Background: Ecological studies in predominantly European populations have reported higher cancer survival in areas of higher solar ultraviolet (UV) B irradiation, perhaps due to a cancer protective effect of vitamin D synthesized photochemically in the skin. Such studies have not been done in developing countries, perhaps because of lack of cancer registries that can do outcome follow-up. One minus the mortality-to-incidence ratio (1-MIR), however, can be used as a measure of survival, and MIR as a measure of fatality, in developing country cancer registries. We analyzed the association between ambient solar UVB and MIR in China.
Methods: National cancer registration data in 32 counties of China in 2004-2005 were used to estimate MIR by age, sex, and area. The accuracy of 1-MIR as a measure of survival was assessed in the Cixian County cancer registry. Contemporary satellite measurements of cloud-adjusted ambient UVB intensity at 305 nm were taken from an NASA database and spatial Kriging methods used to estimate the average daily irradiance in each county. We estimated mortality hazard ratios (HRs) per 10 mW/m(2) of UVB for all cancers together, and the ten commonest cancer types by fitting a generalized linear model assuming mortality had a binomial distribution conditional on the sum of mortality and incidence, adjusted for sex, age, and location.
Results: The 5-year survival proportions for the main cancer types were in good agreement with 1-MIR in Cixian County. MIR ratios for all cancers combined were inversely associated with ambient UVB in men (HR = 0.96, 95% CI 0.93-0.99) and women (HR = 0.91, 95% CI 0.88-0.94) and in urban (HR = 0.95, 95% CI 0.94-0.96) and rural areas (HR = 0.90, 95% CI 0.87-0.93). Similar inverse associations were present for cancers of esophagus, stomach, and bladder in both sexes together and breast cancer in women. They were present in urban residents for all major cancers except liver cancer, bladder cancer, and breast cancer in women. For rural residents, most HRs were <1.0 but, with the exception of breast cancer, their upper 95% confidence bounds were >1.0.
Conclusion: Ambient UVB was significantly inversely associated with MIR for all cancers together and four of ten cancer types. Solar UVB may increase survival from some cancers in China.