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Background
Fibrosing mediastinitis (FM) is an idiopathic condition in which a fibrosing or inflammatory process infiltrates mediastinal soft tissues, often leading to bronchial and/or vascular obstruction. FM is commonly due to granulomatous processes such as sarcoidosis or prior histoplasmosis infection, in which case it often contains calcification, but it may also be caused by other inflammatory processes, such as IgG4-related disease. Accurate diagnosis of FM is challenging due to its rarity and ability to simulate malignancy.

Educational Goals / Teaching Points
Describe typical imaging features of fibrosing mediastinitis including location and vascular involvement. Describe less common imaging features of fibrosing mediastinitis, including presentation as a focal mass. Describe imaging features of malignancy that can mimic fibrosing mediastinitis and clues that can help the radiologist suggest the correct diagnosis.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Fibrosing mediastinitis is often infiltrative with predilection for pulmonary venous stenosis, although other structures may be involved such as the bronchi, SVC, or esophagus. Presence of calcifications suggests FM but is often absent in nongranulomatous conditions. Right-sided predominance is also commonly associated with granulomatous FM but typically absent in other causes. FDG uptake on PET/CT is variable and may overlap with malignant etiologies. Less commonly, FM presents as a focal mass, in which case it is often impossible to distinguish from malignancy.

Conclusion
Awareness of fibrosing mediastinitis as a diagnosis is important to suggest to the clinical team. Occasionally, the diagnosis may be made confidently by imaging alone, although tissue sampling is often necessary for confirmation. Knowledge of malignancies that can simulate FM is important so that those patients are not dismissed without tissue sampling.