Geographical factors

The location of a population may have a profound influence on its incidence of a given cancer. Standardized tumour registries have allowed detailed comparisons. Some commonly quoted examples include:

• the mortality from stomach cancer is 7 times higher in Japan than the United States
• the incidence rate of skin cancer is 200 times higher in northern Australia than India
• the incidence rate of lung cancer is 35 times higher in England than Nigeria

For these examples, it is thought that exposure to an environmental agent in a particular location results in the increased risk, e.g. sun exposure in Australia. In support of this is the evidence from migrant studies. Immigrants commonly acquire with successive generations the risk for a given cancer related to their new location.

Yet, it is often hard to extricate geographical factors from other differences which characterise communities, e.g. ethnic origin or national conventions. Hence, Mormons in Utah in the U.S. have a lower incidence of respiratory, gastrointestinal and genital cancer than other communities in the same state. Equally, the death rate from breast cancer is higher in Denmark than Sweden despite their close apposition; social differences in the number or pregnancies and breast feeding may have a role.