Chronic diseases such as cardiovascular disease and cancer are the most common public health threats in the 21st century. Cancer accounted for about 19% of the total disease burden in Australia in 20101 and the Australian community pays about $3.8 billion per year in direct health system costs.2 With the increasing and ageing population, the burden caused by cancer and the direct costs to the community will only increase. More and more people are now living with a diagnosis of cancer, and this also has indirect costs in support and follow-up services. Fortunately, in spite of the large and increasing impact of chronic diseases, they are also the most preventable diseases.3 Only about 5%–10% of cancers are due to genetic or inherited disorders, with the remainder being due to external factors (environmental factors or those related to human behaviour).4 The best evidence for this is from studies of people who have migrated.5,6 Clearly, not all external factors associated with cancers are understood or quantified. Recognising the need to address this lack of knowledge, the World Cancer Research Fund (WCRF) and the American Institute for Cancer Research (AICR) assembled an international group of cancer experts in 2004. Experts in cancer epidemiology, nutrition, public health and cancer biology systematically examined the association between food, nutrition and physical activity (including body fatness) and the prevention of cancer.3 They then calculated population-attributable fractions (PAFs) for the specific cancers included in their review.7 These PAFs represent the proportion of cancer incidence that could be prevented if poor nutrition and diet, physical inactivity and obesity were eliminated, while leaving other risk factors unchanged. While the theoretical impact of primary prevention is substantial, motivating populations to improve their health status is difficult. Therefore, unless a concerted and significant effort is made to invest in and implement powerful preventive measures, the impact of primary prevention on reducing total cancer incidence over the coming decades will probably be relatively small.8 Preventive measures are unlikely to be initiated by governments and policymakers in Australia unless they are viewed as high priority. Typically, this requires quantitative evidence. Our aim was to estimate the number of projected cancer diagnoses in Australia in 2025 that would be preventable solely due to improvements in diet and physical activity, and then consider the implications of taking action to reduce the future cancer burden.

Methods Cancer incidence data between 1982 and 2007 were obtained,9 with data grouped by cancer site, 5-year age group (up to 85 years or more) and sex. We focused on the cancer types included in the WCRF/AICR report that were therefore demonstrated to have some proportion of cases preventable by improved nutrition and physical activity. These comprised 12 types of cancers: mouth and laryngeal (C00–C02, C32), oesophageal (C15), lung (C33 and C34), stomach (C16), pancreatic (C25), gallbladder (C23 and C24), liver (C22), bowel (C18–C20), breast (C50), uterine (C54 and C55), prostate (C61) and kidney (C64) cancers, along with all cancer types combined (C00–C97, D45, D46, D47.1, D47.3). We did not include male breast cancer cases due to the small number of cases diagnosed each year. We were unable to match two categories of cancer in this study to the definitions in the WCRF/AICR report, due to the availability of Australian cancer incidence data. Therefore, instead of mouth, pharyngeal and laryngeal cancers (C00–C09, C10–C14, C32), we reported mouth and laryngeal cancers only (C00–C02, C32). Based on unpublished data,10 mouth and laryngeal cancers comprised about 54% of the original mouth, pharyngeal and laryngeal cancers group. Similarly, we included all uterine cancers (C54 and C55), although the WCRF/AICR report only included endometrial cancers (C54.1), representing about 82% of all uterine cancers.10 We used a two-stage process to estimate the number of cancers that would be diagnosed in 2025 and that could be prevented by diet and physical activity. First, we estimated the number of cancers that would be diagnosed in 2025 by applying the current age- and sex-specific incidence rates to the age- and sex-specific population projections. This method assumes that the age- and sex-specific cancer incidence rates, averaged over the years 2005–2007, will remain constant until 2025. The eventual validity of this assumption cannot be determined, but a similar modelling process was used recently for a major study,11 and the overall Australian cancer incidence rates have increased by less than 2% per year since 1998.9 Second, we multiplied the projected numbers of cancers by the published PAF estimates (percentages) from the WCRF/AICR report. The Australian Bureau of Statistics (ABS) has published three main series (A, B and C) of population projections for the Australian population from 2006–2101.12 Series B largely reflects current trends in fertility, life expectancy at birth, net overseas migration and net interstate migration, and series A and C are based on high and low assumptions of these variables, respectively. The number of projected cancer cases for each cancer type was obtained by applying series B population projections according to the following formula, where i represents the calendar year (2008, 2025), j is sex (males, females) and k is the 5-year age group. To improve stability, we used the average incidence rate over the combined 2005–2007 period as the current incidence rate. We incorporated a crude measure of quantitative uncertainty about these projected estimates by calculating corresponding estimates based on the ABS series A and C population projections. However, it is important to note that the greatest uncertainty with projections is not with “statistical noise” (random error), but lies with the unquantifiable bias that might occur if the age- and sex-specific incidence rates in the future are substantially different from those of the present. To estimate the number of preventable cancer cases for each cancer type, we multiplied the year-specific projected number of cancer cases by the corresponding PAF for that cancer. These data were obtained from the WCRF/AICR report (see Box 1).13 Separate estimates were published for the United States and the United Kingdom, and we applied the average of those two estimates to the Australian data since they shared similar socioeconomic, cultural and environmental characteristics.

Results Based on current trends in cancer incidence, population growth and ageing, our projections suggest that about 170 000 Australians will be diagnosed with cancer in 2025. This is an increase of almost 60% on the incidence in 2007 (almost 110 000) (Box 1 and Box 2). As shown in Box 1, the ageing of the population means that the gradient for cancer incidence projections is steeper than for population projections. By applying the WCRF/AICR preventability estimates, we suggest that by 2025, almost 43 000 of these cancers could be prevented through improvements in diet and physical activity levels, including their impact on obesity. The different population projection series had only a minimal impact on these projections of cancer incidence (Box 2). There were some differences between 2007 and 2025 in the ranking of cancer types according to the number of preventable cancers, reflecting the number of projected cancers and the preventable fraction of them. We estimated that bowel cancer would have the most cases in 2025 that would be preventable by changes in nutrition and physical activity. The middle estimate was 10 049 cases (Box 2). This was followed by female breast cancer with 7273 preventable cases. While only a relatively small proportion of prostate cancers are considered preventable (16%), the very high number of projected cases in 2025 meant that 4882 cases would have been preventable by 2025. Lung cancer (5736) and mouth and laryngeal cancers (2054) were also in the top five cancers whose incidence could be reduced by changes in diet and physical activity.