Background: Breast cancer (BC) has been increasing globally, though it is unclear whether the increases are seen across all age groups and regions and whether changes in rates can be primarily attributed to decreasing fertility rates. We investigated age-specific trends in BC incidence and mortality from 1990 to 2017, worldwide and by region, and evaluated whether incidence trends are explained by decreases in fertility. Methods: We used country-level data to examine trends in BC incidence and mortality rates from 1990 to 2017 by region and age group. Linear mixed models were used to estimate age-specific rates from baseline models of year and were compared to fertility-adjusted models for incidence. Results: The global BC mortality rate increased overall by 0.23% per year (95% CI=0.20, 0.25), with statistically significant increases in the under 50 and 70 and over age groups, and in 5 out of 7 regions. The global BC incidence rate increased overall by 1.44% per year (95% CI=1.42, 1.47), with statistically significant increases in all age groups, and in 6 out of 7 regions. After adjusting for fertility, the incidence annual percent change (APC) remained statistically significant (APC=0.84, 95% CI=0.81, 0.88), in all age groups, and in 6 of 7 regions. Interpretation: The global increase in BC mortality is seen in most age groups and regions. The global increase in BC incidence is seen in all age groups and is highest in women under 50; increases remained in most regions even after considering declining fertility rates.
IMPORTANCE During the past several decades, breast cancer incidence has been increasing for women younger than 40 years. The increase matches the decrease in parity, an established breast cancer risk factor, but secular trends in incidence have not been examined prior to the 1970s. OBJECTIVE To examine whether secular trends in parity explain the increase in breast cancer incidence among US women aged 25 to 39 years from 1935 to 2015. DESIGN, SETTING, AND PARTICIPANTS This population-based cohort study used populationbased aggregate-level data from the Connecticut Tumor Registry (CTR) to examine breast cancer incidence and age-standardized rates among women aged 25 to 39 years from 1935 to 2015. National mean live births were calculated using birth data from the National Vital Statistics System (NVSS) from 1930 to 2015 (allowing for 5-year lag). Linear regression was used to compare a baseline model of year estimating age-adjusted breast cancer incidence rate with a model that adjusted for parity constructs. MAIN OUTCOMES AND MEASURES Breast cancer incidence rates among women aged 25 to 39 years from 1935 to 2015. RESULTSAmong women in Connecticut aged 25 to 39 years from 1935 to 2015, incidence of breast cancer for women aged 25 to 39 years increased 0.65% (95% CI, 0.53%-0.77%) per year, from 16.3 breast cancer diagnoses per 100 000 women in 1935 to 38.5 breast cancer diagnoses per 100 000 women in 2015. This increase began nearly 4 decades before the secular decrease in parity (mean [SD] parity peaked at 2.26 [0.87] live births per woman in 1966 and in 2010 had decreased to 1.41 [0.71] live births per woman). Age-specific parity trends explained only 0% to 4% of the variability in incidence over time. CONCLUSION AND RELEVANCEThese findings suggest that breast cancer incidence for women aged 25 to 39 years has been significantly increasing since the 1930s and cannot be attributed to changes in parity over time.
A lthough colorectal cancer (CRC) incidence has decreased over time in adults aged 50 years, largely due to advancements and uptake in screening, incidence has dramatically increased over the past 4 decades in adults aged < 50 years in the United States. 1 Previous studies have only evaluated age-specific trends in CRC incidence from the mid-1970s or later, providing only a few years of data before the documented increase in early-onset CRC. This makes it difficult to establish the temporal relationship between potential risk factors and CRC incidence, given the induction time of most cancers. For example, it is not known if early-onset CRC incidence was increasing before the rise in obesity rates in the United States, which also began to increase across age groups in the mid-1970s. 2 To further inform hypotheses about the etiology of CRC, we evaluated age-and sex-specific trends in CRC incidence rates from 1935 to 2017 using the oldest population-based cancer registry in the United States, which includes almost 4 more decades of data compared with national cancer registries.
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