The CEA levels cannot be the sole criteria of estimating either the tumor burden or the response. So, they should not be taken or considered of a very high prognostic value. These tumor markers should be a part of the other assessment. Since your tumor had shrunk and decreased probably that has more importance than anything else. While 15.3 is otherwise also not high, but as I said with the other thing taken in assessment, you have a fair response. For your type of cancer, it is not very reliable also. Read these resources about CEA;
Please feel free for your follow up questions.