What can this cea pattern mean on chemotherapy for colon cancer mets 65 before chemo, 80 after round 1, 55 after round 2?

It's improving!: All it really means is the CEA is going in the right direction ... falling after chemotherapy. You can not reliably speculate on what the number changes might say about the magnitude of improvement. There is too much individual variability and unpredictability for that. I would be pleased that these changes have occurred. Best wishes as you continue your treatment:)

Progression disease: CEA as a tumor marker is only effective in monitoring and not diagnosing cancer. If in presence of known disease the CEA continues to rise then progression of disease being monitored. When CEA goes from 80 to 55 after round 2 of chemo then there an effective method of defining regression of metastatic disease. If CEA rises then change of protocol would be required.