A 68-year-old woman with breast cancer metastases presents with progressive dyspnea. Chest CT shows a right-sided pleural effusion with nodular pleural thickening and enhancement. Which imaging feature best distinguishes malignant pleural effusion from benign parapneumonic effusion?

Explanation

## Imaging Distinction: Malignant vs Parapneumonic Pleural Effusion ### Radiological Features on CT **Key Point:** **Pleural nodularity with focal enhancement** on contrast-enhanced CT is the most specific imaging feature for malignant pleural involvement. This reflects tumor implants on the visceral and parietal pleura. **High-Yield:** Malignant pleural effusion imaging hallmarks: - Nodular or irregular pleural thickening (>3 mm) - Heterogeneous enhancement after IV contrast - Focal pleural nodules or masses - Often unilateral - May show pleural calcification (mesothelioma) - Associated mediastinal or hilar lymphadenopathy common Benign parapneumonic effusion features: - Smooth, thin pleural thickening (<3 mm) or no thickening - Homogeneous fluid attenuation - Loculation and septation (from fibrin deposition) but NO nodularity - Usually ipsilateral to pneumonia - Adjacent consolidation or ground-glass opacities - Mediastinal lymph nodes reactive (mild enlargement only) ### Pathophysiologic Basis Malignant effusions arise from **direct pleural invasion or metastatic implants**, creating a nodular, enhancing surface. Parapneumonic effusions result from **reactive inflammation** secondary to adjacent pneumonia — the pleura remains smooth but may form adhesions (loculation) without nodular disease. ### CT Technique Importance **Clinical Pearl:** Contrast-enhanced CT (arterial or portal venous phase) is essential to detect pleural enhancement. Non-contrast CT may miss subtle nodularity. Pleural nodules that enhance suggest malignancy; smooth thickening without nodules favors benign etiology. **Mnemonic:** **MALIGNANT pleural signs = Nodules, Enhancement, Irregular thickening, Lymphadenopathy** ### Diagnostic Yield When pleural nodularity is present, malignancy is suspected in >90% of cases. Absence of nodularity does not exclude malignancy (10–15% of malignant effusions have smooth pleura), but presence of nodules is highly specific. [cite:Harrison 21e Ch 297; Robbins 10e Ch 15]