Breast cancer mortality trends in Australia: 1921 to 1994
Catherine L Smith, Anne Kricker, Bruce K Armstrong
Objective: To analyse breast cancer mortality
trends in Australia and to see if mammographic screening has yet led to
a reduction in mortality.|
Design: Retrospective analysis of trends in mortality rates from breast cancer in Australian women between 1921 and 1994, and in potentially explanatory variables such as fertility, body size, age at menarche, and screening.
Results: Changes in breast cancer mortality in Australian women could not be explained by chance variation alone. Mortality rose steadily (average annual increase, 1.0%) to 1940-1944, fell to the 1960s and early 1970s, and rose (average annual increase, 0.3%) to the late 1980s. Between 1985-1989 and 1990-1994, breast cancer mortality fell by 3.2% in women 50-69 years of age (the target age group for mammographic screening) and by 4.2% in women 25-49 years of age. There was almost no change ( - 0.2%) in breast cancer mortality in older women in this period. The proportion of women screened in all age groups increased substantially between 1988 and 1994; nearly 65% of women in the target age group had had at least one mammogram by 1994. Decreases in fertility were followed by increases in mortality, and vice versa.
Conclusions: Trends in breast cancer mortality have probably been influenced by changing fertility, nutrition and body-size increases among Australian women. Improvements in stage at diagnosis and treatment have probably moderated the upwards pressure on mortality caused by an increasing incidence. Recent falls in mortality could be expected to continue as more women participate in the mammographic screening program. This trend should be more clearly evident in the second half of the 1990s.
Breast cancer mortality in women in most developed countries,
including Australia, has levelled off or fallen in recent years.1 In Britain, an important
fall in mortality since 1989, considered too early to be the result of
mammographic screening, is thought to be the result of improved
observations have been made in the United States.3 |
We examined trends in breast cancer mortality and breast cancer risk factors in Australia between 1921 and 1994 to see if we could explain recent mortality changes and, in particular, to see if mammographic screening has yet led to decreased breast cancer mortality in the target age group for screening (50-69 years).
Age-specific and age-standardised mortality rates from breast
cancer per 100 000 woman-years, standardised to the world
population, were calculated for one four-year period (1921-1924)
and 14 five-year periods (1925-1929 to 1990-1994) from annual
mortality and population data from the Australian Bureau of
Statistics. Rates for 1995 have subsequently been released.
Estimates of the number of women who had had at least one mammogram by the end of each year from 1984 to 1994 were obtained as follows:
Breast cancer mortality (Box 1a) increased in Australia between the
periods 1921-1924 and 1940-1944 (average annual increase, 1.0%),
fell to a post-war low around 1960, and increased slowly again to a peak
in 1985-1989. The average annual increase in rates from 1970-1974 to
1985-1989 was 0.3%. Thereafter, rates fell by 0.5% a year between
1985-1989 and 1990-1994. The recent fall in annual age-standardised
rates has been from 20.7 per 100 000 woman-years in 1990 and 1991, to
19.6 per 100 000 in 1992, 1993 and 1995, and 19.2 in 1996; in 1994 the
mortality rate was 20.3 per 100 000 woman-years.
Age-specific mortality rates for women aged 25-49 years showed little change between 1921 and 1994 (Box 2). In women 50 years of age and older, breast cancer mortality rose steadily to reach initial peaks at different times between 1935-1939 and 1950-1954. These peaks occurred in women born between 1865 and 1885. Mortality in women 50 years and older then fell to a post-war low between 1950-1954 and 1985-1989 depending on age group and occurring in women born around 1890 to 1905. Thereafter, mortality rates again rose and appeared to peak in 1980-1984 or 1985-1989 for each age group of women from 50-54 years to 70-79 years. There was little evidence of this later mortality rise in women older than 80 years.
In the target age group for mammographic screening (50-69 years), the age-standardised mortality rate increased by 4.0% (95% CI, - 0.1 to 8.3) from 1980-1984 to 1985-1989 and then fell by 3.2% (95% CI, - 6.9 to 0.6) to 1990-1994. In women 25-49 years of age, rates increased by 7.5% (95% CI, 0.4-15.0) from 1980-1984 to 1985-1989 and then fell by 4.2% (95% CI, - 9.9 to 1.9) to 1990-1994. Women 70 years of age and older showed only weak evidence of change from the early to the late 1980s (1.3%; 95% CI, - 3.1 to 6.0) and almost no evidence of a fall in the 1990s ( 2 0.2%; 95% CI, - 4.2 to 4.0).
Our analysis of mammographic screening data is summarised in Box 3.
The percentages of women screened between 1988 and 1994 increased in
all age groups, most notably in women aged 50-69 years. The beginning
of most pilot projects in late 1988 and early 1989 and of the National
Program in 1991 led to a substantial change in the age distribution of
screening mammograms. There was a nearly sevenfold increase in the
cumulative numbers of women in the target age group who had been
screened once or more, from around 138 000 in 1988 to nearly one million
by 1994. The estimated number of women under 50 years of age who had been
screened (which was twice the number in the target group in 1988)
increased only 3.7 times (to an estimated 1 140 000) by 1994.
By 1994, about 70% of women in their 50s and 58% of women in their 60s had had an initial screen. In all, 54% of women in their 40s and 22% of women younger than 40 were estimated to have been screened once by 1994. In older women, the proportion screened reached 25% at 70-74 years of age and 6% at 80 years and older.
|Fertility fell at all ages from 1921-1924 or 1925-1929, to reach a minimum in most age groups in 1930-1934 or 1935-1939 (Box 1b), and then rose to reach a peak between 1945-1949 and 1970-1974. Rates again fell from these peaks. Minima were reached in 1975-1979 or 1980-1984 in those aged between 30 and 44 years, with subsequent rises to 1990-1994. There was little evidence that any of these trends related better to cohort of birth than time period.|
Compared with many other countries, breast cancer mortality in
Australia has varied little over the past 75 years.1 However, the clear trends we found
cannot be explained by chance fluctuation. The recent fall in the
early 1990s, while apparently quite definite in women aged 40-69
years, could be a chance occurrence and will need to be shown for
several more years for it to be declared a "real" trend.
There are three broad classes of explanation for these trends, represented under the subheadings below. Changes in how cause of death is registered and coded can influence reported cause-specific mortality rates, but no substantial changes of this kind are known to have occurred in Australia.1,12 We have specified "real" incidence (below) because recent apparent increases in breast cancer incidence have probably been the result of increased screening.3,13 These increases will not cause increases in mortality because they reflect either earlier diagnosis of breast cancers, or the diagnosis of lesions that would otherwise never have been detected.13
Incidence trends: Whether changes in incidence caused changes in mortality would be most easily determined by comparing incidence and mortality trends. Incidence of breast cancer in New South Wales (representing about one-third of Australian women) changed little from 1972 to 1983, but increased steadily from 1984, and by 1995 was nearly 50% higher than it was in 1983.14 The greatest increase was in women in the target age group for mammographic screening (50-69 years). As there was no parallel increase in mortality during that period, the observed increase in incidence has probably been caused by screening.
Incidence was not measured in Australia before 1972. However, it is most likely that incidence rates in Australia, as in several other countries,15-17 were increasing and thus underlie the steady increase in mortality from 1921 to the peak of the mid 1940s. Incidence rates in Australia were probably also increasing in the post-war period, as in other populations of European origin,18,19 when Australian mortality rates were falling or stable.
Fertility: Breast cancer is associated with late age at first birth, childlessness and low parity.13 Box 1 shows that Australian trends in rates of mortality from breast cancer moved in the opposite direction from those of fertility rates, with changes in mortality occurring a few years after those in fertility.
Australian women born in the 1840s were at the forefront of a transition to lower family sizes in English-speaking countries.10 This falling fertility could have produced the increasing mortality from breast cancer from 1921 onwards. The peak breast cancer mortality in the 1940s occurred in women born before 1885; the highest proportions of unmarried and childless women seen in Australia up to the 1940s were among those born in 1871 to 1876.10
The upward trend in fertility after 1935, which peaked in 1955-1964, started about 10 years before mortality began to fall in the mid 1940s. Fertility again fell in the late 1960s and 1970s to a new low in the 1980s, with a pronounced shift during that period to later childbearing; mortality began to rise again in the 1980s.
Body size and age at menarche: Each 5-cm increase in average height in adult women has been estimated to increase breast cancer risk by 10%.13
From the early 1900s to about 1980, net increases of 8-9 cm in height and 10 kg in weight20,21 could have contributed appreciably to increases in mortality in women born from about 1895 to 1935 (evident in overall mortality from breast cancer between 1970-1974 and 1985-1989).
A fall in age at menarche is also associated with an increase in breast cancer incidence.13,22 Trends in Australia, probably similar to the 2-3 months' fall per calendar decade seen in the United Kingdom and United States in the 100 years to about 1950,13,22,23 would have been expected to increase breast cancer rates. Younger age at menarche is very likely caused by increased height and body mass index, perhaps because menarche depends on attainment of a critical body mass.22,24
Diet: Dietary changes may have affected breast cancer rates by way of changes in body size, and possibly by other means.25
Alcohol consumption is associated with increased risk of breast cancer, being 35% higher in women who have 2-4 drinks and 67% higher in those who have more than four drinks a day compared with women who drink little or no alcohol.26 The high alcohol consumption among Australian women of the early 1800s was not equalled again until 1989, when more than 50% of women over 18-20 years were consuming up to two drinks a day.26,27 However, it has been estimated that no more than 3% of breast cancers in Australia in 1990 were the result of drinking more than two drinks of alcohol a day.26
Physical activity may reduce risk of breast cancer.27 The proportion of Australian women who participate in any recreational exercise (around 70%) appears not to have changed in recent times.28 Changes over time in the physical activity associated with running a household and in paid employment have not been measured.
Migration: Breast cancer rates vary six-fold internationally.29 Migration of women from countries of higher (UK) and lower (eastern Europe, southern Europe, Asia) breast cancer mortality than in Australia has varied, but the net effect of migration on breast cancer mortality rates has probably been small.
Oestrogen use: Use of the oral contraceptive pill increased rapidly in Australia after its introduction in 1961.30 If the Pill has caused an increase in breast cancer mortality, it would have done so mainly in younger women who were current or recent users.31 The increase in mortality in the late 1980s, however, was mostly in women over 50 years of age. Falling mortality from breast cancer in the generations of women who first used oral contraceptives in the US, UK and Sweden also suggests no major effects of the Pill on breast cancer rates.32,33
Trends to smaller breast cancers and fewer axillary node metastases
over nearly 100 years34-36
are probably the results of increased access to and use of care. The
trend to more localised disease is probably continuing because of
Such trends would have caused downward pressure on breast
Mortality from breast cancer in Australian women in the target age group for screening (50-69 years) fell by 3.2% between 1985-1989 and 1990-1994. This may be the result of screening, although the same or larger falls in mortality also occurred in younger women, for whom there is little evidence that mammographic screening reduces breast cancer mortality.38
|The effectiveness of radical mastectomy as the primary treatment for breast cancer has probably changed little in the past 100 years. However, advances in anaesthetics and operating conditions38 as well as the recent use of adjuvant chemotherapy and hormonal therapy3,39 have almost certainly increased survival.|
|While no certain conclusions can be drawn about the causes of changes in breast cancer mortality since 1921, it is probable that the increase to the mid 1940s was caused mainly by rapidly falling fertility in the latter part of the 19th and the early 20th centuries. Subsequent increased fertility, and earlier diagnosis, may have contributed to the fall in mortality from 1940-1944 to 1960-1964. The increasing mortality in women born between 1895 and 1935 was probably caused by nutritional factors leading to increases in body size and resultant earlier age at menarche. However, this incidence-driven increase in mortality was probably moderated by increasing survival with earlier diagnosis and, more recently, improved treatment. Improved treatment is probably the reason for cross-sectional falls in mortality between 1985-1989 and 1990-1994 in women up to 69 years of age. Early effects of mammographic screening may have contributed to these falls, but should be more clearly evident in the second half of the 1990s.|
|Australian Bureau of Statistics data on mortality from breast cancer from 1921 to 1994 were supplied by Mr Paul Jelfs from the national mortality database at the Australian Institute of Health and Welfare, Canberra.|
(Received 29 Jan, accepted 21 Jul, 1997)
Authors' detailsNational Health and Medical Research Council National Breast Cancer Centre, Sydney, NSW.
Catherine L Smith, MPH, Statistician;
Anne Kricker, PhD, Epidemiologist.
Cancer Control Information Centre, NSW Cancer Council, Sydney, NSW.
Bruce K Armstrong, DPhil, FRACP, Director.
Reprints will not be available from the authors.
Correspondence: Dr A Kricker, NHMRC National Breast Cancer Centre, PO Box 572, Kings Cross, NSW 2011.
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Received 3 March 2021, accepted 3 March 2021
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