Definition of Malignant Pleural Effusion
A malignant pleural effusion (MPE) is defined as a pleural effusion in which malignant cells are identified in the pleural fluid or pleural tissue. 1, 2
Diagnostic Criteria
The diagnosis of MPE requires positive cytologic or tissue confirmation of malignant cells to establish the diagnosis definitively. 3 This is critical because:
- Even in patients with known malignancy, up to 50% of pleural effusions are benign, making cytologic or histologic confirmation essential before labeling an effusion as malignant 3
- The presence of malignant cells in the effusion indicates poor prognosis and advanced disease 1, 2
Key Distinguishing Features
Fluid Characteristics
- Almost all malignant pleural effusions are exudates, though rarely they may have transudative characteristics 1, 2
- The fluid may be serous, hemorrhagic, or chylous (particularly in lymphoma) 2
Diagnostic Confirmation Methods
- Pleural fluid cytology is positive in 31-60% of cases on first sample 1, 2
- If the first cytology is negative, it should be repeated a second time, as an additional 27% of cases are diagnosed from the second specimen 1
- Pleural biopsy adds approximately 7% diagnostic yield when cytology is negative 1
- Both cell blocks and fluid smears should be prepared for examination 1
Important Clinical Context
Paramalignant vs. Malignant Effusions
It is crucial to distinguish true malignant effusions from paramalignant effusions, which occur in cancer patients but are not directly caused by pleural metastases. 2 Paramalignant effusions result from:
- Postobstructive pneumonia 2
- Thoracic duct obstruction causing chylothorax 2
- Pulmonary embolism 2
- Transudative effusions from atelectasis or low oncotic pressure 2
- Treatment-related causes (radiation therapy, chemotherapy drugs) 2
Prognostic Significance
- The presence of malignant cells in pleural fluid indicates poor prognosis with median survival of 3-12 months after diagnosis 1, 4, 3
- For non-small cell lung cancer, malignant pleural effusion upstages the disease to stage 4 5
- Median survival after first thoracentesis is approximately 6-7 months, though this varies widely 1