1 Division of Environmental Health Science, Mount Sinai School of Medicine, New York, NY.
2 Department of Epidemiology, School of Public Health, University of North Carolina, Chapel Hill, NC.
3 Cancer Epidemiology Services at the New Jersey Department of Health and Senior Services, Trenton, NJ.
4 Fred Hutchinson Cancer Research Center/University of Washington, Seattle, WA.
5 Division of Cancer Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA.
6 Office of Dietary Supplements, National Institutes of Health, Bethesda, MD.
7 Applied Research Branch, National Cancer Institute, Bethesda, MD.
8 Department of Biostatistics, Rollins School of Public Health, Emory University, Atlanta, GA.
9 Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, MD.
Received for publication December 13, 2001; accepted for publication March 27, 2002.
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ABSTRACT |
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breast neoplasms; case-control studies; receptors, estrogen; receptors, progesterone; risk factors
Abbreviations: Abbreviations: BMI, body mass index; CI, confidence interval; ER, estrogen receptor; PR, progesterone receptor; WHR, waist-to-hip ratio.
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INTRODUCTION |
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Clinical implications of tumor hormone receptor status have prompted investigators to examine whether risk factors for breast cancer vary by hormone receptor status (7). Epidemiologic studies examining hormone-related breast cancer risk factors in relation to either the ER or PR status of the tumors have consistently shown that the ER+ tumor risk is positively associated with older age, White (vs. Black) race, and nulliparity (7, 8). More recent research has focused on determining whether the etiologies of breast cancer tumors classified by joint ER/PR status differ, with the assumption that ER+PR+ tumors are more hormonally sensitive (916).
Previous studies reported that risk of ER+PR+ breast cancer is positively associated with nulliparity, a later age at first birth, a later age at menarche, and a higher body mass index (BMI; weight in kg/height in m2), but that these factors were inversely related to the risk of ERPR tumors (9, 10, 1215). These observations suggest that tumors subclassified by joint steroid receptor status may actually represent distinct forms of breast cancer with differing etiologies. A recent international comparison of age-specific breast cancer incidence rates by hormone receptor status led Yasui and Potter (17) to hypothesize that hormone-related factors associated with a western lifestyle may be more strongly related to ER+PR+ breast cancer than to other tumor subtypes and that these associations may vary by menopausal status. The Carolina Breast Cancer Study additionally explored whether the heterogeneity among tumor subtypes varied by menopausal status (14). Among premenopausal women, a high waist-to-hip ratio (WHR) was associated with an elevated risk of ER+PR+ tumors, but WHR was unrelated to ERPR tumor risk. Neither a family history of breast or ovarian cancer nor medical radiation exposure to the chest was related to ER+PR+ tumors, yet both factors increased ERPR tumor risk. Because of the limited number of premenopausal women in previous studies (1315), data from a large group of mostly premenopausal women were analyzed to examine whether breast cancer risk factors were associated with and varied according to tumor types subclassified by joint hormone receptor status.
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MATERIALS AND METHODS |
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Eligible women were 2044 years of age and were residents of either the metropolitan area of Atlanta, Georgia; the three-county area surrounding Seattle, Washington; or one of five central New Jersey counties. Cases were newly diagnosed, between May 1, 1990, and December 31, 1992, with either in situ or invasive breast cancer. In this population, risk factor profiles for in situ and invasive cases were found to be similar (27). Controls were identified by using the modified Waksbergs method of random digit dialing (28) and were frequency matched to the expected case distribution by 5-year age group and geographic site. Relevant institutional review boards approved all protocols.
In-person interviews were completed by 1,668 breast cancer cases (85.7 percent), with subject and physician refusals (6.6 and 5.8 percent, respectively) the primary reasons for nonparticipation. Subsequently, two cases were found to be ineligible. A total of 1,505 controls (78.7 percent of those selected) participated in the interview; subject refusal was the primary reason for nonparticipation (12.9 percent). The overall response rate for controls was 71.2 percent (random digit dialing screener response rate multiplied by interview response rate). Cases who did not have a telephone (n = 21) and controls previously diagnosed with in situ or invasive breast cancer (n = 4) were excluded to maintain case-control comparability.
Data collection
Signed informed consent was obtained. A structured questionnaire (average administration time, 70 minutes) was used to collect information on sociodemographic factors, reproductive and menstrual histories, hormone use, alcohol consumption, cigarette smoking, recreational exercise, medical history, and family history of breast cancer. Interviewers measured height, weight, and waist and hip circumferences (20). For cases, ER and PR status (classified as positive, borderline, negative, or unknown) as well as stage and grade of disease were either obtained from Surveillance, Epidemiology, and End Results (SEER) reports (Atlanta and Seattle) or abstracted from medical records in a manner compatible with SEER protocols (New Jersey).
Data analysis
Tumors for which hormone receptor status was borderline (ER, 2.7 percent; PR, 1.3 percent) were coded as receptor positive (10). Breast cancers were characterized by their joint ER and PR status (ER+PR+, ER+PR, ERPR+, ERPR, or ER/PR unavailable when either receptor status was unknown).
Chi-square tests were used to determine whether risk factors were associated with tumors classified by joint receptor status as well as with the availability of receptor information (29). Unordered polytomous logistic regression (SAS PROC CATMOD; SAS Institute, Inc., Cary, North Carolina) was used to determine odds ratios and 95 percent confidence intervals for each joint steroid receptor subgroup compared with the same control group (30). In addition to providing information on etiologic inference, this technique identifies sources of heterogeneity between tumor subgroups.
Estimates were adjusted for age and geographic site (hereafter referred to as the age- and center-adjusted model). Joint receptor breast cancer risk was examined in relation to the following characteristics believed to influence risk via a hormonal pathway: BMI at interview and at 20 years of age, WHR, age at menarche, menopausal status (postmenopause defined as no menstrual period for at least 6 months prior to a cases diagnosis date and to a controls random digit dialing identification date), gravidity, abortion or miscarriage, parity, age at first birth, lactation, oral contraceptive use, years since last oral contraceptive use, cigarette smoking, usual alcohol intake, and moderate and vigorous exercise at three time periods (1213 years of age, 20 years of age, year prior to interview) as well as the average of these three time periods (24). Family (mother or sister) history of breast cancer, education (high school or less/posthigh school but no college degree/at least a college graduate), and race (White/Black) were also examined. The 89 cases and 104 controls whose race/ethnicity was unknown or who reported a racial/ethnic background for which there were too few women to enable meaningful analyses were excluded from further consideration. Thus, 1,556 cases and 1,397 controls were available for analysis.
After a thorough examination of characteristics measured on a continuous scale, cutpoints were selected to capture the underlying relation with the fewest categories needed to maximize the stability of estimates. Covariates were entered as indicator variables, and, in general, continuous variables were dichotomized at the median by using the distribution of controls. Subjects for whom values for a variable were missing were excluded from any analyses pertaining to that variable.
Pairwise differences between regression coefficients among the steroid-receptor case subgroups for a risk factor were assessed by using a Wald chi-square statistic (31). Incorporating the covariance between parameter estimates from the polytomous model into the chi-square statistic provides a more powerful comparison of the coefficients than does a chi-square statistic based on separate logistic models, where the covariance is assumed to be zero (31).
To determine whether confounding accounted for the associations observed in the age- and center-adjusted models, all characteristics for which risk estimates were significant, along with age and geographic site, were simultaneously included in a single model (hereafter referred to as the multivariate-adjusted model). This latter model also included risk factors found to have statistically significantly different regression coefficients among steroid receptor subgroups. Estimates from other models, including one containing all of the breast cancer risk factors under consideration, regardless of significance, as well as a model with just the risk factors that were found to have risk estimates that differed significantly between steroid receptor subgroups, were not materially different (data not shown). To adjust for stage of disease, case-only models were fit. In these models, each receptor subgroup was compared with the ER+PR+ subgroup.
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RESULTS |
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Many of the odds ratios estimating the association for hormonal (age, WHR, current cigarette smoking, and recreational exercise at 1213 years of age and during the year prior to interview) and sociodemographic (race and education) characteristics in relation to ERPR tumors were in the opposite direction of the risk estimates observed for ER+PR+ tumors, albeit in some instances risk estimates were extremely close to the null value. Heterogeneity was most pronounced between the ER+PR+ and ERPR beta coefficients for age (3034 vs. 4044 years), race, and recreational exercise at 1213 years of age.
Age at menarche, menopausal status, alcohol consumption, and family history of breast cancer similarly influence breast cancer risk, regardless of tumor steroid subtype. Nulliparous women were at increased risk of all tumor types except ERPR+. An inverse association was observed between months of lactation and each of the hormone receptor tumor subtypes, with the strongest risk reduction observed for ER+PR tumors (multivariate-adjusted odds ratio = 0.29, 95 percent confidence interval (CI): 0.13, 0.66). Finally, ever use of oral contraceptives was only modestly associated with an elevated risk of both ER tumor types but neither ER+ tumor type. When time since last oral contraceptive use was explored, risk was highest for women who used them during the 5-year interval prior to interview. However, the magnitude of the association was actually the weakest for ER+PR+ tumors; the age- and center-adjusted odds ratios for oral contraceptive use within the 5 years prior to interview versus never use of oral contraceptives were 1.33 (95 percent CI: 0.95, 1.86), 1.82 (95 percent CI: 0.96, 3.45), 2.40 (95 percent CI: 1.17, 4.94), and 1.72 (95 percent CI: 1.14, 2.60) for ER+PR+, ER+PR, ERPR+, and ERPR tumors, respectively.
Associations for the risk factors examined in relation to the discordant receptor tumors (ER+PR or ERPR+) were not consistently more similar to those observed for either ER+PR+ or ERPR tumors. In addition, the beta coefficient for ER+PR in relation to several risk factors differed from the beta coefficients for the other tumor subtypes; however, no clear pattern emerged. Since fewer women were diagnosed with the discordant tumors, our ability to compare these two subgroups with one another as well as with ER+PR+ and ERPR tumors was limited. Associations for unclassified tumors differed from those observed for the other tumor subgroups regarding the womens age, education, WHR, lactation, and exercise at 1213 years of age. These findings are difficult to interpret since unclassified tumors are a mixed group of hormone receptor tumor subtypes.
Analyses restricted to premenopausal women (88 percent of the population) as well as analyses reconducted by excluding women with in situ (14 percent) and unknown-stage (2 percent) tumors yielded results essentially unchanged from those presented in table 3 (data not shown). Adjustment for tumor stage in case-only models did not materially modify the risk estimates obtained from unadjusted models (data not shown). Because of the previously noted association between race and tumor steroid subtype (32, 33), estimates with (table 3) and without (data not shown) adjustment for race were compared. In general, risk estimates were similar in the two models; however, with adjustment for race, the difference between the ER+PR+ and ERPR tumors beta coefficients for parity and age at first birth no longer remained. Small stratum-specific sample size prevented a formal evaluation for interaction, but examination of the odds ratios did not suggest an interaction between race and either of these reproductive factors (data not shown).
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DISCUSSION |
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Our studys findings provide weak support for the hypothesis promulgated by Potter et al. (10, 17): ER+PR+ and ERPR breast cancer tumors have different risk factor profiles. In our study, the strongest evidence supporting this hypothesis, although limited, was that the regression coefficients for ER+PR+ and ERPR tumors were different for age, race, and exercise at 1213 years of age. Other evidence for differences in risk factor profiles is provided by several characteristics (age, WHR, current cigarette smoking, and recreational exercise at 1213 years of age and during the year prior to interview), with risk estimates in opposite directions for the two tumor subtypes. On the other hand, the lack of heterogeneity among tumor subgroups for many of the reproductive and menstrual characteristics commonly considered to influence breast cancer risk through a hormonally mediated pathway provides little support for this hypothesis (35). Failure to provide stronger support may be due to the limited power of our study to detect such subtle subgroup heterogeneity.
If tumors classified by their joint hormone receptor status represent different stages of the same disease, then control for stage and tumor size would be expected to influence the observed associations. Risk estimates remained essentially unchanged in analyses considering tumor stage, providing additional support for the hypothesis that tumors of differing steroid receptor statuses may be etiologically distinct. Tumor size was not examined because of concerns about the reliability of these data. Earlier studies adjusting for tumor stage (14) or size (10) found that these characteristics did not influence risk estimates.
Our findings do not agree with previous reports of stronger associations for ER+PR+ tumors and hormone-related characteristics (9, 10, 1215). Inconsistency may reflect differences in study design or populations. Breast cancer risk factors and the underlying biologic mechanisms may vary with menopausal status (14, 36). The discussion therefore focuses on comparing our results with those of other studies that included premenopausal women, since our population was 88 percent premenopausal.
Our finding of a reduced risk of ER+PR+ tumors among women with a higher BMI is consistent with one (14) but not the other (15) known study of premenopausal women. In studies of premenopausal breast cancer risk that do not consider hormone receptor subtype, an inverse or even no association is often observed for BMI (3740). Premenopausal obesity has been associated with menstrual cycle irregularities resulting in lower estrogen and progesterone levels, which may lower breast cancer risk (38). Abdominal fat is considered more metabolically active than peripheral fat (41). Central fat has been associated with increased estrogen and testosterone levels but decreased sex hormone-binding globulin levels (38). Additionally, it has also been linked with insulin resistance, which may in turn be associated with an elevated breast cancer risk (42). However, we did not observe an association between WHR and any of the hormone receptor subgroups. This finding contrasts with the previous report of a twofold risk of ER+PR+ breast cancer tumors in relation to a higher WHR (14) as well as with two recent studies not considering hormone receptor status that observed positive associations between WHR and premenopausal breast cancer risk (43, 44).
As observed in the Carolina Breast Cancer Study, we found that nulliparous women were at increased risk of ER+PR+ tumors (14). In contrast, nulliparous women were also at increased risk of ERPR tumors in our study but not in the Carolina Breast Cancer Study. In addition, the increased ER+PR+ tumor risk for older versus younger age at first birth that we noted is not consistent with the null association reported in the Carolina Breast Cancer Study (14). A later versus earlier age at first birth has been hypothesized to increase breast cancer risk since pregnancy-related increases in estrogen and progesterone levels (45) are not offset by the benefits associated with an earlier age at breast cell differentiation (46).
Physical activity has been hypothesized to reduce breast cancer risk by a variety of mechanisms, including an influence on menstrual cycle (47, 48) or body size characteristics (37). A reduced risk of ER+PR+ tumors was observed in these data for higher levels of participation in recreational exercise at 1213 years of age but not for exercise in the year prior to the interview. In a previous population-based case-control study conducted in Los Angeles, California, an inverse association between lifetime recreational physical activity levels and all steroid receptor tumor subgroups except ERPR+ was observed among premenopausal women (15). Explanations for our observation of an increased risk of ER+PR tumors in relation to recreational exercise in the year prior to interview is unclear and may have been due to chance.
Unlike the premenopausal findings from the Carolina Breast Cancer Study (14), we observed no differences in ER+PR+ and ERPR tumors in relation to cases age at menarche, ever/never oral contraceptive use, and family history of breast cancer. Our finding of a nonsignificant elevated risk across all hormone receptor status subgroups, except for ER+PR tumors, in relation to higher consumption of alcoholic drinks also disagreed with the null findings (13) and the nonsignificant reductions in risk (14) observed in the two known previous studies of premenopausal women. As suggested previously (25), the strong decreased risk of ER+PR breast cancer for current versus never cigarette smokers may have been a chance finding, particularly in light of the large number of comparisons we made. Finally, neither our results nor those of the Carolina Breast Cancer Study (14) support differences among these two breast tumor subgroups for a previous history of an abortion or miscarriage.
Bias must be considered when interpreting our findings. Response rates were higher among cases than controls. However, selection bias from control nonparticipation is unlikely to explain our findings of heterogeneity among tumor subgroup associations with some, but not other, risk factors. Since each case group was compared with the same control group, any selection bias would be expected to similarly affect the estimates among the tumor subgroups. Most of the associations for ER+PR+ tumors were in the expected directions based on previous studies of breast cancer risk (35), and it is extremely unlikely that recall bias issues would apply to only those cases in a specific hormone receptor status subgroup. In contrast, our WHR findings were not in the expected direction. This unexpected finding may have been due to a higher refusal rate among heavier women who were invited to be controls. Data from this study suggest otherwise; no differences in self-reported weight were found between participants who completed the full study interview and those willing to complete only a short nonrespondent questionnaire (49).
Distributions of the breast cancer risk factors were generally similar for cases for whom hormone receptor status information was and was not available, which also argues against selection bias. Women with in situ and lower-grade tumors were more likely to have an unknown hormone receptor status than were women with other stage or grade tumors. In the study population from the Womens Interview Study of Health, associations for in situ versus regional/distant tumors differed for only nulliparity, BMI, and alcohol consumption (27). Excluding in situ or unknown-stage tumors did not modify the risk estimates shown in table 3.
Numerous laboratories determined ER and PR status, primarily by using the dextrose-charcoal-coated biochemical assay as opposed to immunohistochemical techniques. The higher prevalence of dextrose-charcoal-coated testing reflects the time period (19901992) in which this study was conducted. Despite generally high agreement between these two methods, differences in classification as well as interlaboratory variability may account for discrepancies among studies findings (50). Although prediction of prognosis and response to hormonal treatment by hormone receptor status is relatively consistent, misclassification of hormone receptor status may make it more difficult to disentangle whether the more subtle etiologic relations vary among hormone receptor subgroups (50).
The methodological strengths of this study to examine whether associations differed according to tumor subgroups of steroid receptor status include analyses based on the largest known sample size of premenopausal women to date. However, subgroup analyses undertaken in this study were hindered by decreased power to detect associations of small magnitude. Other study strengths include the population-based design, the wide range of breast cancer risk factors available for analyses, and the use of a standardized anthropometric protocol. In general, our findings did not strongly support the notion that many of the established or suspected hormonal breast cancer risk factors differ regarding their relations with ER+PR+ versus ERPR breast cancer tumors in younger women.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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