Welcome to HCM.
The diagnosis of cancer in the esophagus
has already been in established in your father, probably by doing endoscopy
and taking a biopsy
from the growth detected on endoscopy.
Malignancy markers such as CEA are used to study whether the malignancy remains stationary or subsides or is progressing. Comparisons are made in the serial study of the Marker results in that particular patient.
What the doctor is doing is perfectly right.
Wishing your father all the best.