Well my mom had colon cancer. It was only in her liver at the time of diagnosis. Her cea at the tine was 41. Her oncologist didn't repeat her cea for two months after that but then it was 17. We didn't want to happy only on cea report but her pet scan was also negative n lesions were disappearing.
Her cea started to rise gradually. We asked around alot n everyone was of the opinion that cea is not a good marker of cancer progression or mets etc. But that was not right in my mom's case. The doctors missed two new lesions that had formed one time. The second time, they let her have surgery saying there's nothing new on the scan to worry about cea and it turned out to be peritoneal metastasis.
I have personally stopped trusting doctors about these things but i do trust my friends who have themselves or one of their family members have been through this. What I've seen is that cea as a cancer marker is not reliable in everyone. But you should look at its trends throughout your cancer journey. If it has always elevated in reponse to disease progression and decreased with treatment then its a good marker for you and can be of alot of help.