A lung cyst is an air- or fluid-filled pocket that forms within the lung tissue. These structures are often discovered incidentally during imaging performed for other reasons, such as a routine chest computed tomography (CT) scan. While finding any abnormality can cause concern, the majority of lung cysts are benign. However, a small subset of these cystic lesions can be associated with or represent a primary lung malignancy, requiring careful evaluation.
Understanding Lung Cysts
Lung cysts form from various non-cancerous origins. Many are the result of air trapping and tissue destruction, such as bullae or blebs, which are air-filled sacs common in individuals with emphysema or chronic obstructive pulmonary disease (COPD). Bullae typically have very thin walls and represent damaged alveoli that have coalesced into a larger pocket.
Other benign cysts are remnants from past infections or inflammatory processes. A pneumatocele, for example, is a thin-walled, air-filled cyst that can form following severe pneumonia or trauma. Granulomas, which develop after fungal or bacterial infections like tuberculosis, can sometimes cavitate and appear cystic on imaging. Some cysts are congenital, such as a bronchogenic cyst, formed from an abnormal budding of the bronchial tree during fetal development. These diverse causes show that a lung cyst is a descriptive finding rather than a specific diagnosis, often being a harmless consequence of prior disease or developmental anomaly.
Features That Suggest Malignancy
While most cysts are benign, certain characteristics seen on imaging raise suspicion that a lung cyst may be cancerous. A true primary lung cancer rarely presents as a simple, thin-walled cyst, but malignant tumors, particularly adenocarcinomas, can form in relation to a pre-existing cystic air space.
The most concerning feature is the presence of a solid component within the cyst, often called a mural nodule. A nodule growing along the inner wall suggests an invasion of cancerous cells. Cysts that show irregular, thick, or asymmetrical wall thickening are also highly suspicious for malignancy, contrasting with the smooth, thin walls of benign cysts. Additionally, any rapid change in the cyst’s size or appearance warrants immediate investigation, as aggressive growth is a hallmark of cancerous lesions.
Diagnostic Procedures for Evaluation
Determining whether a suspicious lung cyst is benign or malignant requires a systematic approach utilizing advanced imaging and tissue sampling. High-Resolution Computed Tomography (HRCT) is the initial and most informative tool. It provides detailed cross-sectional images that allow for precise measurement of cyst wall thickness and the detection of subtle internal features, such as endophytic or exophytic solid components.
For lesions that show solid components, a Positron Emission Tomography (PET) scan may assess the tissue’s metabolic activity. Cancer cells are typically more metabolically active and show increased uptake of the radioactive tracer, suggesting malignancy. PET scans are most useful when the solid component or mural nodule measures at least eight millimeters, as smaller lesions may not show sufficient uptake.
The definitive method for confirmation is a tissue biopsy, which obtains a small sample of the cyst wall or solid component for microscopic analysis. Biopsies may be performed using a needle guided by CT imaging or through a transbronchoscopic approach. If the lesion is difficult to access or imaging features are inconclusive, a surgical biopsy may be necessary to remove the entire structure. For cysts that are indeterminate but not highly suspicious, sequential monitoring with repeat CT scans over several months is often employed to watch for changes in size or morphology.
Management and Follow-Up
Once a definitive diagnosis is established, the management plan follows one of two paths. If the cyst is confirmed to be benign, the approach depends on the underlying cause and symptoms. Many simple, benign cysts require only routine follow-up. However, if a benign cyst causes complications, such as a recurrent pneumothorax, a surgical procedure like pleurodesis may be recommended to prevent future episodes.
If the diagnostic workup confirms the cystic lesion is malignant, the patient is diagnosed with lung cancer, and treatment is initiated immediately. Treatment is individualized but generally involves approaches used for other forms of lung cancer. These may include surgical removal of the affected lung part, radiation therapy, or systemic treatments like chemotherapy or targeted drug therapy. A structured follow-up plan ensures that a benign lesion remains stable or that a malignant one is treated promptly.