Mediastinal seminoma is the second most common primary germ cell tumor of the anterior mediastinum (after teratoma). It constitutes 25-40% of all mediastinal germ cell tumors. The vast majority of patients are young men aged 20-35 years; it is exceedingly rare in women. There is a strong association with Klinefelter syndrome (47,XXY) — men with this syndrome have a 50-fold increased risk of mediastinal seminoma. Clinical presentation is often asymptomatic; large masses may cause chest pain, cough, dyspnea, and superior vena cava (SVC) syndrome. Serum beta-hCG may be mildly elevated in 10-20% of patients, but AFP must be normal — elevated AFP suggests a non-seminomatous germ cell tumor (NSGCT) component and changes the treatment approach. On CT, it appears as a large, lobulated, homogeneous or mildly heterogeneous, well-marginated mass in the anterior mediastinum. Necrosis and calcification are rare (unlike teratoma and NSGCT). Prognosis is generally favorable — 5-year survival is 80-90% with radiotherapy and/or chemotherapy.
Age Range
20-40
Peak Age
30
Gender
Male predominant
Prevalence
Rare
Mediastinal seminoma arises from primordial germ cells that become ectopically located in the mediastinum during embryonic migration. Normally, germ cells migrate from the yolk sac to the gonads; however, during this migration, cells may become arrested in the mediastinum (or retroperitoneum, pineal region) and undergo malignant transformation. Seminomas are histologically identical to gonadal seminomas — composed of large, uniform cells with clear cytoplasm and lymphocytic infiltration. The homogeneous imaging findings reflect the uniform cellular architecture: same cell type, regular vascularization, and minimal necrosis/hemorrhage. The homogeneous enhancement on CT is explained by the tumor's regular neovascularization and low necrosis rate — a rapidly growing tumor structure that maintains adequate vascular supply. The absence of calcification relates to seminomas not exhibiting mesenchymal differentiation (unlike cartilage/bone formation in teratomas). The high sensitivity to radiotherapy and chemotherapy results from defects in DNA damage repair mechanisms in seminoma cells.
Large, homogeneous, well-marginated mass in anterior mediastinum + young male patient + normal AFP = seminoma triad. Absence of calcification and fat excludes teratoma and NSGCT. This triad combination allows treatment planning even before tissue diagnosis.
On non-contrast CT, mediastinal seminoma appears as a large (usually 5-15 cm), homogeneous, soft tissue density (30-50 HU) mass in the anterior mediastinum. Margins are generally well-defined and lobulated. Necrosis and calcification are inconspicuous or absent — this is an important distinguishing feature from teratoma (fat + calcification) and NSGCT (heterogeneous, necrotic). The mass may fill the anterior mediastinum and encircle bilateral pulmonary arteries and superior vena cava. Pleural or pericardial effusion may accompany.
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A homogeneous, well-marginated, lobulated soft tissue density mass measuring approximately ___ × ___ cm is seen in the anterior mediastinum; no significant calcification or necrosis is identified.
In the arterial phase, mediastinal seminoma shows homogeneous, mild-to-moderate enhancement. The enhancement pattern is uniform, differing from heterogeneous enhancement seen in NSGCT and thymoma. In large masses, rare peripheral minimal low-attenuation areas may be seen, but significant central necrosis is not expected. The mass may encircle great vessels (SVC, brachiocephalic veins, aorta) but generally does not invade the vessel wall — luminal patency is maintained. Mediastinal and hilar lymphadenopathy may accompany.
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In the arterial phase, the mass shows homogeneous mild-to-moderate enhancement; no significant necrotic areas are identified.
In the portal venous phase, seminoma maintains its homogeneous enhancement. Encasement of great vessels (SVC, brachiocephalic veins) is best evaluated in this phase — seminoma may compress and encircle vessels but rarely invades the vessel wall (patent despite luminal narrowing). Pericardial thickening or effusion, pleural effusion, and mediastinal lymphadenopathy should be assessed. Relationship to pulmonary arteries and branches is important for surgical planning.
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In the portal venous phase, the mass maintains homogeneous enhancement and encases the SVC; the vessel lumen shows ___ degree narrowing but appears patent.
On T1-weighted images, mediastinal seminoma shows isointense or slightly hypointense signal relative to muscle, with homogeneous signal. No areas of internal hemorrhage or necrosis are identified — this homogeneity is a characteristic feature of seminoma. No fat content is present (unlike teratoma). MRI is superior to CT for evaluating relationship with great vessels and mediastinal invasion. Uniform enhancement is seen on contrast-enhanced series.
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On T1-weighted images, the anterior mediastinal mass shows isointense to slightly hypointense signal relative to muscle with homogeneous signal; no fat content or hemorrhagic signal is identified.
On T2-weighted images, mediastinal seminoma shows mildly to moderately hyperintense, homogeneous signal. No markedly hyperintense necrotic or cystic areas are identified. The T2 signal is more moderate compared to the marked hyperintensity seen in lymphoma. Mild heterogeneity may be seen in the presence of fibrous bands, but this is generally minimal. T2 sequences are important for evaluating relationship with pericardium, pleura, and great vessels.
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On T2-weighted images, the mass shows mildly to moderately hyperintense homogeneous signal; no significant necrotic or cystic areas are identified.
On diffusion-weighted imaging (DWI), mediastinal seminoma shows high signal with low values on the ADC map — restricted diffusion is present. High cellularity and high nucleus-to-cytoplasm ratio restrict free movement of water molecules in the extracellular space. This finding is important for treatment response assessment — successful response after chemotherapy/radiotherapy manifests as ADC increase (cell death → water release).
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On DWI, the mass shows high signal with low values on the ADC map — restricted diffusion is present, consistent with high cellularity.
On PET-CT, mediastinal seminoma shows intense, homogeneous FDG uptake (SUVmax typically 5-15). The FDG uptake pattern is homogeneous — different from heterogeneous uptake in NSGCT. PET-CT is used in three important roles in seminoma: (1) diagnosis and staging — detection of distant metastases, (2) treatment response assessment — viability of residual mass after chemotherapy, (3) recurrence detection. FDG negativity in post-chemotherapy residual masses strongly predicts absence of viable tumor.
Report Sentence
On PET-CT, the anterior mediastinal mass shows intense homogeneous FDG uptake (SUVmax: ___); distant metastasis ___.
Criteria
Composed of single cell type, no NSGCT component. AFP normal, beta-hCG normal or mildly elevated (<200 mIU/mL). Most common subtype (90%).
Distinct Features
Homogeneous mass on CT, minimal necrosis, no calcification. Intense homogeneous FDG uptake on PET-CT. Highly sensitive to radiotherapy and chemotherapy. 5-year survival >90%.
Criteria
Mass diameter >10 cm or >5 cm transverse. Same histology but higher stage and worse prognosis. Increased risk of SVC syndrome.
Distinct Features
Mild heterogeneity, minimal necrosis may be seen in large masses. Residual mass after chemotherapy is common — viability should be assessed with PET-CT. Combined chemotherapy (BEP/EP) is first-line.
Criteria
Develops on 47,XXY karyotype or mosaic Klinefelter syndrome. Usually diagnosed at younger age (15-25 years). 50-fold increased risk.
Distinct Features
Imaging features identical to classic seminoma. Accompanying gynecomastia, hypogonadism, tall stature are clinical clues. Hematological malignancy risk also increased.
Criteria
Contains isolated syncytiotrophoblastic giant cells within pure seminoma. Beta-hCG mildly elevated (usually <1000 mIU/mL). AFP remains normal.
Distinct Features
Imaging findings largely overlap with classic seminoma, may be slightly more heterogeneous. Treatment response and prognosis similar to classic seminoma.
Distinguishing Feature
Teratoma contains fat + calcification + soft tissue (three-component mass); seminoma lacks fat and calcification. Teratoma is heterogeneous on CT, seminoma is homogeneous.
Distinguishing Feature
Thymoma typically >40 years, may be encapsulated; seminoma in young men, non-encapsulated. Myasthenia gravis association should be sought in thymoma.
Distinguishing Feature
Hodgkin lymphoma usually bilateral asymmetric lymphadenopathy; B symptoms accompany. Seminoma is a well-defined, lobulated single mass.
Distinguishing Feature
Thymic carcinoma more aggressive, heterogeneous, necrotic, invasive; seminoma well-marginated and does not invade vessels. Thymic carcinoma at more advanced age.
Urgency
urgentManagement
medicalBiopsy
NeededFollow-up
specialist-referralMediastinal seminoma is highly sensitive to treatment, but rapid diagnosis and treatment initiation are important. Diagnosis is confirmed by CT-guided core biopsy or mediastinoscopy. First-line treatment is cisplatin-based chemotherapy (BEP) or radiotherapy. If residual mass >3 cm after chemotherapy, viability should be assessed with PET-CT. Elevated AFP indicates NSGCT component and changes treatment protocol.
Mediastinal seminoma responds excellently to chemotherapy (BEP regimen). 5-year survival exceeds 90%. Testes should be normal (excluded by bilateral testicular US). AFP must be normal — elevated AFP suggests a non-seminomatous component and changes treatment strategy.