In early September, startling cancer research findings hit the news.
A study press release headline touted a “global surge” in new cancer cases among younger people over the past 3 decades.
Many major news outlets parroted the “striking” finding. “Cancer cases in under-50s worldwide up nearly 80% in 3 decades, study finds,” The Guardian reported.
The analysis, published in BMJ Oncology, plumbed data from the Global Burden of Disease 2019 study to determine changes in cancer incidence and deaths among people aged 15-49 years across 204 countries.
The team found that, between 1990 and 2019, global cancer cases in this younger group had increased by almost 80% and cancer deaths had risen by nearly 28%. The authors flagged diet, alcohol, and tobacco as “the main risk factors” underlying the early-onset cancer trend.
But
The global population has increased by 46% between 1990 and 2019, but the study calculations are “based on absolute numbers rather than age-standardized rates,” said Montserrat García-Closas, MD, MPH, PhD, professor of epidemiology at the Institute of Cancer Research, London, who weighed in on the findings via the U.K.-based Science Media Centre. That means “these numbers do not account for changes in demographics such as increases in population size or aging of the population.”
The study researchers reported 1.82 million early-onset cancer cases in 1990 and 3.26 million global cases in 2019, which led to the reported increase of 79.1%.
Similarly, the authors calculated the change in cancer deaths globally using absolute, not population or age-adjusted, numbers: 0.83 million cancer deaths in 1990 and 1.06 million in 2019 led to the reported increase of 27.7%.
But when population growth is considered, the story changes dramatically. The population-adjusted calculations indicate that the global incidence of early-onset cancers only rose about 6% over the past 30 years while cancer deaths actually fell 25% in that time, according to calculations done by Medscape using the study’s supplemental data.
Experts commenting via the BMJ website also noted the flawed calculations. “Epidemic news, but no epidemic of cancer,” Henrik Møller, MD, lead epidemiologist for the Danish Clinical Registries, and colleagues wrote, highlighting the “misleading” 79% figure. When accounting for population growth in Nordic countries, Dr. Møller and colleagues found a 1% average annual increase in the cancer incidence rate and a 2.5% decrease in the cancer mortality rate.
This news organization reached out to the study’s corresponding authors, Kefeng Ding and Xue Li, to ask why they used absolute numbers instead of population-adjusted numbers for their calculations, but they did not respond in time for publication.
In their analysis, however, the researchers did note that “the study still has several limitations” that could affect the results, such as variations in the quality and availability of data provided by different countries.
The study, for instance, compared the Solomon Islands with the other 203 nations and concluded that the Solomons had the highest age-standardized death rate for early-onset cancer (82.9 per 100,000). However, this tiny South Pacific nation, whose population is scattered across 350 islands, did not start collecting cancer data until 2008 and founded its first oncology unit in 2019.
The authors also reported the “sharpest increases” in cancer cases diagnosed between 1990 and 2019 in the United Arab Emirates (1,127.6%), Qatar (1,089.5%), and Saudi Arabia (896.0%); however, those numbers do not seem possible, given population growth during that time, and may instead reflect reporting or other changes in those countries.
Although the overarching conclusion may be misleading, some of the numbers ring true, especially for breast cancer. The researchers found that the incidence of early breast cancer increased nearly 18% – from 11.2 to 13.2 per 100,000 – between 1990 and 2019.
This increase is “consistent with what is happening” in the United Kingdom, said Stephen Duffy of Queen Mary University of London, also weighing in via the Science Media Centre. Since the United Kingdom does not routinely screen women under 50, this rise “is not due to increased diagnostic activity.”
Darren Brenner, MD, associate professor in oncology at the University of Calgary (Alta.), said in an interview he agreed that the breast cancer trends look accurate.
In a 2020 study, Brenner and colleagues found that breast cancer diagnoses in women under 40 had increased significantly between 2000 and 2015, at a rate of 0.66% per year. “Given that breast cancer at a younger age is associated with worse outcomes, the results are troubling,” Brenner and colleagues concluded at the time.
The experts commenting via Science Media Centre reported no conflicts of interest.
A version of this article first appeared on Medscape.com.