Can carcinoembryonic antigen replace computed tomography in response evaluation of metastatic colorectal cancer?

Introduction: Response Evaluation Criteria in Solid Tumours (RECISTs 1.1) define computed tomography (CT) as the gold standard in response evaluation of patients with metastatic colorectal cancer (mCRC) who are undergoing chemotherapy. The aim of this study was to evaluate whether carcinoembryonic antigen (CEA), which is cheaper and easier to perform, can replace repeated CT.Material and methods: The study included 66 patients with non-resectable mCRC participating in a phase I-II study. CEA values were determined, and CT images were taken every 2 months. CT images were externally and retrospectively reviewed according to the RECIST 1.1 criteria. Different cut-off values for CEA change in percent (DeltaCEA%) compared with baseline or nadir value underwent testing to find patients with disease control (that is stable disease, partial or complete response) at 2, 4, 6 and 8 months, in order to identify those who could have continued with chemotherapy based on CEA values alone. CT verification is needed in progressive disease (PD), and therefore identifying PD patients was our secondary endpoint.Results: The results showed that by using a cut-off value of 0 for DeltaCEA%, disease control was seen in all patients at all measuring points (negative predictive value (NPV)=1.0). Secondarily, increasing CEA was able to identify all PD patients (sensitivity (Se)=1.0) and in 50-74% of the patients increasing CEA provided a lead time to PD on upcoming CT. It was possible to replace CT with CEA in all patients with decreasing CEA, meaning that 23-47% of CT scans could have been avoided at any given time point.Conclusion: When the CEA level at a certain measuring point is the same or lower than CEA at baseline or at nadir (the measuring point with the lowest CEA value) during treatment, CEA can replace CT.