Background: The season of diagnosis of several forms of cancer has been observed to impact survival, supporting the hypothesis that vitamin D3 has a protective role in cancer survival. All previous studies demonstrating this seasonality were performed in European populations residing at latitudes upwards of 50°N. This study investigated whether seasonality of prognosis persists in populations residing in the lower latitudes of the contiguous United States (Latitude 21°N to 48°N). Material/Methods: The 5-year survival data of 19 204 female breast cancer, 6740 colorectal cancer, and 1644 prostate cancer cases was analyzed. Results: Female breast cancer patients exhibited improved survival when diagnosed in the summer as compared to the winter at all latitudes (Hazard Ratio [HR]: 0.940, 95%; Confidence Interval [CI]: 0.938 to 0.941, P=0.002). Colorectal cancer and prostate cancer also exhibited a similar seasonal pattern (HR: 0.978, 95% CI: 0.975 to 0.980, P=0.008 and HR: 0.935, 95%, CI 0.929 to 0.943, P=0.006), respectively, when the analysis was restricted to northern regions. Conclusions: These observations contribute to the mounting evidence that vitamin D3 may affect the progression of cancer. Data also suggest that vitamin D3 status at the onset of treatment may synergistically improve the prognosis of several cancer types.

Keywords: Colorectal Neoplasms - epidemiology, Breast Neoplasms - epidemiology, Geography, Linear Models, Proportional Hazards Models, Prostatic Neoplasms - epidemiology, SEER Program, Seasons, United States - epidemiology