Impact of Vitamin D Levels on Progression-Free Survival and Response to Neoadjuvant Chemotherapy in Breast Cancer Patients: A Systematic Review and Meta-Analysis

Background: Breast cancer remains the leading cause of cancer-related deaths among women despite advances in early detection. Neoadjuvant chemotherapy (NACT) is now standard for early-stage BC, with vitamin D (VD) emerging as a potential prognostic biomarker considering its positive pleiotropic effects. This review and meta-analysis assess the impact of baseline VD levels on outcomes in BC patients undergoing NACT. Methods: Inclusion criteria required patients to be over 18 years of age, have a pathologically confirmed BC diagnosis, and have their VD levels assessed prior to chemotherapy. Studies were included if they reported odds ratios (ORs) for response and/or hazard ratios (HRs) for PFS with 95% confidence intervals (CIs). A comprehensive literature search of PubMed/MEDLINE and Scopus/ELSEVIER (2014-2024) was conducted, and data were analyzed using fixed- and random-effects models, with Forest plots illustrating the results. Study quality and potential biases were assessed using the MINORS, NOS, and RoB2 scales, and statistical heterogeneity was evaluated with I-2 statistics and funnel plots. Results: Six studies were included in the analysis. All studies addressed stages II and III, with three also including stage I. The meta-analysis covered data from 722 patients regarding NACT response and 1033 patients for PFS. The results revealed a 22% reduction in the likelihood of non-response to NACT associated with adequate VD levels (low/deficient VD vs. high/sufficient VD; OR: 0.78; 95% CI: 0.30-1.25; p = 0.001) and a 35% reduction in progression risk with sufficient baseline VD levels (low/deficient VD vs. high/sufficient VD; HR: 0.65; 95% CI: 0.33-0.97; p < 0.001). Conclusions: These findings highlight the significance of maintaining adequate vitamin D levels in BC treatment and encourage further studies to unravel the role of VD on cancer biology.