A JAMA Oncology paper estimating the global burden of cancer is getting a lot of attention in the press. The study’s key findings are:
In 2015, there were 17.5 million cancer cases worldwide and 8.7 million deaths. Between 2005 and 2015, cancer cases increased by 33%
A 33% increase in cancer cases!!! There must be an epidemic!!! Why are so many people getting cancer all of a sudden!!!
It’s time for everyone to calm down a bit. Let’s break down these numbers a bit more. Of the 33% increase in cancer cases over the last 10 years, 16% is due to population aging, 13% to population growth, and 4% due to age-specific rates. Let’s take these one by one.
Cancer is a disease that largely occurs in old age. One good way to avoid getting cancer is to die young. Countries with high infant mortality rates, war, contagious disease (HIV, Zika, etc.) are likely to have lower cancer rates. This is not because they are healthier, but because people die before reaching an age when they would get cancer. This concept is known as the the competing risks. Thus, observing that more people are dying of cancer could mean that people are living longer and are less likely to be dying from other diseases.
The second component of cancer growth is population growth. If cancer incidence as a proportion of the population were constant, clearly an increase in the number of people living on the planet would increase the number of cancer cases. Population growth in and of itself, however, is not a bad thing.
Now we get to the bad news: Age-specific mortality rates increased by 4%. That means, for a given person aged 50-54, or aged 55-59, the likelihood they would get cancer increased by 4%. However, this statistic blurs differences between countries. Age-specific mortality rates decreased by 0-10% in most of the Americas and Western Europe, decreased by 10-20% in Russia and Eastern Europe, and decreased by more than 20% in China. Many African countries, in contrast, saw increasing age-specific morality rates.
Further, if you look at the age-standardized absolute years of life loss metric (AS-YLL), for 24 of 31 cancers the AS-YLL decreased (i.e., there were fewer years of life lost) and for 9 of 31 cancers there was no statistically significant change in AS-YLL.
In this post, I do not mean to play down the severity of cancer. It is a serious disease and additional efforts are needed to improve the care and outcomes of cancer patients. What I do want to highlight, however, is that it pays to did a bit deeper into the statistics and not always rely on the headlines.