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The variation in the occurrence of cancer types between different parts of the world gives some indication of the proportion of cancers that could be prevented by modifying specific harmful lifestyle or environmental factors. Removal of HPV infection would substantially reduce the burden of cervical cancer; smoking and indoor and outdoor air pollution explain over two-thirds of lung cancer incidence. Yet, for many cancers, the causes remain largely unknown. Only 5–20% of all prostate, colorectal and breast cancers could be prevented by better diet, increased physical activity, or reduced alcohol consumption.

Variations in incidence rates largely reflect differences in distribution of risk factors.

Average of the 5 highest and lowest incidence rates (age-standardized rate per 100,000) by cancer site and sex, 2003–2007 (Example cancer registries labeled with highest and lowest rates)

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Chart showing average of the 5 highest and lowest incidence rates (age-standardised rate per 100,000) by cancer site and sex, 2003–2007 which indicates that variations in incidence rates largely reflect differences in distribution of risk factors

Across the globe there are striking geographical differences in cancer occurrence, mortality and survival.

In addition to differences in risk factors, higher awareness in the population combined with more widespread early detection practices also explain the national and regional variation for some cancers.

Finally, regional differences in incidence will be reflected in mortality. Yet death from cancer is also influenced by early detection and access to adequate treatment. For example, less access to diagnosis and treatment facilities for prostate cancer is partly responsible for the higher mortality observed in low-income settings. On the other hand, for cancers for which treatment does not greatly affect survival, e.g. liver cancer, the regional mortality profile mimics that of incidence.

The causes of a large portion of commonly diagnosed cancers in Western populations remain unknown.

Estimated number of new cancer cases (2012) and percent attributable to unknown risk factors by cancer site

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Illustration showing new cancer cases attributable to known preventable risk factors by cancer site which indicates that a large part of the causes for the most common cancers in Western populations remains unknown

‟Cancer varies between different populations, and every type is rare in some part of the world. Many specific causes are now known (to explain these differences), but a large proportion of global variation for common cancers remains unexplained.”

Peto J. Nature, 2001