By Dr. Nathan Schachter

This year, the Canadian Cancer Society reported that nearly 50% of Canadians will get cancer in their lifetime, and 1 in 4 will die from it. This is expected to result in healthcare burdens of unprecedented magnitude, not to mention the emotional trauma surrounding cancer diagnosis and treatment. The enormity of this health crisis begs a fundamental but crucial question: How many cancer cases are avoidable?

While links between cancer incidence and environmental factors have been known for decades, scientists have struggled to quantify the precise cancer risk that is ‘preventable’. For example, although we are aware that smoking is strongly linked to lung cancer, it is still unclear what percentage of lung tumors could be altogether prevented if smoking and pollution did not exist.

Therefore, if we continually pursued healthy lifestyle choices and avoided exposure to cancer-causing agents, could we outrun the risk of cancer?

The first answer to this question appeared in early 2015, when Dr. Bert Vogelstein’s research group at Johns Hopkins University attempted to estimate the contribution of environmental factors to cancer risk1. Knowing that 5-10% of cancers have a hereditary component, they sought to understand whether the remaining percentage of cancers is caused by extrinsic factors (i.e. environmental causes such as cigarette smoke) or intrinsic factors (i.e. unavoidable biological mishaps).

Their findings, published in the renowned scientific journal ‘Science1, were cause for both concern and dismay. Vogelstein’s group suggested that mutations which accidentally occur during normal cell division could explain 2/3 of cancer cases. In other words, ~65% of cancer risk is unavoidable and based solely by intrinsic factors. This hypothesis, dubbed the ‘Bad Luck Theory’ of cancer, made international headlines and provoked strong opposition from health advocacy groups worldwide. After all, these conclusions certainly appeared to undermine the message that healthy living is an effective means of mitigating cancer risk.

Given the impact of these conclusions, it was only a matter of time before other researchers offered opposing theories and in January of 2016, the Hannun group published a strong rebuttal in ‘Nature2 – another top-tier science journal. After re-examining some assumptions made by the Vogelstein group, Hunnan and his colleagues incorporated other statistical, mathematical, and genetic data to draw a new set of conclusions:  That ~90% of cancer risk is attributed to extrinsic factors and can largely be avoided.

While these studies offer extreme and opposing estimates of our capacity to prevent cancer, several findings about cancer prevention from both groups were identical:

1) Avoiding exposure to carcinogens such as cigarette smoke, sunlight, and excessive alcohol irrefutably lowers your risk of developing many common forms of cancer.

2) Maintaining a healthy diet and body weight reduces cancer incidence in the general population.

3) Preventing infection by certain viruses (e.g. HPV, Hepatitis B & C) and bacteria (e.g. H. pylori) has the potential to eliminate a number of avoidable malignancies.

Despite ongoing debate about the true number of preventable cancer cases, scientific data suggest these strategies are still the best way to lower your lifetime risk of cancer and avoid development of preventable malignancies.

  1. Tomasetti and Vogelstein (2015) Variation in cancer risk among tissues can be explained by the number of stem cell divisions. Science. doi: 10.1126/science.1260825.
  2. Wu et al. (2016) Substantial contribution of extrinsic risk factors to cancer  development. Nature. doi: 10.1038/nature16166

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