Worldwide, the incident cases of female breast cancer and ASIRs showed annual increases, and the higher the SDI, the higher the incidence. According to our prediction, ASIRs will continue to increase in the future, except for the high-income regions; the global ASIR will reach 47.36 per 100 000 persons in 2035. For most countries and regions, ASIRs will continue to be the highest among people aged over 70 years. However, in Indonesia and the Russian Federation, rates will remain the highest among people aged 50 to 69 years.
Of the six selected countries, only the USA showed a downward trend in ASIRs from 1990 to 2019, whereas India and China were the two countries where the ASIRs increased every year. ASIRs remained stable or declined slightly in the Russian Federation, Japan, and Germany over the past decade. However, despite the decline, the USA still had the highest ASIR among the six countries in 2019, whereas India had the lowest.
The USA’s success in breast cancer rates reduction was influenced by both period and cohort effects, especially among those born after 1960. The rise in breast cancer incidence rates in the early 1990s might reflect changes in women’s reproductive patterns at the time, such as delaying childbirth and declining fertility, both of which are associated with breast cancer risk. The increasing use of mammography screening was also a major factor, with screening rates rising from 29% in 1987 to 70% in 2000. However, since the late 1990s, the USA’s ASIRs demonstrated a clear declining trend, especially for invasive breast cancer, which showed a sharp drop (nearly 13%) between 1999 and 2004. This sharp drop could be attributed to the decline in menopausal hormone therapy after the publication of a randomized trial by the Women’s Health Initiative that linked menopausal hormone therapy with an increased risk of breast cancer. Another factor may be the slight decline in mammography screening rates since 2000. Notably, the decrease in the incidence rates of breast cancer was largely confined to white women in the USA, mainly to estrogen-receptor positive subtypes. From 2012 to 2016, the incidence of hormone receptor (HR)-positive cancers gradually increased, whereas HR-negative tumors decreased by 1.5–2.6% each year. The reasons for this difference in trends remain unclear but may relate to changes in subtype-specific risk factors. For instance, parity is associated with a lower risk of HR-positive breast cancer but a higher risk of triple-negative breast cancer. meanwhile, the fertility rate in the USA declined from 118 births per 1,000 women in reproductive age to 69.4 in 2007, and then continued to decline to 60.3 in 2017.
Germany recorded increased ASIRs from 1990 to 2009 and then showed a slow downward trend from 2010 to 2019, with little evidence of period and cohort effects. Changes in women’s reproductive behaviors in the 1990s (e.g., lower fertility rate and later childbearing) might be the major reason for the increase in breast cancer incidence, although this period was also marked by the increase in overweight and obesity rates among young German women. The national breast cancer screening program in Germany started in 2005 and was completely implemented nationwide until 2009. A study compared the breast cancer incidence rates before the program began and 10 years later, and the results indicated that breast cancer incidence typically peaked with the introduction of mammography screening, driven mainly by an increase in early-stage breast cancer. In the long run, the increase in ASIRs after the introduction of the mammography screening program might indicate over-diagnosis, a highly concerning adverse consequence. The temporary reduction observed before 2006 may be due to a decline in the prescription of hormone replacement therapy (HRT), after two large studies, reported that HRT increases the risk of breast cancer. According to the World Bank, the fertility rate in Germany began to show an upward trend from 2006, rising from 1.33 births per woman in 2006 to 1.57 in 2018. This increase may explain downward trend in the incidence of breast cancer in Germany after 2010, as infertility is a risk factor for the increased incidence of breast cancer.
Japan has unfavorable trends in breast cancer incidence, with significant period and cohort effects across 30 years. The ASIR peaked in women aged 45–49 years, consistent with the highest mammography screening rate in this age group, suggesting that a screening effect might play an important role in the increased incidence rate. Another possible explanation is that a general tendency toward later marriage and declining birth rates, as well as changes in the lifestyle of Japanese women, has led to an increase in the number of women at higher risk of breast cancer. Indeed, breast cancer incidence is on the rise as younger generations adopt Western culture, diet, and habits. A study by the Japan Public Health Center indicated that a Westernized dietary pattern (characterized by a high intake of red meat, refined grains, potatoes, and fat) is associated with an increased risk of breast cancer in Japanese women. Atomic bombing and nuclear radiation may also have contributed to the incidence rates, although these factors are more localized. In recent decades, the incidence in Japan had shown a declining trend. As mentioned before, the incidence in women aged 45–49 years was higher than that in women aged >50 years. With the aging of the population, the proportion of women over 50 years old increases, which might be one reason for the recent decreasing trend in ASIR.
The ASIR of female breast cancer in Russia continued to increase over the study period, rising sharply between 1990 and 1995, then falling back and then rising again at a slower rate. The increasing incidence could partially be explained by changing fertility rates in Russia, which fell sharply in the early 20th century and reached its lowest point in the 1950s. In the post-World War II generations, the incidence was relatively stable, which could be attributed to several balancing factors, including a decline in fertility, increase in obesity prevalence, and changes in dietary habits, as well as improved diagnostic methods and accidental early detection. Environmental factors, such as radiation, may also be at play. A study on the Techa River cohort (suffered from radioactive wastes) reported a significant temporal trend of breast cancer incidence, characterized by a linear dose-response relation. The fallout from ionizing radiation has also been significant, with women living in the Bryansk Oblast experiencing a significantly increased risk of breast cancer in the decades after the Chernobyl accident. A related cohort study found a dose-dependent relation between radiation and breast cancer incidence, and younger women are identified as at a higher risk when exposed.
China showed increasing ASIRs for the study period, driven by significant period and cohort effects. Higher incidence rates were observed among older women and more recent birth cohorts. These trends could be attributed to changes in lifestyle patterns, such as dietary change, decreased physical activity, and increased obesity rates. Data from the National Chronic Disease and Risk Factor Surveillance showed that the overall prevalence of obesity in Chinese adults tripled from 2004 to 2014 (14.0%). The increasing obesity rate is reported as a consequence of nutrition transitions and changes in physical activity. Reproductive patterns were also likely to contribute to the ASIR increase in breast cancer; China implemented the one-child policy in the 1970s. Compared with older generations, younger women reported earlier ages at menarche, later ages at menopause, delayed childbearing, lower fertility rates, and less breastfeeding. According to a large-scale cohort study comparing women born in the 1930s and 1970s in China, the age of menarche was 1.8 years earlier (16.1 vs. 14.3), age of first birth was 6 years later (urban: 19.0 vs. 25.9; rural: 18.3 vs. 23.8), breastfeeding duration was 4 to 5 months shorter (urban: 16 vs. 11; rural: 18 vs. 14), and menopause was 1.4 years later (47.9 vs. 49.3) in the younger cohort. Another factor that should not be ignored is the increase in breast cancer screening rates. There has been a marked improvement in the number and effectiveness of national cancer registries between 2003 and 2011—the number of registries rose from 35 to 243, and the covered population increased from 4.34–13.01%—as well as an improvement in the quality of data collected.
India had the lowest ASIR of breast cancer among the six selected countries, although the ASIR in India continued to rise, influenced by period and cohort effects. The change in trend might be explained by lifestyle changes via economic liberalization since 1991, and period-specific declines were observed in more recent generations. The increasing trend among older women might be related to diagnostic improvement or increased awareness of the disease, including improved diagnosis and self-referral for screening, which have led to a higher average risk of breast cancer diagnosis. The increase in incidence could also be partially owing to changes in risk factors, including reproductive, dietary, and other lifestyle patterns associated with economic growth. In the case of reproductive factors, the marriage rate of Indian women by the age of 18 years declined from 54.2% in 1992–1993 to 44.5% in 2005–2006, parity declined from 3.39 live-born children per women to 2.26, and contraceptive use increased from 1.2–3.1% during the same period. The incidence rates, education level, and income are higher in urban areas compared with rural areas, suggesting that access to care and use of preventive care might be issues that affect cancer incidence. Other epidemiological studies have suggested that obese or overweight women are at a higher risk of breast cancer. According to the India National Family Health Survey, the percentage of married women aged 19–49 years who were overweight or obese increased from 11% in 1995–1996 to 18% in 2015–2016, which may also contribute to the increase in breast cancer incidence.
Based on the prediction of the breast cancer incidence in women worldwide, across four regions with different income levels, and in several countries, the global incidence of breast cancer will continue to increase, and the change pattern presents a certain correlation with economic development. Developed countries, such as the USA, have a relatively high incidence, although incidence has started and will continue to decline in the future, especially in middle-aged and older women. Developing countries, such as India, have a relatively low incidence but it will continue to increase in the future, and will be higher than that of some of the developed countries. Therefore, for developing countries, attention should be paid to the prevention and treatment of breast cancer to control further increases in incidence. Non-genetic, modifiable risk factors should be highlighted in these regions and countries. Income levels, education, and insurance status, which might affect access to health care and influence breast cancer stage at diagnosis, should also be improved. Other measures include changing reproductive patterns, such as having a first child at a younger age, increasing parity, and extending breastfeeding for one or more years; reducing menopausal HRT use; and promoting lifestyle changes, such as reducing smoking and drinking alcohol, exercising regularly, eating a healthy diet, and staying in shape. Developed countries, where breast cancer incidence has shown a downward trend, already have a fairly sophisticated strategy for prevention, such as chemical and surgical prophylaxis for people with genetic risk factors. If the downward trend continues, these countries will complete the transition from high incidence and low mortality to low incidence and low mortality.
Our study has several strengths and limitations. First, to our knowledge, this work is the first attempt to compare and analyze breast cancer incidence in multiple countries and to predict the incidence at the national, regional, and global levels. However, owing to the limitations of the GBD database, we could not obtain the original data to conduct more accurate analyses, particularly detailed analyses accounting for the clinical classification of breast cancer. Second, although the APC model was applied to analyze the morbidity of female breast cancer based on GBD 1990–2019, this GBD-based study is not a cohort study. Therefore, large-scale cohort studies in different countries are strongly needed to establish location- and time-specific RRs. Third, predictions based on current incidence trends are based on the assumption that trends observed in the past will continue in the future, thereby ignoring the impact of risk factor prevalence and trends; thus, our predictions involved a degree of uncertainty. Nonetheless, when comparing our predicted data with the published data, the two are remarkably close, which proves that our prediction is relatively accurate.