Fibrosing mediastinitis

Rare fibroinflammatory reaction in the mediastinum that progressively encases and narrows mediastinal structures (vessels, airways). Often post-infectious (classically histoplasmosis in North America; TB can be relevant in Asia) but can be IgG4-related or idiopathic.

Pearls

• Two patterns: focal calcified granulomatous (post-infectious) vs diffuse infiltrative non-calcified.
• Symptoms come from obstruction: SVC syndrome, dyspnoea, haemoptysis, recurrent pneumonia.
• Can mimic malignancy clinically and radiologically, but calcified, encasing pattern is a clue.

Key imaging features

CT (workhorse)

• Infiltrative, ill-defined soft tissue in mediastinum or hila, often centred on prior nodal stations.
• Dense or stippled calcifications are common in granulomatous/post-infectious type.
• Key finding is encasement/stenosis:
  • SVC and brachiocephalic veins (SVC syndrome, chest wall collaterals)
  • Pulmonary arteries/veins (pulmonary hypertension, pulmonary oedema, haemoptysis)
  • Main bronchi (airway narrowing, post-obstructive infection, lobar collapse)
• May see unilateral volume loss, perfusion asymmetry, pleural effusion.

MRI

• Fibrotic tissue often low signal on T2 (more "burnt out" fibrosis).
• Variable enhancement; more enhancement can suggest active inflammatory component.

FDG PET-CT

• Variable uptake: can be low in inactive fibrosis, higher if ongoing inflammation.
• Helpful mainly to exclude aggressive malignancy and guide biopsy target.

Top differentials

• Lymphoma/metastatic nodes: bulky discrete nodal masses, less encasing fibrosis, calcification usually absent unless treated.
• Sarcoidosis: symmetric hilar/mediastinal nodes, perilymphatic lung nodules; fibrosis pattern differs.
• IgG4-related disease: multi-organ involvement (pancreas, salivary glands, retroperitoneum), less calcification; may respond to steroids.
• Radiation fibrosis: matches radiation field and history.