Mediastinal bronchogenic cyst is a congenital cystic lesion arising from foregut budding anomaly during embryological development. Most commonly located in the subcarinal (50-65%) and right paratracheal region. It is a thin-walled, fluid-filled, non-enhancing cystic structure. Content may be serous or proteinaceous/mucinous — proteinaceous content can mimic soft tissue density on CT (40-80+ HU). MRI is the gold standard for confirming the cystic nature of these high-density cysts: T2 hyperintensity is present in all cyst types, T1 signal depends on content (proteinaceous = T1 hyperintense). Usually discovered incidentally; airway/esophageal compression or infection rarely develop.
Age Range
15-50
Peak Age
30
Gender
Equal
Prevalence
Uncommon
Bronchogenic cysts arise from abnormal budding of the foregut during embryological development (weeks 3-7) — during branching of the tracheobronchial tree, an isolated cystic structure forms and settles in the mediastinum. The cyst wall contains respiratory epithelium (ciliated pseudostratified columnar epithelium), cartilage, smooth muscle, and mucous glands — this histological composition precisely mimics the bronchial wall. Cyst content accumulates from continuous secretion of mucous glands; serous fluid shows water density (0-20 HU), while proteinaceous/mucinous content increases density due to protein, calcium oxalate, and cholesterol crystals (40-80+ HU) — this high density can be confused with a solid mass on CT. On MRI, T1 signal depends on protein concentration: low-protein fluid is T1 hypointense (like free water), high-protein fluid is T1 hyperintense (protein molecules shorten T1 relaxation of water molecules — paramagnetic effect). On T2, all bronchogenic cysts are hyperintense because free water content is high. Since the cyst is avascular, it shows no enhancement — this feature provides definitive differentiation from solid lesions. Subcarinal location (50-65%) corresponds embryologically to the abnormal budding point at the tracheal bifurcation at the carina level.
Thin-walled, non-enhancing cystic lesion in subcarinal location — density ranges between water (0-20 HU) and milk (40-80+ HU). Confirmation of cystic nature of high-density cysts by MRI establishes the diagnosis.
Well-defined, thin-walled (<3 mm), round or oval cystic lesion in the subcarinal region. Homogeneous internal structure, no septation or solid component. Density varies depending on content: serous cysts at water density (0-20 HU), proteinaceous cysts at soft tissue density (40-80+ HU). Milk of calcium may be seen in rare cases.
Report Sentence
A well-defined, thin-walled, homogeneous cystic lesion measuring approximately ...x... mm (...HU) is observed in the subcarinal region, and bronchogenic cyst should be primarily considered.
No enhancement is observed in cyst content or wall on contrast-enhanced CT. Pre- and post-contrast density values remain the same. This finding definitively differentiates cystic from solid lesion — critically important especially in high-density proteinaceous cysts (40-80+ HU).
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No enhancement is observed in the lesion on contrast-enhanced series, consistent with cystic structure; solid lesion has been excluded.
Homogeneous hyperintense signal on T2-weighted sequences. This finding is consistently present in all bronchogenic cysts — regardless of serous or proteinaceous content. Definitively confirms cystic nature and is the gold standard for differentiating high-density cysts from solid lesions on CT.
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The lesion shows homogeneous hyperintense signal on T2-weighted sequences, consistent with cystic content.
Signal on T1-weighted sequences depends on cyst content: serous fluid is T1 hypointense (water-like), proteinaceous/mucinous fluid is T1 hyperintense. High T1 signal is a pathognomonic finding of proteinaceous content and is the most critical MR finding confirming the true cystic nature of high-density cysts on CT.
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The lesion shows hyperintense signal on T1-weighted sequences, consistent with proteinaceous/mucinous cyst content.
No true restricted diffusion is observed in cyst content on diffusion-weighted imaging. In proteinaceous cysts, T2 shine-through effect may show high signal on DWI but ADC map values are high — proving absence of true restriction. Abscess or solid tumor shows low ADC values.
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No true restricted diffusion is detected in the lesion on diffusion-weighted series; ADC values are high, consistent with cystic content.
No enhancement is observed in cyst wall or content on contrast-enhanced MR series. Thin smooth wall shows no signal increase after gadolinium. This finding is typical of bronchogenic cyst without infection or malignant transformation.
Report Sentence
No enhancement is observed in cyst wall or content on contrast-enhanced MR series, consistent with uncomplicated bronchogenic cyst.
In rare cases, calcium accumulation is observed within the cyst — termed milk of calcium. Gravity-dependent fluid-calcium level may form. Should not be confused with calcified granuloma or teratoma.
Report Sentence
Gravity-dependent calcific fluid level (milk of calcium) is observed within the cyst content.
Criteria
CT density 0-20 HU, MRI: T1 hypointense, T2 hyperintense. Pure serous fluid content. Cystic nature is already evident on CT.
Distinct Features
Diagnosis on CT is usually straightforward as water-density cyst is clearly visible. MRI is rarely needed. Complication risk is low.
Criteria
CT density 40-80+ HU, MRI: T1 hyperintense, T2 hyperintense. Mucinous/proteinaceous fluid content with high protein concentration. May be confused with solid mass on CT.
Distinct Features
Enters solid mass differential on CT — MRI confirmation is mandatory (T2 hyperintensity + absence of enhancement). Absence of enhancement and T2 hyperintensity exclude solid lesion.
Criteria
Cyst wall thickening (>3 mm), wall enhancement, pericystic inflammation, air-fluid level (abscess formation). Clinical: fever, chest pain, leukocytosis.
Distinct Features
Unlike simple cyst, shows wall enhancement. May be confused with abscess — history of cyst on prior imaging is important. Surgical indication is established.
Criteria
Located within lung parenchyma rather than mediastinum (15-20%). Lower lobes more commonly involved. Same imaging features apply but location differs.
Distinct Features
Appears as solitary cystic lesion within lung parenchyma. Infection and air-fluid level develop more frequently than mediastinal type. May be confused with pulmonary sequestration.
Distinguishing Feature
Pericardial cyst is located at the cardiophrenic angle (usually right); bronchogenic cyst is subcarinal. Pericardial cyst is almost always water density (<20 HU), proteinaceous content is rare.
Distinguishing Feature
Thymoma is located in the anterior mediastinum and shows enhancement; bronchogenic cyst is subcarinal/middle mediastinum and shows no enhancement. High-density bronchogenic cyst on CT may be confused with thymoma but MRI T2 hyperintensity provides definitive differentiation.
Distinguishing Feature
Lymphoma is a solid mass, shows enhancement, has heterogeneous T2 signal and diffusion restriction. Bronchogenic cyst is homogeneously T2 hyperintense, no enhancement, no diffusion restriction. Multiple lymphadenopathy favors lymphoma.
Distinguishing Feature
Teratoma contains fat-fluid level, calcification, and fat density (fat: -20 to -100 HU) — this heterogeneous appearance is easily differentiated from the homogeneous structure of bronchogenic cyst. Teratoma is located in the anterior mediastinum.
Distinguishing Feature
Schwannoma is located in the posterior mediastinum (paravertebral), shows solid enhancement, may extend into neural foramen. Bronchogenic cyst is in the middle mediastinum, shows no enhancement. Schwannoma may be T2 heterogeneous (Antoni A/B areas).
Urgency
routineManagement
surveillanceBiopsy
Not NeededFollow-up
12-monthAsymptomatic bronchogenic cysts are generally managed conservatively — annual CT or MRI follow-up is sufficient. Symptomatic cysts (airway compression, dysphagia, infection) require surgical resection — video-assisted thoracoscopic surgery (VATS) is the preferred approach. In infected cysts, antibiotherapy + surgery is planned. Although malignant transformation is extremely rare, it has been reported in long-term unmonitored cysts. Prophylactic resection of uncomplicated cysts is controversial.
Bronchogenic cyst is generally benign and follow-up is sufficient in asymptomatic cases. Surgical excision is performed in symptomatic cases (compression, infection). Infected bronchogenic cyst may mimic an abscess. Malignant transformation is extremely rare.