Comparison of Contralateral Breast Background Parenchymal Enhancement on MRI Before and After Neoadjuvant Chemotherapy According to Molecular Subtypes in Unilateral Breast Cancer

<b>Background/Objectives:</b> To evaluate changes in background parenchymal enhancement (BPE) in the contralateral breast on MRI before and after neoadjuvant chemotherapy (NAC), stratified by molecular subtype in patients with unilateral breast cancer. <b>Methods:</b> This study retrospectively analyzed 116 individuals diagnosed with unilateral breast cancer by biopsy, all of whom underwent breast MRI examinations before and after neoadjuvant chemotherapy. Contralateral breast BPE was graded into four levels (BPEC: 1 = minimal, 2 = mild, 3 = moderate, 4 = marked) by two readers in consensus. Histopathological features and BPE reduction were compared according to molecular subtype. <b>Results:</b> BPE showed a reduction across all molecular subtypes after NAC. In ER-positive cancers, BPEC shifted from 26/16/28/30% to 68/28/4/0%; in HER2-positive cancers, from 37.8/26.7/22.2/13.3% to 73.3/20.0/6.7/0%; and in triple-negative breast cancers, from 47.6/14.3/23.8/14.3% to 76.2/14.3/9.5/0%. Compared to the ER-positive cancer, the reduction in BPE over time was significantly greater in the HER2-positive cancer group (Estimate = 0.48, <i>p</i> = 0.0168) and TNBC (Estimate = 0.55, <i>p</i> = 0.0321), suggesting that the extent of BPE decrease varied by subtype. <b>Conclusions:</b> The extent of BPE reduction on breast MRI following NAC varies significantly across different molecular subtypes of breast cancer.