Donald Maxwell Parkin

Data from 20 years of cancer registration in Harare (Zimbabwe) are used to investigate the risk of cancer in the white population of the city (of European origin), relative to that in blacks (of African origin). In the absence of information on the respective populations-at-risk, we calculated odds of each major cancer among all cancers, and took the odds ratios of whites to blacks. Some major differences reflect obvious phenotypic differences (the very high incidence of skin cancer—melanoma and nonmelanoma—in the white population), whereas others (high rates of liver cancer, Kaposi sarcoma and conjunctival cancers in blacks) are the result of differences in exposure to infectious agents. Of particular interest are cancers related to lifestyle factors, and how the differences in risk are changing over time, as a result of evolving lifestyles. Thus, the high risk of cancers of the esophagus and cervix uteri in blacks (relative to whites) and colorectal cancers in whites show little change over time. Conversely, the odds of breast cancer, on average four times higher in whites than blacks, has shown a significant decrease in the differential over time. Cancer of the prostate, with the odds initially (1991–1997) 15% higher in whites had become 33% higher in blacks by 2004–2010.

The Kampala cancer registry is the longest established in Africa. Trends in incidence rates for a 20-year period (1991–2010) for Kyadondo County (Kampala city and a rural hinterland) illustrate the effects of changing lifestyles in urban Africa, and the effects of the epidemic of HIV-AIDS. There has been an overall increase in the risk of cancer during the period in both sexes, with incidence rates of major cancers such as breast and prostate showing particularly marked increases (3.7% and 5.2% annually, respectively). In the 1960s cancer of the oesophagus was the most common cancer of men (and second in women), and incidence in the last 20 years has not declined. Cancer of the cervix, always the most frequent cancer of women, has shown an increase over the period (1.8% per year), although the rates appear to have declined in the last 4 years. HIV prevalence in adults in Uganda fell from a maximum in 1992 to a minimum (about 6%) in 2004, and has risen a little subsequently, while availability of antiretroviral drugs has risen sharply in recent years. Incidence of Kaposi sarcoma in men fell until about 2002, and has been relatively constant since then, while in women there has been a continuing decline since 2000. Other HIV related cancers—non-Hodgkin lymphoma of younger adults, and squamous cell carcinoma of conjunctiva—have shown major increases in incidence, although the former (NHL) has shown a small decline in incidence in the most recent 2 years.

This chapter summarises the results of the preceding sections, which estimate the fraction of cancers occurring in the UK in 2010 that can be attributed to sub-optimal, past exposures of 14 lifestyle and environmental risk factors. For each of 18 cancer types, we present the percentage of cases attributable to one or all of the risk factors considered (tobacco, alcohol, four elements of diet (consumption of meat, fruit and vegetables, fibre, and salt), overweight, lack of physical exercise, occupation, infections, radiation (ionising and solar), use of hormones, and reproductive history (breast feeding)). Exposure to less than optimum levels of the 14 factors was responsible for 42.7% of cancers in the UK in 2010 (45.3% in men, 40.1% in women) – a total of about 134 000 cases. Tobacco smoking is by far the most important risk factor for cancer in the UK, responsible for 60 000 cases (19.4% of all new cancer cases) in 2010. The relative importance of other exposures differs by sex. In men, deficient intake of fruits and vegetables (6.1%), occupational exposures (4.9%) and alcohol consumption (4.6%) are next in importance, while in women, it is overweight and obesity (because of the effect on breast cancer) – responsible for 6.9% of cancers, followed by infectious agents (3.7%). Population-attributable fractions provide a valuable quantitative appraisal of the impact of different factors in cancer causation, and are thus helpful in prioritising cancer control strategies. However, quantifying the likely impact of preventive interventions requires rather complex scenario modelling, including specification of realistically achievable population distributions of risk factors, and the timescale of change, as well as the latent periods between exposure and outcome, and the rate of change following modification in exposure level.