1 Institute of Public Health, University of Copenhagen, Denmark.
2 Statistics Denmark, Copenhagen, Denmark.
3 Department of Oncology, Aalborg Hospital, Denmark.
4 Institute of Public Health, Department of Biostatistics, University of Copenhagen, Denmark.
Correspondence: Hella Danø, Institute of Public Health, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen N, Denmark. E-mail: H.Danoe{at}pubhealth.ku.dk
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Abstract |
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Methods In all 1 402 225 women in Denmark were individually followed up for death, emigration, and incident breast cancer in 19701995. Of the 1 402 225 women included in the study, 730 549 were economically active in 1970, and 480 379 women were both married and economically active. Socioeconomic status was assessed based on the occupation in 1970.
Results For all women classified by their own socioeconomic group, the standardized incidence (SIR) and the standardized mortality ratios (SMR) were highest in academics (SIR = 1.39, SMR = 1.29), and lowest in women in agriculture (SIR = 0.77, SMR = 0.75). For married, economically active women classified by their own socioeconomic group the SIR and SMR were highest in academics (SIR = 1.40, SMR = 1.44) and lowest in women in agriculture (SIR = 0.76, SMR = 0.76). Classified by their husbands socioeconomic group, the SIR and SMR were highest in women married to academics (SIR = 1.21, SMR = 1.16) and lowest in women married to men in agriculture (SIR = 0.79, SMR = 0.79). From 1970 to 1995, the risk of developing breast cancer increased by 38% in women aged 5064. All social groups contributed to this increase, the increase being 45% in unskilled workers, and 26% in academics.
Conclusion During the last quarter of the 20th century academics had the highest risk of breast cancer in Denmark. The size of the social gradient in breast cancer occurrence depended on the measure used. The time trends in social distribution will result in breast cancer becoming more frequent.
Accepted 9 July 2002
Breast cancer is the most frequent cancer in women in the Western world.1 In Denmark, the incidence increased from 43 per 100 000 women in 1945 to 83 per 100 000 women in 19962 (World standard population), whereas the mortality increased only slightly from 23 per 100 000 women in 1945 to 27 per 100 000 women in 1996.3 Similar diverging trends with increases in breast cancer incidence and stable or declining trends in breast cancer mortality are seen in many countries.46 With breast cancer now affecting almost one in ten women, it is important to know how this disease burden is shared among women in society. Our objectives were to study: (1) the socioeconomic differences in breast cancer incidence and mortality in Denmark, (2) how different socioeconomic groups have contributed to the increasing incidence, (3) whether the diverging trend between breast cancer incidence and mortality reflects different socioeconomic distributions of breast cancer cases and breast cancer deaths, and (4) to compare measures of socioeconomic status based on own and spouses occupation, respectively.
We addressed these questions by studying the socioeconomic distribution of breast cancer incidence and breast cancer mortality in Danish women during the last 25 years.
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Material and Methods |
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Second, to measure changes over time cumulative breast cancer incidence rates for women aged 5064 years were calculated by adding the rates for 5-year age group and multiplying by five (Figure 2). The 95% CI were calculated under the assumption that the observed number of cases followed a Poisson distribution. Third, we calculated SIR and SMR values for married, economically active women by their own and by their husbands socioeconomic group using married, economically active women as the reference population (Figure 3
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Results |
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All women by own socioeconomic group in 1970, reference population all women
The SIR in 19701995 was highest in academics (SIR = 1.39; 95% CI: 1.281.52), and lowest in women in agriculture (SIR = 0.77; 95% CI: 0.740.81) (Table 1). Incidence rates in between were found for functionaries (SIR = 1.16; 95% CI: 1.141.18), urban trade (SIR = 1.11; 95% CI: 1.071.14), skilled (SIR = 1.04; 95% CI: 0.921.18), and unskilled workers (SIR = 0.89; 95% CI: 0.870.91). The SIR was close to unity for the large group of economically inactive women (SIR = 0.97; 95% CI: 0.950.98).
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There was a wider socioeconomic gradient in breast cancer incidence from 1.39 to 0.77 than in breast cancer mortality from 1.29 to 0.75.
The breast cancer risk increased from 19701975 to 19901995 in all socioeconomic groups. This is illustrated by the cumulative incidence for the 5064-year-old women for each of the 5-year periods (skilled workers excluded due to small numbers) (Figure 2). In 19701975, the cumulative incidence per 100 person-years was 4.1 (95% CI: 2.85.5) in academics, 3.2 (95% CI: 2.93.4) in functionaries, 2.9 (95% CI: 2.53.3) in urban trade, 2.6 (95% CI: 2.52.7) in economically inactive, 2.2 (95% CI: 2.02.4) in unskilled workers, and 2.1 (95% CI: 1.72.5) in agriculture. In 19901995, the cumulative incidence had increased to 5.6 (95% CI: 4.27.1), 5.1 (95% CI: 4.95.4), 4.5 (95% CI: 4.05.0), 3.9 (95% CI: 3.74.1), 3.9 (95% CI: 3.74.2), and 3.1 (95% CI: 2.73.6), respectively. However, while the cumulative incidence had increased by 26% in academics and by 33% in women in agriculture from 19701975 to 19901995, it had increased by 38% in functionaries, and by 45% in unskilled workers.
Married, economically active women classified according to their own socioeconomic group, reference population married, economically active women
The SIR was highest in academics (SIR = 1.40; 95% CI: 1.25 1.58) and lowest in women in agriculture (SIR = 0.76; 95% CI: 0.720.8) (Table 2). Incidence rates in between were found for functionaries (SIR = 1.13; 95% CI: 1.111.16), urban trade (SIR = 1.08; 95% CI: 1.051.12), skilled (SIR = 0.93; 95% CI: 0.781.11), and unskilled workers (SIR = 0.87; 95% CI: 0.850.9).
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Married, economically active women by husbands socioeconomic group in 1970, reference population married, economically active women
The SIR was highest in women married to academics (SIR = 1.21; 95% CI: 1.141.28) and lowest in women married to men working in agriculture (SIR = 0.79; 95%CI: 0.750.82) (Table 2). Incidence rates in between were found for functionaries (SIR = 1.12; 95% CI: 1.091.15), urban trade (SIR = 1.08; 95% CI: 1.041.12), skilled (SIR = 0.99; 95% CI: 0.951.03), and unskilled workers (SIR = 0.89; 95% CI: 0.860.92). The SIR was close to unity for economically active women married to economically inactive men (SIR = 1.01; 95% CI: 0.941.08). In the group of women married to economically inactive men, the age composition was atypical with many women in the younger and older age groups but very few middle-aged women. Direct age standardization changed the estimate from 1.01 to 0.97. Otherwise the pattern across groups was unchanged by direct age standardization.
The SMR was highest in women married to academics (SMR = 1.16; 95% CI: 1.051.28) and lowest in women married to men working in agriculture (SMR = 0.79; 95% CI: 0.740.85). Mortality rates in between were found for women married to functionaries (SMR = 1.09; 95% CI: 1.041.14), men in urban trade (SMR = 1.05; 95% CI: 1.001.11), skilled (SMR = 1.01; 95% CI: 0.951.08), and unskilled workers (SMR = 0.93; 95% CI: 0.880.98). In the group of women married to economically inactive men direct age standardization changed the estimate from 1.04 to 1.08, leaving the pattern across groups unchanged.
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Discussion |
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Denmark has unique possibilities for studying cancer incidence and cancer mortality by socioeconomic status due to complete population-based health registers and comprehensive use of the personal identification number.7 In the present study, the measure of socioeconomic group was based on occupation recorded at the 1970 census. At the 1970 census, it was compulsory for heads of households to fill out self-administered questionnaires. The questionnaire data were centrally coded in the Danish Statistical Office. No control survey of the 1970-census data was undertaken, but when a sample of the original census questionnaires was retrieved for a nested case-referent study, no coding error of occupation was detected.8
A weakness of the present analysis is that the socioeconomic group was measured only at one point in time. However, although some women will have changed job during the 25-year follow-up period, changes between the major socioeconomic groups used in the present study are unlikely because the jobs in these groups require either educational skills (e.g. for academics, most functionaries, and skilled workers) or property, (e.g. in agriculture) unlikely to be acquired after entry into the labour market. The socioeconomic group is less likely to be stable over time for women economically inactive in 1970. They include a small group of students or unemployed, and a large group of housewives. Many housewives entered the labour market in Denmark during 19701995. In 1970, 41% of women aged 2044 were economically inactive.9 In these same generations aged 4064 in 1991, only 27% were economically inactive.10
The social gradient in breast cancer incidence and mortality in Denmark is unlikely to be due to social differences in cancer detection. Health care is free of charge, socioeconomic status is not associated with utilization,11,12 and Nørredam et al.13 found breast cancer patients from different social groups to have similar distributions of stage of disease at time of diagnosis. Furthermore socioeconomic status was assessed before the cancer diagnosis; i.e. a potential bias caused by a downward drift in the social hierarchy as a result of the disease has been avoided. Part of the increase in breast cancer incidence from 1970 to 1995 could be due to increased diagnostic activity. Mammography screening was, however, only introduced in 1991 in the municipality of Copenhagen, and in 1993 in Funen, covering in total 18% of Danish women aged 5069.14
In studies of breast cancer incidence, socioeconomic status has been measured by length of education,1519 area of residence,2022 occupation and education,23 and occupational groups.2427 In studies of breast cancer mortality, socioeconomic status has been measured by education,28,29 occupation,30,31and area of residence.32 Both the highest incidence and the highest mortality have almost uniformly been found in women with a long and higher education, and the Danish data followed this well-known pattern.
The size of the socioeconomic gradient has, however, varied considerably among studies. Our study illustrates that different gradients may derive from use of different methodologies. We analysed both women classified by their own socioeconomic group, and married, economically active women classified by their husbands and their own socioeconomic group, and we analysed both breast cancer incidence and breast cancer mortality. We found that the size of the social gradient in breast cancer depended on the measure. The largest gradient was found for breast cancer mortality among married, economically active women classified by their own socioeconomic group, from 1.44 in academics to 0.76 in women working in agriculture. The smallest gradient was found for breast cancer mortality in married, economically active women classified by socioeconomic group of their spouse, from 1.16 in women married to academics to 0.79 in women married to men working in agriculture.
We found similar socioeconomic differences among all women and among married, economically active women. For married, economically active women the gradient was steeper across the womens own socioeconomic group than across their spouses. For married, economically active women in Finland breast cancer mortality differentials were wider according to own occupation than according to spouses occupation.33 For England and Wales, data were available only for the entire group of married women. The range of breast cancer mortality here was wider according to womens own socioeconomic class than according to that of her spouse.34 All available data therefore show the widest range of breast cancer mortality when women were classified by their own socioeconomic group.
Our study included 51 721 incident breast cancer cases but only 21 576 breast cancer deaths, reflecting the considerable gap between the risk of contracting the disease and the risk of dying from the disease. The burden of deaths was, however, distributed fairly similarly to the burden of disease. Auvinen et al. showed significant differences in breast cancer survival across social classes in Finland 19711985.35 The Danish data do not point to a similar pattern. Denmark introduced a nation-wide standardization of breast cancer treatment in 1977 including adjuvant chemotherapy. This resulted in equal survival throughout the country for breast cancer patients diagnosed in the period 19781987,36 which may suggest that breast cancer patients across socioeconomic groups have benefited equally from the breast cancer treatment in Denmark. Survival data by socioeconomic group are not available for Denmark.
Breast cancer incidence has been on a rapid increase in Finland from 1970 to 1995.23 Trends in socioeconomic difference have been studied with a methodology similar to the one applied in this study. For Finland, a decline in socioeconomic difference has been reported for breast cancer incidence,23 and breast cancer mortality.29 Socioeconomic differences in breast cancer incidence and mortality have also been studied with the same methodology in England and Wales using data from the 1971 Longitudinal Study Cohort.24,30 The trend from 1976 to 1989 showed a fairly unstable pattern across social class, but the results indicated a change from high incidence and mortality in women in non-manual classes to high incidence and mortality in those in manual classes.
In Denmark, the number of newly diagnosed breast cancer cases in women aged 5064 increased by 42% from 3649 in 19701975 to 5196 in 19901995. A particularly marked increase of 172% was seen in the number of cases in functionaries. This reflected the combined effect of a 38% increase in the cumulative incidence for 5064-year-old functionaries and a larger proportion of functionaries in the younger generations. Women who were functionaries in 1970 constituted 14% of 5064-year-old women in 1970, whereas women who were functionaries in 1970 constituted 25% of the 5064-year-old women in 1990.
In the long term, the social gradient in breast cancer incidence is therefore probably due to diminish as a combined effect of: (1) an increasing proportion of well-educated women, and (2) a spread of the risk pattern of well-educated women to other social groups.
As the well-educated women are presently the group at highest risk of breast cancer this equalization process is then expected to lead to more breast cancer cases.
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Conclusion |
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The number of newly diagnosed breast cancer cases in women aged 5064 increased by 42% in Denmark from 19701975 to 19901995. All social groups contributed to this development, but with an uneven growth rate. Although the rank order between socioeconomic groups remained unchanged during these 25 years, it is expected to diminish with time, and the disease is, as a result of the same process, expected to become more frequent.
KEY MESSAGES
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Acknowledgments |
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References |
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