Rising cancer rates in younger generations signal urgent need for early-life prevention strategies

By Tarun Sai LomteReviewed by Susha Cheriyedath, M.Sc.Aug 4 2024

In a recent study published in The Lancet Public Health, researchers assessed cancer incidence and mortality trends in the United States (US).

Previously, the authors reported an increased incidence of eight cancers over time in successively younger birth cohorts in the US. Besides, people born between 1965 and 1980 might have increased incidence rates of all leading cancers combined and specific cancers, such as colorectal, kidney, thyroid, leukemia, and uterine corpus. However, a comprehensive analysis of trends in cancer incidence and mortality by year of birth or birth cohort is lacking for contemporary generations.

Study: Differences in cancer rates among adults born between 1920 and 1990 in the USA: an analysis of population-based cancer registry data. Image Credit: SewCreamStudio / Shutterstock

About the study

In the present study, researchers analyzed incidence and mortality trends of cancers by birth cohort in the US. They acquired incidence data from the North American Association of Central Cancer Registries for 34 cancers diagnosed at ages 25–84 between 2000 and 2019. Data on mortality were obtained from the National Center for Health Statistics; mortality analysis was restricted to 25 cancers.

Population estimates were acquired from the US Census Bureau. Cases were classified according to the International Classification of Disease Oncology and into molecular, histological, and anatomical subtypes. Birth cohort trends in cancer rates were evaluated using age-period-cohort models, adjusted for age and period effects. Five-year intervals were used to categorize age groups and cancer incidence/death period.

Nominal birth cohorts were created, resulting in partially overlapping birth cohorts. For example, the 1990 birth cohort represented the experiences of those born approximately in 1990, with four-fifths of cases/deaths involving those born during 1987-93. Cohort rate ratio curves were generated. Further, incidence (IRR) and mortality rate ratios (MRR) were computed for each cohort relative to the reference cohort.

The team analyzed cancers for which IRRs increased with successive birth cohorts. Further, cancers with a reversal of IRR trends between younger and older cohorts (e.g., decreasing IRR for older cohorts but increasing for younger cohorts) were examined. The mean annual percentage changes were calculated from 2000 to 2019.

Findings

Overall, data for over 23.65 million cancer patients and 7.34 million cancer deaths between 2000 and 2019 were analyzed. IRRs increased for eight cancers in successive birth cohorts. The incidence rate of pancreas, kidney and renal pelvis, and thyroid cancers was 2–3 times increased in the 1990 birth cohort compared to the 1955 birth cohort. Females in the 1990 birth cohort had elevated incidence of cancers of the intrahepatic bile duct and the liver.

Further, the incidence of non-human papillomavirus (HPV)-associated pharyngeal or oral cancers was higher in females in the 1985 birth cohort. There was a significant birth cohort effect on the incidence trends of eight cancers. Mortality data were available for eight cancers. Birth cohort trends in mortality rates fluctuated, decreased, or plateaued in 1955 to 1990 birth cohorts for all these cancers except for female intrahepatic bile duct and liver cancer.

In adults aged 25–49, pancreatic, kidney, renal pelvis, and small intestine cancers had the fastest annual increases in incidence rates. By contrast, bimodal patterns emerged for intrahepatic bile duct and liver and non-HPV-associated pharyngeal and oral cancers in females, with rapid increases in those aged 30–39 or 55–64. Further, mortality rates stabilized or decreased in people aged 25–49, except for intrahepatic bile duct and liver cancer in females in the 35–39 age group.

Reversals in IRRs, viz., increased IRRs in younger cohorts following a decrease in older cohorts, occurred for nine cancers (estrogen receptor-positive positive breast, gallbladder, uterine corpus, testicular, colorectal, non-cardiac gastric, male anal, male Kaposi sarcoma, and other biliary cancers). The cancer incidence rate in the 1990 birth cohort was 12% to 169% higher compared to the birth cohort with the lowest rate. MRRs generally had similar trends as the incidence rates.

Conclusions

Taken together, the findings revealed increases in incidence rates for 17 cancers in progressively younger birth cohorts. Mortality trends for colorectal, testicular, gallbladder, uterine corpus, and female intrahepatic bile duct and liver cancers mirrored incidence trends. The growing incidence in successive younger cohorts suggests increases in carcinogenic exposure in early life or youth. Further research is required to identify the underlying risk factors and inform prevention strategies.