To retrospectively investigate the characteristics of background parenchymal enhancement (BPE) in the contralateral breast following neoadjuvant chemotherapy (NAC) and whether BPE could help predict tumor response in early stage of advanced breast cancer.

Data from 161 patients who were diagnosed with unilateral breast cancer and then underwent NAC before surgery were analyzed retrospectively from August 2014 to December 2016. All the patients underwent both bilateral breast MRI scan with contrast enhancement. Two experienced radiologists independently categorized the patients' levels of BPE into four categories (1=minimal, 2=mild, 3=moderate, 4=marked) at baseline and after the 2nd cycle of NAC. All the patients were divided in to pathologic complete response (pCR) group and non-pCR group according to the histopathologic tumour response. The status of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) were recorded before underwent NAC. The t test and Pearson chi-squared test were used to compare the baseline characteristics of the pCR and Non-pCR groups. The kappa test was used to test the inter-observer agreement. The Wilcoxon test was used to test for changes of BPE categories after NAC. The Mann-Whitney U test was used to test the difference of BPE between pre-and post-menopausal status. Associations were evaluated using Binary logistic regression models.

Fifty nine patients achieved pCR, and 102 patients had residual disease (non-pCR). Age, tumor size, distribution of size, menopausal status and lymph node showed no significance between pCR and non-pCR groups (all P>0.05), while only ER/PR status and HER2 status had a significant difference (P>0.05 in both). Inter-observer agreement regarding BPE categorization was moderate and substantial before and after NAC (Kappa value 0.644 and 0.708). The level of BPE was higher in premenopausal than post-menopausal women both at baseline and after the 2nd cycle of NAC (P<0.01). Decreased BPE was observed in 106 cases (premenopausal 60 cases and postmenopausal 46 cases), and no change in BPE were observed in 55 cases (premenopausal 27 cases and postmenopausal 28 cases). A significant reduction in BPE was observed after the 2nd NAC cycle in the overall cases, pre-and post-menopausal cases (all P<0.01). Logistic model showed that hormonal receptor (HR) negative and HER-2 receptor at baseline and the change of BPE after NAC were the independent factors for predicting pCR.

Regardless of the menopausal status, BPE showed a reduction after NAC, and it can serve as an additional imaging biomarker of tumour response at an early stage of NAC.

Breast neoplasms; Magnetic resonance imaging; Background parenchymal enhancement; Neoadjuvant chemotherapy; Treatment outcome