A “worrying” progression, figures that send chills down your spine, a “spectacular” increase… The increase in the number of cancers among those under 50, recently revealed by an international research team, has been widely reported Wednesday, September 6 in the media. With 3.26 million cases detected in 2019, tumors have increased by 79% in thirty years in this age group. Something to be afraid of, especially since deaths have also increased in this population, by around 28% over the same period, according to this same study. At this age, should we not be preserved from this affection, one of the evils of the century?
If cancer is indeed an increasingly significant pathology in the world, and a public health priority, the estimate published Tuesday in the scientific journal British Medical Journal (BMJ) gives the impression that the risk of contracting it has also increased by 79%. And so that a mysterious evil would gnaw at the young. A misconception, despite appearances: “There is no epidemic of cancer in younger people. These figures do not allow us to understand the evolution of the risk”, nuance Philippe-Jean Bousquet, epidemiologist at the Institute National Cancer Center (Inca).
If the increase reported in the BMJ stomach, it is largely explained by the increase in the world population, a much more important factor than a possible increase in the “carcinogenicity” of the products we consume, of the air we consume. breathes, or genetic propensities to develop tumours. There were 5.3 billion people in 1990, compared to 7.7 today. This increase, of around 45%, mechanically affects the number of cases identified. Yet essential, this point, well specified by the authors of the study, has not always been mentioned in the articles on the subject.
How to assess the risk?
Another statistical pitfall: the study, carried out on 29 tumors from the Global Burden of Disease database, does not compare the evolution in young people with that in older people. However, it is in this age group, by far, that cancers are the most numerous – almost two out of three tumors occur in those over 75 in France, according to the Inca. “This does not mean that the work is bad, it is simply a choice of researchers. By focusing on young people, we better understand the disparities between countries, we can follow the disease, from diagnosis to therapy. C This is the purpose of the study”, specifies Steven le Gouill, director of the Institut Curie.
Finally, the statistical registers are very unequal from one country to another. Some are very comprehensive, others quite partial. There may be gaps, which give rise to large variations from one year to the next, and missing data, another important element to take into account, also recalled by the researchers. Also, the older you get, the rarer the tumors become, which again makes the data more difficult to interpret. In France, official Inca records show no incidence below 40 years because its experts consider that it is not possible to draw general conclusions with a number of cases which remains limited..
Would the news be good, after all? No, the risk of cancer increases at all ages. “But to account for the notion of risk, oncologists rather look at the incidence rate, that is to say the number of cases for an equal population, per 100,000 people,” continues epidemiologist Philippe-Jean Bousquet . In these terms, the most common cancer in the world in women, breast cancer, only jumps, according to data collected in the BMJ study, by 8.1% in those under 50. . The second, that of the skin, “only” 4.3%. The third, that of the cervix, “only” 1.2%. Alcohol consumption, smoking, and lifestyle habits (diet, sedentary lifestyle, exposure to the sun, etc.) seem to be the main causes, even if scientists do not know everything that causes these pathologies, and that new carcinogens can emerge.
Thinking in terms of incidence allows us to better understand the phenomenon, but this indicator must also be taken with caution: “The incidence rate is very dependent on the way we diagnose. The more we test, the more we find”, specifies Catherine Hill, epidemiologist at Gustave Roussy. And to cite the example of the explosion in cases of thyroid cancer detected in South Korean women between the years 2000 and 2007: + 340%. However, 90% of these cancers were due to overdiagnosis according to a study carried out by a French team and published in The New England Journal of Medicinein 2016. These tumors, sometimes removed with the help of major operations and treatments, would not have been bothersome in reality – a cancer can remain stable and asymptomatic.
A two-speed world
Conversely, there are countries where certain cancers are not screened, and are identified at death. Hence this paradox: for the same population, there are more deaths in the least developed countries, but there are more cases in the most developed countries. The latter detect more and earlier, due to better health systems and prevention policies. “The study reveals a two-speed world, and the dynamic could increase, while the new drugs placed on the market are sold at exorbitant prices,” regrets Steven le Gouill, director of the hospital complex at the Curie Institute.
Beyond reminding us that the benefits of drinking less, of not smoking, of eating healthy, of playing sports, and this from a very young age, the work published in the BMJ above all allow us to realize the differences in exposure to carcinogens, depending on the place in the world: “There are countries where tobacco consumption remains higher, like in France, others where it is decreasing. The air is more polluted in India than in France, and we generally eat less well in the United States than here”, lists epidemiologist Philippe-Jean Bousquet. Thus, alcohol accounts for 16.7% of the risk factors for dying from colorectal cancer in Western Europe, compared to less than 2% in North Africa and the Middle East.
Although there is no general epidemic of cancer among young people, early-onset cancers are increasing at a worrying rate in certain regions of the world. This is the case for colorectal cancers, which last March were the subject of a scientific post in the journal Science, summarizing the latest work on the issue, and calling for further research. Specialists do not know exactly what is due to the increase observed, especially marked in the United States and increasingly in the West. But, it is likely that it is explained “by the increase in consumption, from an early age […] red and processed meat, and sugary drinks”. And in France? “For the moment, this trend is not confirmed in France, says Philippe-Jean Bousquet, of the National Cancer Institute. But we are watching closely, because the eating habits of the French are also deteriorating”.