Objective: To evaluate inter-observer agreement and the predictive value of tumor size measurements using MRI for breast cancer under neoadjuvant chemotherapy (NAC) by comparing the measurements of the longest diameters (LD), total enhanced volumes (TEV) and washout volumes (WOV).Methods: Thirty-seven female breast cancer patients were prospectively enrolled from August 2008 to October 2010. Two of these patients had locally advanced disease. MRI examinations were acquired within 2 weeks before and after NAC. Interim scans were also conducted in 30 patients. Tumor resection was undertaken within 2 weeks after the cessation of NAC. MRI images were independently measured for LD, TEV and WOV by two experienced radiologists. Inter-observer agreement was evaluated using concordance correlation coefficients (CCCs). Tumor sizes after NAC were evaluated relative to their initial sizes for early prediction of a pathological complete response (pCR).Results: The CCCs were 0.93 (CI: 0.90-0.95) for LD, 0.98 (CI: 0.97-0.98) for TEV and 0.99 (CI: 0.991-0.996) for WOV. All measurements had high inter-observer agreement, but the CCCs were significantly increased in the aforementioned order (P < 0.0001). WOV measured after the completion of chemotherapy had significant discriminating ability (P = 0.0056) when evaluated using receiver operating characteristic analysis, and was found to be superior to LD (P = 0.045). The average WOV size was significantly smaller in pCR cases than in non-pCR cases (P = 0.016).Conclusion: Computer-aided detection-generated tumor volumes had significantly higher inter-observer concordance than conventional LD measurements. WOV measurements had the highest concordance, and WOV could better predict pCR after NAC at smaller tumor sizes. (c) 2011 Elsevier Ireland Ltd. All rights reserved.