Breast cancer survival and season of surgery: an ecological open cohort study

BACKGROUND: Vitamin D has been suggested to influence the incidence and prognosis of breast cancer, and studies have found better overall survival (OS) after diagnosis for breast cancer in summer–autumn, where the vitamin D level are expected to be highest. OBJECTIVE: To compare the prognostic outcome for early breast cancer patients operated at different seasons of the year. DESIGN: Open population-based cohort study. SETTING: Danish women operated 1978–2010. CASES: 79 658 adjusted for age at surgery, period of surgery, tumour size, axillary lymph node status and hormone receptor status. STATISTICAL ANALYSIS: The association between OS and season of surgery was analysed by Cox proportional hazards regression models, at survival periods 0–1, 0–2, 0–5 and 0–10 years after surgery. A two-sided p value <0.05 was considered statistical significant. RESULTS: Only after adjustment for prognostic factors that may be influenced by vitamin D, 1-year survival was close to significantly associated season of surgery. 2, 5 and 10 years after surgery, the association between OS and season of surgery was not significant. LIMITATIONS: Season is a surrogate measure of vitamin D. CONCLUSIONS: The authors found no evidence of a seasonal variation in the survival after surgery for early breast cancer. Lack of seasonal variation in this study does not necessarily mean that vitamin D is of no importance for the outcome for breast cancer patients.