CEACAM5 and squamous cell carcinoma: Considering histology, a marker’s reduction improved PFS in adenocarcinoma patients (CEA,7.1 vs 1.9 months, p = 0.013; CYFRA21-1, 7.9 vs 1.9 months p < 0.001; NSE 5.9 vs 1.9 months, p = 0.067), while in patients with squamous carcinoma PFS was improved only in patients with CYFRA21-1 reduction ≥ 20% (6.1 vs 1.7 months, p = 0.032).