Thymoma
Summary
Primary thymic epithelial neoplasm of the anterior mediastinum with a spectrum from encapsulated to locally invasive and pleural/pericardial spread. Frequently associated with autoimmune disease, especially myasthenia gravis.
Clinical associations
- Myasthenia gravis is common
- Other classic associations:
- Pure red cell aplasia
- Hypogammaglobulinaemia (Good syndrome)
Imaging features
CXR
- Anterior mediastinal mass or mediastinal widening; may be subtle if small.
CT
- Typically well-defined, round/lobulated anterior mediastinal soft tissue mass.
- Can be heterogeneous (cystic change, haemorrhage, necrosis).
- Calcification can occur: capsular or coarse (not specific).
Features suspicious for invasion
- Irregular/lobulated margins with loss of clear fat planes (suggestive, not definitive)
- Pericardial thickening/effusion or direct contact with deformity
- Great vessel encasement or clear infiltration
- Pleural nodules/pleural thickening (drop metastases) or pleural effusion
- Clue: phrenic nerve palsy (raised hemidiaphragm)
MRI
- Useful for local invasion (pericardium, vessels) when CT equivocal.
- Chemical shift: lack of opposed-phase signal drop supports tumour over thymic hyperplasia (hyperplasia typically drops).
FDG PET/CT
- Uptake is variable; more useful to look for pleural/extrathoracic disease or alternate diagnosis than to "grade" thymoma.
Key differentials
- Thymic rebound/hyperplasia: preserved thymic shape, symmetric; opposed-phase signal drop.
- Thymic carcinoma: more aggressive appearance, nodes/metastases more likely.
- Lymphoma: bulky lobulated mass with widespread nodes, systemic context.
- Germ cell tumour (teratoma): macroscopic fat, fluid, calcification, teeth-like elements.
Zebra
- Osseous metaplasia: very rare; reported in case reports (often females). Can show coarse internal calcification and may mimic teratoma.
