Spectrum of Cystic Mediastinal Lesions: An Imaging Review

Bronchogenic cysts arise from abnormal budding of the tracheobronchial tree during embryogenesis and represent one of the most common congenital cystic lesions of the mediastinum.^12,13 They are typically located in the visceral mediastinum near the carina or mainstem bronchi, although ectopic locations have also been described.^2,3,12 Most lesions are asymptomatic and incidentally detected, while larger cysts may produce cough, chest pain, or compressive symptoms due to mass effect.¹² Approximately 20% of these cysts are intraparenchymal and can cause recurrent infection and, rarely, pneumothorax.¹⁴

On imaging, bronchogenic cysts appear as well-defined, thin-walled cysts with simple fluid (water) attenuation or higher density fluid due to proteinaceous material on CT. These can be occasionally multilocular with thin internal septations. Air or hemorrhage is less common but can be seen within these cysts. On MRI, these lesions are characterized by high T2 signal intensity with variable T1 signal based on proteinaceous content, without enhancement or diffusion restriction (Figure 1).^1,3,14 While most asymptomatic cases may be managed conservatively, symptomatic lesions or when diagnostic uncertainty exists, surgical resection is recommended. Malignant transformation is rare but has been reported.^4,12,14,15

Esophageal duplication cysts result from incomplete recanalization of the foregut and are most commonly located adjacent to the esophagus in the visceral mediastinum.^2,3 These lesions are frequently asymptomatic but may present with dysphagia or chest discomfort depending on size and location.^2,5 Hemorrhage is a common complication of these cysts that can cause rapid enlargement of the cyst and chest pain.¹⁶ Malignant transformation is exceedingly rare.²

Imaging typically demonstrates a unilocular cyst closely associated with the esophageal wall, with homogeneous fluid characteristics on both CT and MRI (Figure 2).^2,3 Surgical excision is generally recommended in symptomatic patients or when the diagnosis is uncertain.⁴

Pericardial cysts arise from incomplete fusion of mesenchymal lacunae during pericardial development and are most commonly located at the right cardiophrenic angle.^13,17 These lesions are usually asymptomatic and incidentally detected, although symptoms may occur due to compression of adjacent structures.¹⁷

On imaging, pericardial cysts appear as well-circumscribed, thin-walled lesions with homogeneous fluid attenuation on CT and high T2 signal intensity on MRI, without enhancement (Figure 3).^3,17 Larger pericardial cysts are detectable on frontal chest radiograph as well-defined basilar opacities silhouetting cardiac borders. The wall of the cyst can sometimes become calcified. There is no established malignant potential, and asymptomatic lesions may be managed conservatively, with intervention reserved for symptomatic or enlarging cysts.¹³

Thymic cysts arise from persistent thymopharyngeal duct remnants and are typically located in the prevascular anterior mediastinum.^10,18 Most lesions are asymptomatic and incidentally detected. While generally benign, multilocular thymic cysts may be associated with underlying thymic neoplasms.^10,18

On imaging, thymic cysts appear as well-defined unilocular or multilocular cystic lesions with homogeneous fluid attenuation on CT and high T2 signal intensity on MRI (Figures 4, 5). MRI is particularly helpful in excluding internal solid or enhancing components that may raise concern for malignancy.^3,10 Surgical resection is considered when imaging findings are atypical or malignancy cannot be confidently excluded.^4,18

Mediastinal lymphangiomas are uncommon benign cystic structures that arise from focal proliferation of lymphatic vessels and are most commonly found in the superior aspect of prevascular mediastinum.¹⁹ They are usually asymptomatic and become apparent when there is mass effect on adjacent structures.²⁰ Complications associated with lymphangiomas include infection, cystic hemorrhage, chylothorax, and chylopericardium. Standard treatment includes surgical excision if symptomatic. Recurrence is highly unlikely after excision. On CT, lymphangiomas appear as low-attenuation cysts with smooth contours and attenuation could be of simple to complex fluid and fat. On MR, they are isointense to muscle on T1-weighted images and hyperintense on T2; however, signal could vary based on cystic and proteinaceous content (Figure 6).

Differentials include necrotic tumors, teratomas, thymic cysts, pericardial or bronchogenic cysts, hematomas, goiters, and infectious fluid collections.²¹

Cystic degeneration may occur within malignant mediastinal tumors such as thymomas, germ cell tumors, lymphomas, or metastatic disease.^2,3,8 These lesions may present incidentally or with systemic or compressive symptoms depending on tumor burden.⁸ Imaging features suggestive of malignancy include irregular or thickened walls, internal septations, heterogeneous or nodular enhancement, and restricted diffusion on MRI.^1,3,9 Management requires tissue diagnosis and appropriate oncologic treatment.^4,22

Cyst-like lesions may develop secondary to infection, hemorrhage, or abscess formation.^1,8 Pseudocysts resulting from pancreatitis can also extend into the visceral mediastinum.²³ Patients may present with fever, chest pain, or systemic signs of infection.^1,3 Imaging typically demonstrates complex cystic lesions with surrounding inflammatory changes and possible enhancement.³ Management includes antibiotic therapy and drainage, with surgical intervention reserved for complicated cases.^4,22

Several noncystic entities may closely mimic mediastinal cystic lesions on imaging and should be carefully considered in the differential diagnosis.^1-3 Pericardial recesses represent normal anatomic variants that may appear as well-defined fluid collections along the pericardium, particularly adjacent to the great vessels.^1,2 Recognition of their characteristic locations, continuity with the pericardial space, and lack of mass effect is essential to avoid misdiagnosis.³

Necrotic or cystic lymph nodes may also simulate mediastinal cysts, particularly in the setting of malignancy, infection, or inflammatory disease.^1,2,22 Unlike simple cysts, necrotic lymph nodes often demonstrate irregular or thickened walls, peripheral or nodular enhancement, and restricted diffusion on MRI, which can aid in differentiation (Figure 7).^9,22

Other important mimics include cystic teratomas and chronic infectious or granulomatous processes.^2,8,10 These entities may demonstrate heterogeneous internal components, fat, calcifications, or variable enhancement patterns, features that are atypical for simple cysts.^2,10 Careful assessment of imaging findings in conjunction with clinical context and multimodality imaging is the key for distinguishing these conditions from true mediastinal cysts.^1,5,22

Management is individualized based on symptoms, lesion growth, imaging characteristics, and diagnostic certainty.^4,12,22,24 Surgical resection is recommended for symptomatic lesions, enlarging cysts, or lesions with suspicious imaging features, whereas conservative surveillance may be appropriate for asymptomatic congenital cysts with classic imaging appearances in patients with reliable follow-up.^4,22