To analyze the relationships between background parenchymal enhancement (BPE) of the contralateral healthy breast and tumor response after neoadjuvant chemotherapy (NAC) in women with breast cancer.

A total of 228 women (mean age, 47.6 years±10 [SD]; range: 24–74 years) with invasive breast cancer who underwent NAC were included. All patients underwent breast magnetic resonance imaging (MRI) before and after NAC and 127 patients underwent MRI before, during (after the 4th cycle of NAC) and after NAC. Quantitative semi-automated analysis of BPE of the contralateral healthy breast was performed. Enhancement level on baseline MRI (baseline BPE) and MRI after chemotherapy (final BPE), change in enhancement rate between baseline MRI and final MRI (total BPE change) and between baseline MRI and midline MRI (early BPE change) were recorded. Associations between BPE and tumor response, menopausal status, tumor phenotype, NAC type and tumor stage at diagnosis were searched for. Pathologic complete response (pCR) was defined as the absence of residual invasive cancer cells in the breast and ipsilateral lymph nodes.

No differences were found in baseline BPE, final BPE, early and total BPE changes between pCR and non-pCR groups. Early BPE change was higher in non-pCR group in patients with stages 3 and 4 breast cancers (P=0.019) and in human epidermal growth factor receptor 2 (HER2)-negative patients (P=0.020).

Early reduction of BPE in the contralateral breast during NAC may be an early predictor of loss of tumor response, showing potential as an imaging biomarker of treatment response, especially in women with stages 3 or 4 breast cancers and in HER2 – negative breast cancers.