Carcinoembryonic antigen (CEA) monitoring of radiation therapy for colorectal cancer.

Serial CEA radioimmunoassays were performed on 16 patients receiving preoperative radiation therapy of rectal cancer or irradiation of recurrent or metastatic colorectal cancer. Radiation therapy of localized colorectal cancer reliably reduced previously elevated circulating CEA titers. Significant decrease of elevated CEA titers with accumulating doses of irradiation may indicate that the bulk of CEA-producing tumor is within the radiation treatment portal and assist in patient management decisions. The decrease of circulating CEA with preoperative radiation therapy was of short duration and may indicate that surgical resection should not be delayed more than 6-8 weeks after irradiation. Because of the high frequency of false positive and false negative results, CEA must be used only in conjunction with other clinical and laboratory parameters.