CEACAM5 and neoplasm: Furthermore, we found that a CEA serum level ≥ 40 ng/mL is the more important factor associated with CNS metastasis development during follow-up (Table 2), independent of clinical stage (IIIB or IV) and absence or presence of liver metastasis, suggesting that association between high CEA serum levels and brain metastasis development are not due to tumor charge but rather to a more invasive phenotype.