Is Minimal Pleural Effusion Clinically Relevant in Lung Cancer?

Minimal pleural effusion (PE; defined as <10 mm thick) poses a diagnostic dilemma in non–small-cell lung cancer (NSCLC) staging. The 7th edition of the TNM staging system classifies malignant PEs as M1a metastasis. However, neither thoracentesis nor pleural biopsies are recommended for minimal PEs. Therefore, minimal PE is not accounted for in NSCLC staging. To address this issue, investigators retrospectively reviewed chest computed tomography scans of 2061 patients with NSCLC at a single hospital in South Korea and identified 272 patients (13.2%) with minimal PEs.

Patients with minimal PE had shorter median overall survival than those without PE (7.7 vs. 17.7 months; P<0.001); fully adjusted modeling showed that the presence of minimal PE was an independent prognostic factor for shorter overall survival (OS; hazard ratio, 1.4; P<0.001). The impact on OS was greater in early-stage disease (HR for stage I, 2.07, vs. HR for stage IV, 1.16; P_interaction=0.001). However, the incidence of minimal PE was lowest in early- versus later-stage NSCLC (5.2% in stage I, 10.9% in stage II, 13.2% in stage IIIA, 23.8% in stage IIIB, and 13.9% in stage IV). The development of minimal PE was associated with shorter OS in stage IV disease (HR, 1.20; P<0.001). The authors also reported both indirect etiologies (e.g., tumor involvement of mediastinal lymph nodes, severe comorbid disease) and direct etiologies (tumor invasion of pleura) in 177 patients (65.5%) and found that those with direct causes of minimal PE had worse overall survival outcomes (HR, 1.75).