Can MRI accurately identify which patients with operable breast cancer will have a pathologic complete response after neoadjuvant therapy?

616 Background: With the introduction of targeted therapy based on tumor subtypes, an increasing number of patients that receive neoadjuvant chemotherapy achieve a pathologic complete response (pCR). Previous studies have shown that the accuracy of MRI is poor at predicting the response to neoadjuvant chemotherapy in locally advanced and often non-resectable breast cancers, where the rate of pCR is low. The purpose of this study is to evaluate MRI’s ability to predict a pCR in operable breast cancers after neoadjuvant therapy. Methods: All patients enrolled in the NSABP B-40, B-41, FB-5 and FB-6 protocols in a single tertiary care centre, that had an MRI done before and after neoadjuvant therapy were reviewed. A radiologist, blinded to the pathology results, interpreted the pre- and post- treatment MRI’s and made a prediction as to whether or not patients would have a pCR. In this study, a true negative was defined as a reading of a complete response on MRI that was confirmed as a pCR on final pathology. pCR was defined as having no residual invasive or in situ disease in the breast. Results: 129 women with a median age of 51 years were identified. 90% had invasive ductal carcinoma; 8% had invasive lobular. 58% were ER+, 21% were triple negative and 21% were Her2+. 16% of patients had a pCR. 25% of patients had no residual invasive cancer in the breast. pCR rates for ER+ tumors was 5%, triple negative 37%, and Her2+ 26%. 19% of patients that had a pCR had a total mastectomy. The sensitivity and specificity of MRI for predicting residual disease were 88% and 52% respectively. The positive predictive value was 90% and the negative predictive value was 46% with an accuracy of 82%. Conclusions: MRI has limited value for determining which patients had a pCR after neoadjuvant chemotherapy, even in operable breast cancers. When residual disease is suspected on MRI, it is unlikely that a pCR has been achieved. Surgical excision following neoadjuvant therapy remains the gold standard to identify which patients have achieved a pCR. Other modalities will need to be used in order to accurately determine which patients would be eligible for studies evaluating non operative management following neoadjuvant therapy.

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