CEACAM5 and Pleural effusion: Among patients who received only chemotherapy as systemic therapy, the MOS of intrathoracic administration group (IAG; i.e., EG and ITG) was longer than that of CG (12 vs. 6 months; p = 0.034), and the MOS of patients with a ratio of carcinoembryonic antigen in pleural effusion (PE‐CEA): CEA in blood (B‐CEA) ≤1 is worse than that of patients with a ratio >1 (4 vs. 12 months, p = 0.021) and that of CG (4 vs. 6 months, p = 0.442).