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Background: To assess the clinical usefulness of serum tumor markers for early detection of distant breast cancer recurrence using FDG-PET/CT. Methods: We retrospectively analyzed 561 consecutive patients who underwent surgery for invasive primary breast cancer and had increased tumor markers (CA 15-3 and CEA) after completion of locoregional therapy. FDG-PET/CT data were reviewed for all cases. CA 15-3 and CEA were evaluated both in a continuous and in a quartile (Q) distribution. The Wilcoxon rank-sum test and logistic regression models were used to evaluate the association between increased tumor marker values and the presence (and type) of distant metastases. Results: The median value of CA 15-3 was 35.0 U/mL (IQR, 29.5–43.0) in cases where no distant metastases were detected, and it was 58.9 U/mL (IQR, 40.0–108.0) in cases where metastases were detected (p < 0.001). The median value of CEA was 6.6 U/mL (IQR, 4.4–10.0) in cases of no metastases and 12.4 U/mL (IQR, 6.9–30.0) in cases of metastases (p < 0.001). Increased levels of both tumor markers (Q3 and Q4) were strongly associated with the presence of distant metastases. The association between CA 15-3 and bone/liver metastases was stronger compared with other types of metastases (p heterogeneity between odds ratios [ORs] = 0.03 for Q3 and <0.001 for Q4), while no relevant heterogeneity between ORs emerged for CEA. Conclusion: Increased tumor marker levels detected in asymptomatic breast cancer patients during adjuvant therapies and follow-up are significantly predictive of distant metastases identified on FDG-PET/CT. [ABSTRACT FROM AUTHOR]