Thymoma is the most common primary neoplasm of the anterior (prevascular) mediastinum, originating from thymic epithelial cells. Peak incidence occurs between ages 40-60 with equal gender distribution. Approximately 30-50% of cases are associated with myasthenia gravis; associations with pure red cell aplasia, hypogammaglobulinemia, and other autoimmune conditions also exist. WHO classification divides thymomas into Types A, AB, B1, B2, and B3. The Masaoka-Koga staging system is critical for surgical resectability and prognosis — Stage I is encapsulated, Stage II shows capsular invasion, Stage III involves invasion of adjacent organs, and Stage IV indicates pleural/pericardial dissemination or distant metastasis. Complete surgical resection is the gold standard treatment; neoadjuvant or adjuvant radiotherapy and/or chemotherapy are added in advanced stages.
Age Range
40-70
Peak Age
55
Gender
Equal
Prevalence
Uncommon
Thymoma originates from cortical and medullary epithelial cells of the thymus gland. The normal thymus plays a critical role in T-lymphocyte maturation and undergoes fatty involution after puberty. In thymoma, these epithelial cells show neoplastic proliferation while mature lymphocytes are also present within the tumor — WHO type is determined accordingly (Type A: minimal lymphocytes, Type B1: lymphocyte-rich). The association with myasthenia gravis is due to abnormal T-lymphocyte education and production of autoantibodies against acetylcholine receptors. On imaging, thymomas typically appear as well-defined, lobulated, homogeneous soft tissue masses; calcification is seen in approximately 25% of cases (coarse, amorphous, or capsular). On contrast-enhanced CT, they show homogeneous or mildly heterogeneous enhancement. Advanced-stage thymomas can extend beyond the capsule to form pleural implants (drop metastases) — these findings are critical for surgical planning. On MRI, the solid component shows intermediate signal on T1 and mildly hyperintense signal on T2; cystic/necrotic areas appear markedly hyperintense on T2.
In a patient with a well-defined, lobulated, homogeneous soft tissue mass in the anterior mediastinum and a diagnosis of myasthenia gravis, the diagnosis of thymoma is highly probable. This clinical-radiological association is the strongest diagnostic clue for thymoma — thymoma is found in 10-15% of myasthenia gravis cases.
Soft tissue density mass localized in the anterior mediastinum, anterior to the great vessels, with well-defined or lobulated contours. Shows homogeneous soft tissue density on non-contrast CT (40-60 HU). Internal cystic/necrotic areas appear hypodense in large tumors. Capsular or amorphous calcification is present in approximately 25% of cases. The tumor typically extends from midline to one side but can be bilateral.
Report Sentence
A well-defined, lobulated, homogeneous soft tissue density mass is seen in the anterior mediastinum anterior to the great vessels, and thymoma should be the leading diagnostic consideration.
Shows homogeneous or mildly heterogeneous enhancement on contrast-enhanced CT. Enhancement degree is typically moderate (solid areas 60-80 HU). In large tumors (>8 cm), heterogeneous enhancement is more prominent due to internal septa and cystic/necrotic areas. WHO Type B2 and B3 thymomas tend to show more heterogeneous enhancement. Invasion or compression of vascular structures (especially brachiocephalic veins) should be evaluated.
Report Sentence
The mass demonstrates homogeneous/mildly heterogeneous enhancement on contrast-enhanced series; the moderate enhancement pattern of the solid component is consistent with thymoma.
In advanced-stage thymoma (Masaoka Stage III-IV), nodular or plaque-like implants are seen along pleural surfaces. These 'drop metastases' indicate transcelomic dissemination and are considered the most characteristic metastatic pattern of thymoma. Pleural implants can be unilateral or bilateral, and pleural effusion may accompany them. Implants appear as soft tissue density enhancing nodules on contrast-enhanced CT. Implants may also be seen on pericardial surfaces.
Report Sentence
Enhancing nodular implants along pleural surfaces are observed, consistent with transcelomic dissemination of advanced-stage thymoma (Masaoka Stage IVA).
On T1-weighted images, the solid component shows intermediate signal (equal to or slightly hyperintense to skeletal muscle). Cystic/necrotic areas show low signal intensity. Hemorrhagic areas may show high signal on T1. Internal septa can be seen as low signal bands on T1. The relationship of the tumor with mediastinal fat is well evaluated on T1 — obliteration of fat planes indicates invasion.
Report Sentence
The solid component of the anterior mediastinal mass shows signal equal to or slightly hyperintense to skeletal muscle on T1-weighted sequences.
On T2-weighted images, the solid component shows mildly to moderately hyperintense signal. Cystic/necrotic areas appear markedly hyperintense. Internal fibrous septa are seen as low-signal bands revealing the lobulated internal architecture. T2 heterogeneity may correlate with tumor size and WHO type — Type B2/B3 thymomas tend to be more heterogeneous. The relationship with adjacent vascular structures (especially superior vena cava, brachiocephalic veins) can be evaluated on T2 through flow voids.
Report Sentence
The mass shows mildly hyperintense signal on T2-weighted sequences with lobulated internal architecture demonstrating internal septa and cystic areas.
The solid component shows mild to moderate diffusion restriction on DWI. ADC values are generally intermediate (higher than thymic carcinoma and lymphoma). Correlation between WHO type and ADC values has been reported — higher-grade thymomas (B2, B3) tend to have lower ADC values. DWI may provide additional contribution to detection of pleural implants beyond conventional sequences.
Report Sentence
Mild to moderate diffusion restriction is observed in the solid component on DWI with intermediate ADC values, consistent with thymoma showing less restricted diffusion than thymic carcinoma.
On FDG PET-CT, thymomas typically show mild to moderate FDG uptake (SUVmax 3-7). There is a positive correlation between WHO classification and FDG uptake — low-grade thymomas (Type A, AB) show low uptake, while high-grade (Type B2, B3) and thymic carcinoma show markedly high uptake. PET-CT is superior to conventional imaging in detecting pleural implants and distant metastases. It is also used for treatment response assessment.
Report Sentence
The anterior mediastinal mass shows mild to moderate FDG uptake on PET-CT (SUVmax: ...); correlation with WHO type is suggested.
Criteria
Medullary thymoma. Composed of spindle/oval epithelial cells with minimal lymphocyte component. Best prognosis among subtypes.
Distinct Features
Homogeneous mass, low FDG uptake, tendency to remain encapsulated (Masaoka I-II)
Criteria
Mixed thymoma. Combination of Type A and lymphocyte-rich Type B components. Prognosis similar to Type A.
Distinct Features
Homogeneous or mildly heterogeneous mass, low to moderate FDG uptake, mixed solid/cystic areas
Criteria
Lymphocyte-predominant thymoma. Rich lymphocyte population resembling normal thymic cortex. Association with myasthenia gravis is frequent.
Distinct Features
Homogeneous solid mass, homogeneous enhancement similar to lymphoma, moderate FDG uptake
Criteria
Cortical thymoma. Large polygonal epithelial cells mixed with lymphocytes. Invasive tendency is increased, prognosis worsens.
Distinct Features
Heterogeneous enhancement, cystic/necrotic areas more common, high FDG uptake, capsular invasion frequent
Criteria
Borderline atypical/well-differentiated thymic carcinoma. Polygonal epithelial cells predominant with few lymphocytes. Invasive tendency is marked.
Distinct Features
Markedly heterogeneous mass, irregular borders, invasion of adjacent structures, high FDG uptake, differential from thymic carcinoma is difficult
Distinguishing Feature
Thymic carcinoma appears more aggressive: irregular borders, prominent vascular invasion, lymphadenopathy, distant metastases. Thymoma typically has smoother borders with more limited invasion. FDG uptake is significantly higher in thymic carcinoma (SUVmax >7).
Distinguishing Feature
Lymphoma typically presents as diffuse, lobulated, confluent lymphadenopathy not confined to the prevascular space. Thymoma shows a well-defined, single-mass morphology. Lymphoma occurs in younger age (20-30) and B symptoms may be present.
Distinguishing Feature
Teratoma shows pathognomonic fat-fluid level, calcification, and soft tissue components on CT (fat+calcification+soft tissue triad). Thymoma lacks fat and tooth/bone-like calcification.
Distinguishing Feature
Thymic cyst appears as a water-density (0-20 HU), thin-walled, non-enhancing cystic lesion on CT. Thymoma contains solid component and enhancement.
Distinguishing Feature
Retrosternal goiter shows continuity with cervical thyroid and high native CT density (~100 HU due to iodine content), prominent enhancement, and calcifications. Thymoma lacks cervical connection and has lower native CT density.
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
6-monthComplete surgical resection (thymectomy) is the gold standard for thymoma treatment. Surgery alone is usually sufficient for Masaoka Stage I-II; adjuvant radiotherapy is added for Stage III, and chemotherapy for Stage IV. Myasthenia gravis screening is mandatory (anti-AChR antibody). Recurrence risk is high with incomplete resection or advanced-stage disease — long-term follow-up (>10 years) is recommended.
Treatment of thymoma is primarily surgical resection. The Masaoka-Koga staging system is used for prognosis and treatment planning. Neoadjuvant chemotherapy and/or radiotherapy may be applied in invasive thymomas. Myasthenia gravis symptoms may improve after thymectomy.