How are almost half of cancer deaths preventable?

The title of a recent Nature News and Views article stated "Almost half of cancer deaths are preventable." In what sense is this true?  The TL;DR is that behavioral, and hence preventable, risk factors such as smoking and drinking alcohol are the biggest contributors to cancer worldwide. Modeling suggests that modifying these behaviors would have a substantial impact on decreasing cancer incidence.

More specifically, researchers found that avoidable risk factors caused almost 4.5 million cancer deaths in 2019, accounting for over 44% of global cancer deaths. Smoking, alcohol use, and high body-mass index (obesity) were the largest contributors.

As background, previous research primarily focused on estimating the cancer burden caused by single risk factors globally or multiple risk factors within one country. However, there was a lack of global estimates that consider a wide range of risk factors. For the first time, a new study reported the global cancer burden attributable to a comprehensive list of behavioral, metabolic, and environmental/occupational risk factors using data from the Global Burden of Diseases, Injuries, and Risk Factors (GBD) 2019. 

GBD is a comprehensive research effort that provides a systematic, scientific analysis of the magnitude and trends of health problems around the world. It quantifies health loss from hundreds of diseases, injuries, and risk factors across different regions, age groups, and genders over time. It aims to provide a comparable assessment of the burden of various health conditions and risk factors to inform health policy and resource allocation. More specifically, GBD 2019 provides estimates on mortality, incidence, years of life lost (YLLs), disability-adjusted life years (DALYs), etc. for 369 causes of death as well as 87 risk factors across 204 countries. 

The risk factors were divided into behavioral, environmental and occupation, metabolic, dietary, and other risks. 

BD 2019 provides estimates on mortality, incidence, prevalence, years of life lost (YLLs), years lived with disability (YLDs), and disability-adjusted life years (DALYs) for 369 causes of death and disability, as well as 87 risk factors and their groups, at the global, regional, and national levels across 204 countries and territories.  Disability-Adjusted Life-Years (DALY) "is a measure of overall disease burden, expressed as the number of years lost due to ill-health, disability or early death."

Estimating the role of various risk factors in mortality and disability involves three separate calculations. First, there is the risk factor exposure which estimates the exposure levels of various risk factors across different populations. Second, researchers determine the relative risks (RRs) associated with different levels of exposure to each risk factor based on data in the literature. Finally, one combines the estimated exposure levels and relative risks to calculate the attributable burden of disease for the risk factors.

The paper focused on cancer as the cause of death, and looked at association with the 87 risk factors, which included behavioral, environmental and occupational, and metabolic risk factors. In 2019, all of the measured risk factors combined to cause 4.45 million deaths and 105 million DALYs globally, representing 44.4% of all cancer deaths and 42.0% of all DALYs. The leading global risk factors for cancer deaths and DALYs in 2019 were smoking, alcohol use, and high BMI (obesity). The risk-attributable cancer burden varied by region and socio-demographic index (SDI). In low SDI (poorer) regions, the top risk factors were smoking, unsafe sex, and alcohol use. In high SDI regions, the top risk factors were smoking, alcohol use, and obesity. From 2010 to 2019, global risk-attributable cancer deaths increased by 20.4%, DALYs by 16.8% with metabolic risks seeing the largest increases.

In terms of incidence (see Figure 1), the most prevalent cancers attributable to risk factors were lung, colon and rectum (CRC), esophageal, liver, stomach, breast, pancreatic, and prostate cancer. These are similar to the most common cancers regardless of risk factor (link). Liver, stomach, and esophageal cancer were much more prevalent in low SDI countries than high SDI countries.

From 2010 to 2019, global cancer deaths from these risk factors increased by about 20%, with excess weight being the largest contributor, especially in lower-income countries.

In summary, roughly half of global cancer deaths in 2019 were preventable in the sense that they were due to avoidable risk factors such as tobacco, alcohol, unhealthy diets, unsafe sex, etc. In total 44.4% of the cancer deaths were linked to one of 87 risk factors, and the vast majority of deaths could be accounted for by preventable risk factors that could be avoided, for example, by changes in behavior or diet. 

Figure 1. "Global deaths from cancers attributable to risk factors in 2019 by sex and SDI. Cancer types are listed from left to right in order of greatest to least risk attributable deaths. SDI=Socio-demographic Index."  Number of deaths are on the y-axis with low SDI country numbers shown above 0, whereas high SDI country numbers are shown below 0 (from Fig. 2 of GBD 2019 Cancer Risk Factors Collaborators, The Lancet, 2022).