Concordance and Discordance Between PET/CT and CEA/CA19-9 Dynamics and Their Predictive Value for Clinical Prognosis in Colorectal Cancer

Aim: The study aimed to evaluate the concordance between treatment-related changes in fluorodeoxyglucose (FDG) positron emission tomography/ computed tomography (PET/CT) -derived metabolic parameters and serum tumor markers [carcinoembryonic antigen (CEA) and carbohydrate antigen 19-9 (CA19-9)] and to investigate the clinical significance of concordant and discordant response patterns in colorectal cancer.Methods: This retrospective study included 81 patients with histologically confirmed colorectal cancer who underwent FDG PET/CT before and after systemic chemotherapy.Metabolic response was defined as a ≥30% maximum standardized uptake value reduction, and ≥20% decrease in CEA or CA19-9 levels defined as tumor marker response.Patients were categorized into concordant-good-response, concordant-poor-response, and discordant-response groups based on PET and CEA responses.Survival was analyzed using Kaplan-Meier and Cox regression methods.Results: Following systemic chemotherapy, treatment responses were evaluated using changes in FDG PET/CT metabolic parameters and serum tumor markers.Significant reductions were observed in all PET-derived metabolic parameters (p