The effects of vitamin D (Vit D) replacement on pathological complete response ( PCR) in breast cancer (BC) patients receiving neoadjuvant systemic chemotherapy (NAC).

e12604 Background: Studies show that Vit D increases apoptosis by inhibiting the proliferation of BC cells. The aims of this study were to prospectively investigate the effects of Vit D replacement on PCR rates in patients receiving NAC. Methods: Breast cancer patients qualified for NAC between June 2019 and September 2020 and consent for the study were given weekly oral 50,000 IU Vit D (study group). The Control group consisted of patients who were given NAC between 2015-2020 and who did not receive Vit D replacement. Tumor characteristics and PCR compared between the study and control groups. Results: A total of 117 patients included in the study (55% in the study group). Both groups had similar patient, tumor, and treatment characteristics. PCR (ypT0/is ypN0 and ypT0/is) rates were significantly higher in the study group (28.1% vs 11.3%, p=0.025 and 37.5% vs 13.2%, p=0.003, respectively). In multivariate logistic regression analysis, estrogen receptor (ER) and HER 2 positivity, and Vit D3 replacement were identified as independent variables affecting PCR. Conclusions: Adding Vit D to NAC in breast cancer patients significantly increases PCR. This effect is more prominent in ER negative and HER2 positive breast cancer patients. Clinical trial information: NCT03986268. [Table: see text]