Etiology and clinical presentation
The pathology develops when the buds of the developing airways separate from the main tracheobronchial tree. As a result, a closed cavity forms, lined internally with respiratory epithelium. Normally, it contains thick mucoid secretion or clear serous fluid.
The disease can manifest at any age, although it is most often asymptomatic. The lesion can reach significant dimensions without visible clinical manifestations. Symptoms usually occur when adjacent anatomical structures—primarily the airways—are compressed.
In rare cases, severe complications can develop, such as hemorrhage into the cyst cavity or its infection. When an infection develops, the wall of the lesion thickens, becomes irregular, and its contents increase in density.
Radiological signs (CXR)
On standard chest radiographs, a bronchogenic cyst appears as a well-circumscribed, single spherical or ovoid mass with homogeneous opacification.
- Contours: Most often unilocular (unilocular), without lobulation.
- Location: Typically projects just below the tracheal bifurcation (carina), often bulging slightly toward the shadow of the right root of the lung. It can also be located anywhere along the trachea and main bronchi.
- Features: Calcification of the walls is extremely rare. A specific sign of duplication cysts is the formation of a fluid level containing "milk of calcium" (milk of calcium).
Computed tomography and magnetic resonance imaging
CT and MRI are the optimal methods for assessing the size, shape, and relationship of the cyst with surrounding mediastinal structures. A distinctive feature of simple bronchogenic cysts is the complete absence of internal contrast enhancement (enhancement).
CT findings:
- Typical variant: Thin-walled lesion with homogeneous density close to that of water.
- Atypical variant: The density may correspond to soft tissue or be uniformly high (due to a high protein content in the fluid). In such cases, unenhanced images help in differential diagnosis from tumors.
MRI findings:
- On T1-weighted images, signal intensity is variable (ranging from low to high) and depends on the protein composition of the contents.
- On T2-weighted images, high signal intensity is consistently observed.
- The method allows the use of fat-suppression sequences, while subtraction images help rule out even minimal contrast enhancement by solid components (which is important to exclude malignant transformation).
Briefly: answers to topical questions
What differential diagnoses should be ruled out in case of atypical density of cyst contents on CT?
With atypical density of bronchogenic cyst contents on CT, the differential diagnosis must be broadened to rule out tumors first. An atypical imaging pattern occurs if the density of the lesion is similar to soft tissue or is uniformly high due to a high protein content in the fluid.
As part of the differential diagnosis on CT, the following are considered:
- Tumours (Tumours) — ruled out if soft-tissue density is present; unenhanced CT images are useful to clarify the diagnosis.
- Peripheral nerve sheath tumours (Peripheral Nerve Sheath Tumours) — can mimic a cyst because they are often homogeneous and have low density due to myelin content.
- Esophageal duplication cysts (Oesophageal Duplication Cysts) — imaging appearance is identical, but the wall of the lesion may be thicker.
To remember the typical localization, use the image of a "drop below the bifurcation". A bronchogenic cyst most often "drips" (forms) right beneath the carina (subcarinally), resembling a smooth drop of water without lobulation on the scan.
Class and exam questions
What does an infected bronchogenic cyst look like on tomograms?
Unlike an uncomplicated (thin-walled) cyst, an infected cyst has a thick, irregular wall that intensely enhances with contrast media. The density of its contents on CT increases.
Is calcification a frequent sign of bronchogenic cysts?
No, wall calcification is rare. However, so-called "milk of calcium" can sometimes be detected inside the cavity.
What else to study on this topic
- Esophageal duplication cysts: clinical presentation and imaging markers
- Neuroenteric cysts and associated vertebral anomalies
- Peripheral nerve sheath tumors (schwannomas, neurofibromas)
- Mediastinal pancreatic pseudocysts
- Sympathetic ganglion tumors: from ganglioneuroma to neuroblastoma
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