a Channing Laboratory, Department of Medicine Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA.
b Department of Health and Social Behavior, and the Harvard Center for Society and Health,
c Department of Epidemiology, and the Harvard Center for Cancer Prevention, Harvard School of Public Health, Boston, MA, USA.
Reprint requests to: Ichiro Kawachi, Department of Health and Social Behavior, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115, USA. E-mail: Ichiro.Kawachi{at}channing.harvard.edu
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Abstract |
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Methods Participants comprised 26 936 postmenopausal women in the Nurses' Health Study ages 4672 who were in paid employment, and who had no previous history of cancer. Multivariate-adjusted regression analysis was used to examine the relationship between job strain (measured by the Karasek Job Content Questionnaire in 1992) and risk of incident invasive and in situ breast cancer.
Results From 1992 through 1994, 219 women were diagnosed with breast cancer. No evidence was found for a relationship between job stress and risk of breast cancer. Compared with women in low strain jobs, the multivariate-adjusted relative risks of breast cancer were RR = 0.78 (95% CI : 0.521.16) for high-strain jobs; RR = 0.76 (95% CI : 0.491.17) for active jobs; and RR = 0.94 (95% CI : 0.671.34) for passive jobs. Although job strain was related to less breast cancer screening among women in highly demanding jobs, it was not associated with tumour size.
Conclusions Job stress was not related to an increase in the incidence of breast cancer in the present cohort of nurses.
Keywords Breast cancer, stress, work
Accepted 10 January 2000
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Introduction |
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Evidence on the relationship between stress and breast cancer has been conflicting.46 Most of the early studies were retrospective, raising the possibility of recall bias. More recent studies of breast cancer have attempted to measure stress immediately before a biopsy procedure.79 Although such designs may be preferred to a retrospective approach, it is possible that the measured level of stress of patients who are interviewed immediately before a breast biopsy would be considerably biased by the stress from the impending procedure.
Findings from studies on stress and breast cancer have ranged from no association,4,7,10 an inverse association,11 to a positive association.12 Greer and Morris13 reported an association between suppressed anger and breast cancer diagnosis, while Schonfield14 found greater life change scores among women with benign tumours compared to those with malignant tumours. These data taken together are at best inconclusive. In addition to inappropriate designs and methodological flaws as discussed by Fox;15,16 the inherent complexities of measuring stress also present a major challenge.17 Nonetheless, persistent reports of the link between perceived stress and breast cancer6,8,9,1821 suggest the value of conducting well-designed studies in this area.
Given the remarkable secular trends in women's participation in the workforce, a potentially important source of stress in women's daily lives is job strain. The Job Content Questionnaire developed by Karasek22 is a widely used, self-reported measure of job stress that has been found to predict health outcomes ranging from diminished functional status to cardiovascular disease.2327 The present study examined the relationship between job strain, defined by the individual's evaluation of her job characteristics, and the incidence of invasive or in situ breast cancer. The data come from a large prospective study of working women.
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Materials and Methodology |
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Population for analysis
A total of 75 453 women responded to a single mailing of the 1992 questionnaire (69% response) that included the Karasek Job Content Questionnaire. Of these, 10 824 women were premenopausal and a further 23 309 women did not provide responses to the Karasek Job Content Questionnaire as they were no longer in paid employment. We excluded from the analyses all women who had reported breast cancer or other cancer on any questionnaire up to 1992 (n = 3137). A further 11 247 women were excluded because of missing information on key covariates, such as body mass index (BMI), age at menarche, and educational attainment. The final study population comprised 26 936 employed women who provided complete information on all covariates of interest. These women were followed as part of the entire Nurses' Health Study cohort through the 1994 questionnaire. For non-respondents to the 1994 questionnaire, we attempted telephone contacts and identified deaths through next-of-kin as well as searches of the National Death Index.
Identification of cases of breast cancer
On each questionnaire, nurses were asked to report any diagnosis of breast cancer and the date of diagnosis. For each case of breast cancer reported, we requested permission to review the relevant hospital records and pathology reports to confirm the diagnosis. Past confirmation checks have revealed extremely high accuracy of self-reported breast cancer in this cohort (over 90%).30 We therefore based our analysis on newly diagnosed invasive and in situ breast cancer cases occurring after the return of the 1992 questionnaire but before 1 June 1994 among postmenopausal women who had not previously reported having cancer.
Measurement of exposure variables
Job Content Questionnaire
Work stress was measured by the Karasek Job Content Questionnaire.22 It is a self-administered instrument developed from the Quality of Employment Surveysnationally representative data bases for job characteristics in the US. The 27-item version of the questionnaire measures seven dimensions of work (Appendix 1) and was administered in 1992 to measure job conditions among the study participants.
The instrument measures both the psychological workload (demands) and the level of control (combined measure of job decision authority and skill discretion) to manage the workload. The Job Content Questionnaire provides four job-strain categories based on combinations of the demand and control dimensions of the job. The demand/control model predicts adverse consequences when high psychological demands are combined with a low level of control over task content (high-strain jobs).
The psychometric properties of the questionnaire have been reported in detail elsewhere.22 The test-retest reliability using occupation as the unit of analysis provides a correlation above 0.9 for all aspects of the scale including decision latitude, skill discretion, decision authority, psychological demands, and social support. The Cronbach's alpha coefficients (the average internal correlations between the different questions that make up the scales) are reasonably high for these dimensions, ranging from 0.59 for psychological demands to 0.83 for social support. The internal scale reliability of the psychological workload measure based on national data was 0.61 for women in a sample of US occupations. The Job Content Questionnaire also differentiates between occupations according to level of control. The job control dimension correlates well with worker trait data on the Dictionary of Occupational Titles (DOT)31 and a high degree of correlation has been reported between objective and self-report measures of job control suggesting the convergent validity of the instrument. The psychological job demand scores, although highly reproducible, discriminate less well between occupations. However, low correlations with stress outside the work place suggest that it is unlikely that strain from other spheres of life contributes to the job stress rating.22 The measure of social support assesses support from both supervisors and co-workers. The total work-based support measure has a reliability of 0.81 for women.22 The demands and control sub-scales are split along the median values of responses to create a 2 x 2 matrix of job conditions. Jobs that are high in demands and low in control (high strain jobs) are posited to have the most deleterious health effects.22 The remaining three categories of jobs defined by the demands/control matrix are: high demands/high control (active jobs); low demands/high control (low strain jobs);and low demands/low control (passive jobs). Following previous research,2327 low strain jobs constituted the reference group for comparison against all other job types. An extension of the demands/control matrix further posits that jobs characterized by high demands, low control, and low social support at work (iso-strain jobs) are associated with poor health outcomes.32 McAbee33 also identified organizational social support as a buffering agent against occupational stress and burnout.
Other covariates
Age, alcohol consumption (ethanol g/week), physical activity, and BMI (kg/m2) were included as continuous variables. Mailed, self-administered questionnaires to measure alcohol consumption are reproducible, valid and have a 0.84 correlation with diet records.34 Recreational physical activity, obtained from the 1992 questionnaire, was assessed from a highly reproducible, validated questionnaire35 that asked about the frequency of women's engagement in eight common activities during the past year. The score is measured in metabolic-equivalent hours (MET-hours) per week. One MET-hour is equivalent to the energy expenditure during one hour of rest. For example, walking at an average pace for one hour is estimated to consume about 3.0 MET units, while jogging or bicycling is estimated to consume about 7.0 MET units. Family history of breast cancer and personal history of benign breast disease were analysed as dichotomous variables (yes/no). Postmenopausal hormone use was categorized as never user of postmenopausal hormone, past user, and current user. Other categorical variables included age at menarche (at 811 years, 12, 13, 14, and 1521 years); type of menopause (natural, or surgicalfurther categorized as both ovaries removed, one ovary removed, or neither ovary removed); a combined variable for parity and age at first birth (nulliparous, 12 births and age at first birth <25 years, 12 births and age at first birth 2534 years, 12 births and age at first birth >35 years, 3+ births and age at first birth <25 years... 3+ births and age at first birth >35 years); and age at menopause (<47 years, 4750, 51).
Additionally, we controlled for the women's educational attainment (registered nursing degree only, bachelors degree, masters degree, doctoral degree), as well as their occupational type (inpatient/emergency room nurse, outpatient nurse, operating room nurse, nurse education, nurse administrator, multiple nursing occupations, other nursing type, and non-nursing occupation).
Statistical analysis
The main outcome, invasive or in situ breast cancer, were incident cases of cancer occurring between 1992 and 1994. The main independent variable was the four types of job conditions, which were entered as dummy variables with the low strain category as the reference. The following additional independent variables were included as covariates in the analyses: age; behavioural factors (alcohol consumption, exercise level, postmenopausal hormone use, and BMI); biological factors (family history of breast cancer, age at menarche, personal history of benign breast disease, parity and age at first birth, type of menopause, and age at menopause); and socioeconomic factors (educational attainment and nursing occupation type). To address issues of stress outside the work environment, we ran separate analyses including a variable for the amount of stress involved in providing care to sick or elderly relatives.
We repeated all the analyses restricting the case set to 182 women with invasive breast cancer to determine whether job strain was associated with a more advanced stage of disease. We also examined whether there was a relationship between level of stress and stage at diagnosis as determined by the presence of positive lymph nodes (yes/no) and the size of the tumour (2 cm versus
2.1 cm).
Finally, we examined the potential influence of social integration on the relationship between job strain and breast cancer by including in the final model a measure of (a) work support, and (b) social networks.36 The presence or absence of worksite support was incorporated into the existing demand-control job type matrix, replacing the previous four categories with eight.
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Results |
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Finally, we examined the influence of two measures of social support on the relationship between job strain and the incidence of breast cancer in two settingssocial support in the work place as well as in other domains of life. Using the high control/low demand/high support job type as the reference category, we found no substantial difference in RR of breast cancer related to any job types. Similar results were obtained from an examination of the social networks in domains outside of work, where the RR of breast cancer associated with job types remained virtually unchanged after we included social networks in the model. In addition, level of social integration was itself unrelated to risk of breast cancer.
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Discussion |
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In a recent commentary, Burke and Goodkin17 listed what they considered to be important elements of a fair test to examine the role of life stresses in the development of cancer. They identified the importance of: a prospective study design; an examination of one specific cancer; the study of psychosocial risk factors that have been shown to be relevant to the aetiology of the particular type of cancer; and the consideration of the interaction between the environmental stressors and the individual's responses. Hilakivi-Clarke et al.18 in an earlier review of the role of lifestyle factors in the onset and progression of breast cancer, also stressed the importance of examining the interaction among stress, personality, and availability of psychosocial support.
The present study prospectively examined the incidence of breast cancer in relation to women's reports of their work environment, using a well-established measure of job strain. Our lack of information on personality factors prevented us from examining interactions between stress and personality. In addition to personality, the potential detrimental effects of stress can be mediated or buffered by the availability of social support. When we examined the influence of social support in the work place as well as the women's overall social network, however, we found no evidence of either a main effect of social ties on breast cancer risk, or a buffering effect of social ties on the effects of job stress.
The lack of an association between job strain and breast cancer may be related to the type of stress investigated or to additional factors unique to this cohort. With regard to the latter, our null findings may be reflective of insufficient heterogeneity within each of the demand/control quadrants despite the overall distribution among the quadrants. Although job conditions were distributed in predictable ways among nursing occupational types (e.g. the highest prevalence of high strain conditions among inpatient, emergency room, and operating room nurses), we lacked information on the specific nursing-related jobs for 30 to 40% of the cohort.
We were concerned that the lack of a relationship between job strain and breast cancer may reflect an association between job type and screening behaviour, of which the latter is related to the incidence of breast cancer. Of the women who were employed between 1990 and 1992, the number of women screened was 7787 (72.1%) in the passive job group, 4653 (73.9%) in the low strain job, 5336 (71.8%) in the high strain job, and 4456 (73.0%) in the active job group. Compared with women in low strain jobs, women in high demand jobs, regardless of the level of control, were less likely to be screened between 1992 and 1994 (high demand/high control OR = 0.90, 95% CI : 0.830.98; high demand/low control job OR = 0.88, 95% CI : 0.820.96). It seems plausible that a demanding job minimizes time and other resources necessary for preventive activities. Despite this apparent lower rate of screening among women in jobs high in demand, we did not observe any relation between job strain and diagnosis of larger tumours.
The validity of these null findings depends in part on the possible pathways between stressor and breast cancer as well as the length of the latent periodthe interval from induction to clinical detection, which has been hypothesized to be 5 years.37,38 A long latent period would suggest that job strain occurring during the previous 2 to 3 years might not predict incidence. A limitation of this study is that we did not collect information on the duration of employment under different job conditions. On the other hand, job conditions assessed in this cohort in 1992 and 1996 demonstrate a moderate degree of stability, with correlation coefficients for job control and job demands of 0.60 and 0.54, respectively.
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Conclusion |
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Appendix |
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Acknowledgments |
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References |
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