Objective: To assess the association of MRI BPE and pathological response in women diagnosed with stage II/III breast cancer submitted to NAC. Methods: This observational and cross-sectional retrospective study was performed in consecutive women who underwent NAC and had MRI exams before and after chemotherapy. The MRI was done before and after 2 weeks of completing NAC. BPE was classified according to ACR-BIRADS 5th edition. The type of BPE before NAC, its changes and the relationship to total pathologic complete response (TpCR) were evaluated. Data were paired with patient age, size on MRI before and after NAC, features of clinical response according to the RECIST criteria, tumor grade and immunohistochemical (IHC) subtypes. MRI assessment included amount of fibroglandular tissue, symmetry of BPE and measurement of tumor at the longest diameter. All images were blinded reviewed by a radiologist. We used for the changes of the BPE the Bowker symmetry test or the McNemar test and to analyze the factors related to the clinical and pathologic responses, logistic regression analysis. The level of significance adopted was 5% (p<0.05). Results: We studied 71 women between 2009 and 2016. The medium age was 37 years old. BPE was symmetrical in 68 women (95.8%). Moderate and marked BPE was present in 28 (39.4%) of the affected breasts and in 25 (34.2%) of the contralateral breasts. After NAC all BPE were symmetrical and just 3% of them were moderate or marked. Regarding the IHC subtype, 40 women (56.3%) were triple negative or HER2 positive, and these women had a higher frequency of TpCR (55% for each, compared to 12.9% in patients with luminal subtypes). We found to be independently associated with pCR: the reduction of BPE (in the affected or contralateral breast) and the molecular subtypes triple negative and HER2 positive Table 1. Multivariate Analysis related to TpCR (n=71).VariableCategoryP-ValueO.R.*CI 95% O.R.*Tumor Size on MR pre-MAC (cm) 0.1710,8590.691-1.068Luminal subtypeLuminal B (ref.)---1.00--- Luminal A0.3120.450.10-2.11 HER2pos/ Luminal B HER20.0055.781.71-19.58 Triple negative0.0493.271.01-10.64Age (years) 0.3870.9820.942-1.023Nottingham grade1 (ref.)---1.00--- 20.0817.830.78-79.16 30.0967.100.71-71.31BPE pre-NAC (S or A)Asymmetric (ref.)---1.00--- Symmetric0.3273.030.33-27.76BPE pre-NAC affected breastMinimal (ref.)---1.00--- Mild0.8120.860.24-3.09 Moderate0.3711.890.47-7.64 Marked0.5911.570.30-8.17BPE pre-NAC contralateral breastMinimal (ref.)---1.00--- Mild0.7130.790.22-2.81 Moderate0.2502.330.55-9.77 Marked0.4701.880.34-10.43BPE ChangeSame/increased (ref.)---1.00--- Reduction0.0263.011.14-7.96* OR (Odds Ratio) = Risk ratio to pCR; (n=26 pCR, n=7 DpCR, n=31 PR e n=7 ED, where Total pCR is pathological complete response (without invasive and DCIS in the breast and axilla) DpCR is pathological response with just DCIS, PR is partial response and ED is stable disease – we haven't progression disease). CI 95% OR = Confidence interval of 95% to risk ratio. Ref.: reference level. Proportional risk models. Conclusion: BPE reduction was significantly associated with TpCR. Nevertheless, patterns of BPE pre-NAC have no association with pathological response. Citation Format: Teixeira SRC, Camargo HSAd, Ramalho S, Natal R, Machado HdC, Camargo MMAd, Azevedo J, Arruda MdS, Negrão EMS, Almeida NR, Teixeira AL, Cabello V, Cabello C. Breast cancer and magnetic resonance imaging (MRI): Background parenchymal enhancement (BPE) predicting response to neoadjuvant chemotherapy (NAC) [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-02-09.