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Purpose: To investigate the relationship between changes in vascularity and metabolic activity measured by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and dynamic (18)F-FDG-positron emission tomography (PET) in breast tumors undergoing neoadjuvant chemotherapy.Materials and Methods: PET and MRI examinations were performed in 14 patients with locally advanced breast cancer (LABC) before and after chemotherapy. Dynamic (18)F-FDG PET measures included (18)F-FDG transport rate constant from blood to tissue (K(1)) and metabolism flux constant (Ki). DCE-MRI measures included initial peak enhancement (PE), signal enhancement ratio (SER), and tumor volume. Spearman rank-order correlations were assessed between changes in PET and MRI parameters, and measures were compared between patients with and without pathologic complete response (pCR) by Mann-Whitney U-test.Results: Changes in glucose delivery (PET K(1)) were closely correlated with changes in tumor vascularity as reflected by DCE-MRI SER (r = 0.83, P < 0.001). Metabolic changes in PET Ki showed moderate correlations with vascularity changes as reflected by SER (r = 0.71) and PE (r = 0.76), and correlated closely with MRI tumor volume (r = 0.79, P < 0.001). Decreases in K(1), Ki, SER, and PE were greater for patients with pCR compared to those with residual disease (P < 0.05).Conclusion: Dynamic (18)F-FDG PET and DCE-MRI tumor measures of tumor metabolism, vascularity, and volume were well correlated for assessing LABC response to neoadjuvant chemotherapy and significantly discriminated pathologic complete responders. Further work is necessary to assess the value of combined PET and MRI for evaluating tumor pharmacodynamics in response to novel therapy. [ABSTRACT FROM AUTHOR]