Seeing the words “cancer” and “bad luck” in close proximity to each other never elicits a positive response. Within hours of the article being published, reports such as “Two-thirds of cancer are due to “bad luck,” study finds” and “Biological bad luck blamed in two-thirds of cancer cases” were making headlines in major new sources. As a cancer biology student, I started receiving questions from perplexed family members and friends asking: “Is cancer really just due to chance?”
Cancer is a very complex and diverse group of diseases. There are over 200 different types of cancer arising in many different tissue types. On top of that, there are several factors that contribute to the development of cancer. Because of this complexity, it is very difficult to describe the cause of cancer as something as simple as “bad luck.” In 2014, the World Health Organization (WHO) reported that half of cancer cases are preventable. If two-thirds of cancer cases are just “bad luck,” then people might think there is little they can do to decrease their cancer risk and risky behaviors might increase. In actuality, numerous reliable studies have shown a link between certain environmental and genetic factors that contribute to cancer. Cancer biologists and oncologists would be worried if the public began to think that the majority of cancer is just by chance, especially since most environmental factors that contribute to cancer (diet, exercise, and tobacco use) can be controlled.
I read the original research paper, by Cristian Tomasetti, PhD and Bert Vogelstein, MD, to see exactly what the authors were trying to say and whether these findings contradict the current understanding of how much genetics, environment, and “bad luck” factor into cancer cases. It become clear that the authors were not talking about the difference in risk between cancer cases, but the difference between the risk of developing certain cancer types. This might seem like semantics, but it is an important point. First, it is important to know the definition of “risk.” Usually cancer risk, or a cancer risk factor, describes something that will increase the likelihood of a person getting cancer. This definition is more about probability than certainty. For example: if you smoke, then the likelihood of developing lung cancer is increased. Therefore, smoking is a cancer risk factor. But as we know, everyone who smokes does not develop cancer. This means that exposure to a risk factor is not a guarantee that you will get cancer. Another important point to note is that the term cancer cases refers to the number of individuals who are diagnosed with cancer. For example, in 2014 there were 224,210 new cases of lung cancer diagnosed in the US. The term cancer type refers to the type of tissue in which the cancer arises (i.e. breast cancer versus bone cancer).
The main question in this article the authors were trying to address was: why are some cancer types more commonly diagnosed (have more cancer cases) than others? For example, why is the lifetime chance of getting lung cancer 6.9% while those for brain and nervous system cancers are only 0.6%? To study this question, they wanted to determine whether the number of times a population of unique, self-renewing stem cells divided in each tissue type was correlated with cancers arising in certain tissues more often than others. After comparing 31 cancer types, their findings stated: “A linear correlation equal to 0.804 suggests that 65% (39% to 81%; 95% CI) of the differences in cancer risk among different tissues can be explained by the total number of stem cell divisions in those tissues.”1 Or simply put, two-thirds of the reason why cancer occurs more often in the colon than in the small intestine is because stem cells in the colon undergo more cell divisions, and therefore have more chances to accumulate random mutations than stem cells in the small intestine.
In the analogy, the 3 factors contributing to a car accident (cancer) could be the condition of the car (genetics), the conditions of the roads (environment), or the length of the car trip (number of stem cell divisions). All of the factors contribute to whether someone gets into an accident or not. If the car has poor brakes, the risk of having an accident is increased. If the roads are icy, the risk of getting into an accident is increased. The study showed that the longer the length of the trip, the more likely you are to get into an accident. Two-thirds of why people get into car accidents (develop cancer) on long trips (in tissues with more stem cell divisions) rather than short trips (in tissues less stem cell divisions) has little to do with the condition of their car (the influence of genetics) or how the roads are (the influence of environment).
Looking at them in this way, the findings make sense. Tomasetti and Volgestein wanted to share these findings because it is important to remember that sometimes accidents (cancers) are not preventable and that other approaches, like early detection and screening, are still important. That does not mean that we have no control over decreasing our chances of getting cancer. Remember, even if the trip is only one mile, if there is an ice storm, then there will be more accidents. A combination of decreasing cancer risks through awareness of environmental factors and early detection will help us in our fight against cancer.
1. Tomasetti C and Vogelstein B. Variation in cancer risk among tissues can be explained by number of stem cell divisions. Science (2015): 347(6217), 78-81.
2. Couzin-Frankel, J. Bad luck and cancer: A science reporter’s reflections on a controversial story. Science (2015): at < http://news.sciencemag.org/biology/2015/01/bad-luck-and-cancer-science-reporter-s-reflections-controversial-story>.
Jasmine Miller-Kleinhenz began her PhD studies in Cancer Biology at Emory University in 2011. Her dissertation work is in the lab of Dr. Lily Yang where she focuses on understanding the mechanisms of drug resistance in Triple-negative Breast Cancer (TNBC) and using theranostic nanoparticles to overcome this drug resistance. At Emory, she serves as President for the GDBBS Student Advisory Council and is a teacher-scholar for the On Recent Discoveries by Emory Researchers fellowship. She is also very interested in community development outside of Emory. She serves as as the coordinator for the Metro Atlanta area for training youth for roles as facilitators of the Junior Youth Spiritual Empowerment Program, whose aim is to empower young people at the neighborhood level to become agents of social change.