CEACAM5 and lung cancer: As early‐generation EGFR‐TKIs such as gefitinib may confer distinct prognostic outcomes, we further evaluated whether baseline serum CEA expression modifies the prognostic effect of first‐line EGFR‐TKI therapy—specifically comparing gefitinib with other EGFR‐TKIs—in patients with EGFR‐mutant lung cancer.