Background: Vitamin D is a secosteroid that exerts immunomodulatory and anti-proliferative effects through the vitamin D receptor (VDR). Because HER2-targeted therapies substantially improve prognosis in HER2-positive breast cancer and introduces a new mechanism of immunotherapy, we hypothesized that successful correction of vitamin D deficiency would be associated with improved disease-free survival (DFS) in patients treated with curative intent. Methods: We performed a retrospective interventional cohort study of patients with early-stage HER2-positive breast cancer treated at Cancer Treatment Centers of America Midwestern Regional Medical Center from 2008 to 2014. Eligible patients had baseline vitamin D deficiency (25-hydroxyvitamin D or D25 50 ng/mL) had the most favorable DFS trends, suggesting a dose response. Conclusions: Failure to correct a vitamin D deficiency was associated with a 1.7-fold higher recurrence risk, although the relationship did not achieve statistical significance. A similar effect size was reported in another retrospective cohort of HER2-positive breast cancer that did achieve statistical significance, and a doubling of pCR rates was seen in two recently completed RCTs in 2025, with benefits particularly seen in the triple-negative and HER2-positive subtypes. Prospective trials evaluating optimized vitamin D repletion in HER2-positive breast cancer are warranted.
Ahn et al. (Thu,) studied this question.