Changes in risk of death from breast cancer with season and latitude

The Norwegian counties can conveniently be divided in three groups with different annual UV exposures and different incidence rates of squamous cell carcinoma (SCC) of the skin. In view of the hypothesis that latitude and season of diagnosis may play a role for breast cancer progression, the prognosis of breast cancer as determined for summer and winter diagnosis, were evaluated in the three residential regions. Two age groups were analysed separately (stratification at 50 years). For all regions, and for all ages, the prognosis was best for women diagnosed in the summer season (Relative risk (RR) of death was 15–25% lower for summer diagnosis versus winter diagnosis). There was no significant seasonal variation of the number of new cases. For women diagnosed before the age of 50, a geographical gradient in cancer prognosis was also found (RR of death 0.6, 95% CI: 0.5–0.7 for cases diagnosed in southeast Norway and RR of death 0.8, 95% CI: 0.6–1.1 for diagnosis in the north of Norway). This is in agreement with a 1.5 times larger annual UV exposures and 3–4 times larger incidence rates of SCC in the southeast region when compared with the north region. For women diagnosed after the age of 50, no significant difference was found between the three regions. Despite a 17% higher vitamin D intake from food in north of Norway no difference in cancer survival was found for diagnosis during winter (when no significant differences in the levels of UV exposure can be detected between regions). The overall data support our earlier hypothesis that season of diagnosis and therapy start improves the survival for breast cancer.