BACKGROUND. This study investigated the role of magnetic resonance imaging (MRI) in evaluation of pathologically complete response and residual tumors in patients who were receiving neoadjuvant chemotherapy (NAC) for both positive and negative HER‐2 breast cancer. METHODS. Fifty‐one individuals, comprised of 25 HER‐2 positive and 26 HER‐2 negative patients, were included in the study. Serial MRI studies were acquired before, during, and after NAC. On the basis of the final MRI, response was determined to be a clinically complete response ([CCR], no enhancement), probable CCR (residual enhancement equal to or less than that of glandular tissue), or residual tumor. All patients received surgery. Pathological outcomes were categorized as 1) no residual cancer, 2) no residual invasive cancer but ductal carcinoma in situ (DCIS) present, or 3) residual invasive cancer. The pathologically complete response (pCR) was defined as no invasive cancer. RESULTS. Complete clinical response as seen through MRI, including CCR and probable CCR, was identified in 35 (35 of 51, 69%) patients. MRI correctly diagnosed pCR in 26 (26 of 35, 74%) patients, including 18 of 19 (95%) patients in the HER‐2 positive group and 8 of 16 (50%) patients in the HER‐2 negative group (P < .005). The accuracy of MRI in identifying pCR varied according to the chemotherapy agent that was administered. MRI was more accurate in identifying pCR in patients who were receiving trastuzumab and less accurate in patients receiving bevacizumab. The high false‐negative rate found in HER‐2 negative patients was associated with residual disease that presented as scattered cells or small foci. In cases with residual bulk tumor, the lesion size, determined by MRI, correlated highly with that found in histopathological measurements (r = 0.93). CONCLUSIONS. MRI may predict pCR with high accuracy in HER‐2 positive patients, but it has a high false‐negative rate in HER‐2 negative patients, particularly in patients who are receiving antiangiogenic agents. Results indicate that the chemotherapy agent should be taken into consideration when using MRI to interpret therapeutic outcomes. More studies are needed to establish the role of MRI in managing, especially surgical planning, patients who are receiving NAC. Cancer 2008. © 2007 American Cancer Society.

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