Discrepancies Between Pathological Tumor Responses and Estimations of Complete Response by Magnetic Resonance Imaging After Neoadjuvant Chemotherapy Differ by Breast Cancer Subtype.

INTRODUCTION: The influence of breast cancer (BC) subtype in discrepancies between pathologic complete response (pCR) and complete response by magnetic resonance imaging (MRI-CR) after neoadjuvant chemotherapy (NAC) have not been discussed well. We evaluated the association between BC subtype and pCR or only residual in situ lesion without invasive cancer (pCR/in situ+ ) in patients with MRI-CR (positive predictive value [PPV]).

MATERIAL AND METHODS: From the data of 716 patients with primary BC who were diagnosed with invasive cancer and treated with NAC and then surgery from January 2009 to May 2014 at St. Luke's International Hospital, 180 patients were determined to have MRI-CR by retrospective chart review. BC subtypes at baseline were classified into 6 subtypes, as strong estrogen receptor (ER++ ), moderately positive ER (ER+ ), negative ER (ER- ), and HER2 status expression.

RESULTS: Three subtypes had PPV (pCR) ≥ 50%: ER- /HER2+ (56.3%, 27/48), ER- /HER2- (57.6%, 34/59), and ER+ /HER2+  (56.2%, 9/16). However, PPV (pCR) for the ER++ /HER2- and ER++ /HER2+ subtypes was < 30%; notably, only 12.0% (3/25) for the ER++ /HER2- subtype, which was significantly low (P < .001) compared with ER++ /HER2- and other subtypes. PPV (pCR/in situ+ ) was significantly low at 20.0% in the ER++ /HER2- subtype (P < .001 compared with other subtypes). PPV (pCR/in situ+ ) in other subtypes was collectively greater than 60%, and was 91.7% in the ER- /HER2+ subtype.

CONCLUSION: We should interpret carefully MRI-CR of NAC to evaluate residual disease for ER++ /HER2- BC.