: T1, T2, SSFP sequences, and fat suppression.
• Differential diagnosis of cystic lesions of the anterior mediastinum.
• Superior vena cava syndrome (SVCS) in aggressive thymic tumors.
• Morphological classification of thymic epithelial tumors (thymomas vs. carcinomas).
• Imaging appearance of Hodgkin lymphoma: nodular sclerosis and specific patterns of lymph node involvement.

Other topics in the «Chest Radiology» section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.