Background
There is growing concern that the incidence of early-onset cancer, including breast cancer, is disproportionately increasing compared to older age groups in some countries in Europe. Early onset, for most cancer types including breast cancer, is usually defined as cancer diagnosed before age 50. The European Society for Medical Oncology defines early-onset breast cancer as that diagnosed before age 40.

Objective
To investigate this further, we conducted a comprehensive analysis of invasive breast cancer incidence by age group, using high-quality data from national and regional cancer registries in 19 European countries.

Methods
Invasive breast cancer cases in women (ICD-10 code: C50) and the corresponding population counts were retrieved from the Cancer Incidence in Five Continents (CI5plus) database and the Association of the Nordic Cancer Registries (NORDCAN). Age-standardised incidence rates (ASIRs) per 100,000 person-years were estimated using the World Standard Population for all ages combined and for truncated age groups (0–39, 40–49, 50–69, and ≥70 years) from 1995 onwards, whenever data were available, and were stratified by country and grouped into four European regions (Northern, Western, Eastern/Baltic, and Southern Europe). The observation periods ranged from 13 to 27 years. Temporal trends were assessed using log-linear regression models to estimate the average annual percent change (AAPC) with 95% confidence intervals, and interaction terms were used to test differences in trends between age groups.

Results
Overall, the highest incidence rates in each age group were observed in Western Europe and the lowest in Eastern/Baltic Europe. Across all regions, ASIRs increased with age, ranging from <15/100,000 in women aged <39 years to >300-400/100,000 in those aged 70+. Roughly half of all cases occurred in women aged 50-69 years, compared to 4-6% of cases in those aged <40. Over the time periods examined, the AAPCs increased in almost all countries among women aged <40 years (mostly between +1% and +2.5%) and those aged 70+ (mostly between +0.5% and +1.5%). In two-thirds of countries, AAPC increases were seen in those aged 40-49 years (mostly between +0.5% and +1.3%) and those aged 50-69 years (mostly between +1.0% and +2.3%). Comparison of trends between age groups showed little significant difference in women aged <50 and ≥50. Detailed examination of the AAPC in women aged <40 compared to the older age groups suggested steeper increases in younger women in most countries, particularly in Western and Northern Europe. However, the magnitude of the contrasts between younger and older ages was modest.

Conclusions
The incidence rates for breast cancer have increased in most countries in Europe for all age groups examined over the time period of interest. While there is some evidence that the observed increases may be greater among younger women compared to older age groups, the difference appears small, and these results should be interpreted with caution, given the lower case numbers in the younger groups. Further examination of incidence rates by age is needed, taking into account any possible changes in methods of detection or diagnosis over time.