An understanding of the etiologic heterogeneity of ovarian cancer is important for improving prevention, early detection, and therapeutic approaches. We evaluated 14 hormonal, reproductive, and lifestyle factors by histologic subtype in the Ovarian Cancer Cohort Consortium (OC3).

Among 1.3 million women from 21 studies, 5,584 invasive epithelial ovarian cancers were identified (3,378 serous, 606 endometrioid, 331 mucinous, 269 clear cell, 1,000 other). By using competing-risks Cox proportional hazards regression stratified by study and birth year and adjusted for age, parity, and oral contraceptive use, we assessed associations for all invasive cancers by histology. Heterogeneity was evaluated by likelihood ratio test.

Most risk factors exhibited significant heterogeneity by histology. Higher parity was most strongly associated with endometrioid (relative risk [RR] per birth, 0.78; 95% CI, 0.74 to 0.83) and clear cell (RR, 0.68; 95% CI, 0.61 to 0.76) carcinomas (P value for heterogeneity [P-het] < .001). Similarly, age at menopause, endometriosis, and tubal ligation were only associated with endometrioid and clear cell tumors (P-het ≤ .01). Family history of breast cancer (P-het = .008) had modest heterogeneity. Smoking was associated with an increased risk of mucinous (RR per 20 pack-years, 1.26; 95% CI, 1.08 to 1.46) but a decreased risk of clear cell (RR, 0.72; 95% CI, 0.55 to 0.94) tumors (P-het = .004). Unsupervised clustering by risk factors separated endometrioid, clear cell, and low-grade serous carcinomas from high-grade serous and mucinous carcinomas.

The heterogeneous associations of risk factors with ovarian cancer subtypes emphasize the importance of conducting etiologic studies by ovarian cancer subtypes. Most established risk factors were more strongly associated with nonserous carcinomas, which demonstrate challenges for risk prediction of serous cancers, the most fatal subtype.

Ovarian cancer is the most lethal gynecologic cancer, with > 152,000 deaths worldwide each year.1 Most ovarian cancers are detected at a late stage and have a poor prognosis. Screening for ovarian cancer did not reduce mortality in two large screening trials.2,3 An understanding of the etiologic heterogeneity of ovarian cancer is critical for development of new prevention strategies.

Although multiple carcinogenic mechanisms for ovarian tumorigenesis have been hypothesized, including incessant ovulation, hormonal stimulation, and chronic inflammation,4-7 the etiology of ovarian cancer is not well understood partly due to its heterogeneous nature. Disease subtypes have been categorized by putative precursor lesions, mutations, and histology.8,9 Low-grade serous, mucinous, clear cell, and endometrioid tumors are believed to arise from inclusion cysts or implants in the ovarian surface epithelium and have KRAS, BRAF, or PTEN mutations. High-grade serous tumors, characterized by TP53 mutations, are believed to arise in the fallopian tube or ovarian epithelium, are more aggressive, and have poorer outcomes than other types.8-10 Due to limited power, individual epidemiologic and biomarker studies usually have considered risk factor associations for all ovarian tumors together. Individual cohorts and individual-level meta-analyses of primarily case-control studies have reported differential associations by subtype for menopausal hormone therapy (HT) use, oral contraceptive (OC) use, parity, smoking, and body mass index (BMI).11-17 To establish etiologic models that account for ovarian cancer heterogeneity, a unified prospective evaluation of multiple ovarian cancer risk factors needs to account for heterogeneity. In the Ovarian Cancer Cohort Consortium (OC3), we evaluated associations of 14 key risk factors with invasive epithelial ovarian cancer risk overall and by histologic subtype based on pooled individual-level data from 5,584 invasive ovarian cancer cases from a combined cohort of > 1.3 million women enrolled in 21 prospective studies.

Study Population

The analysis included women participating in 21 prospective cohort studies from North America, Asia, and Europe (Table 1). Prospective follow-up of ovarian cancer end points through questionnaires, medical records, or cancer registries as well as follow-up for death were required for participation. Minimal required information included age at study entry, OC use, and parity. All studies obtained institutional approval for cohort maintenance as well as participation in the OC3. The OC3 data coordinating center and analytic approaches were approved by the institutional review board of the Brigham and Women’s Hospital (Boston, MA).

Table

Table 1. Characteristics of Cohorts Participating in the Ovarian Cancer Cohort Consortium

Table 1. Characteristics of Cohorts Participating in the Ovarian Cancer Cohort Consortium

Study NameStudy AcronymLocationBaseline Enrollment PeriodBaseline Cohort Size*Median Study Participant Age (years)Median Follow-Up (years)Last Year of Follow-UpInvasive Ovarian Cancer Cases
NIH-AARP Diet and Health StudyAARPUS1995-1997153,06962112006703
Breast Cancer Detection Demonstration Project Follow-Up StudyBCDDPUS1987-198936,2126191999159
Breakthrough Generations StudyBGSUK2001-2014101,869486201475
Canadian Study of Diet, Lifestyle, and HealthCSDLHCanada1991-19992,7455816201090
Campaign Against Cancer and StrokeCLUEIIUS198912,3824622201282
Cancer Prevention Study II Nutrition CohortCPSII-NCUS1992-199365,88462152009533
California Teachers StudyCTSUS1995-199943,77850152010185
European Prospective Investigation Into Cancer and Nutrition StudyEPICEurope1992-2000263,79651132010671
Iowa Women’s Health StudyIWHSUS198630,53761232010263
Multiethnic/Minority Cohort StudyMECUS1993-199816,4745711201175
Nurses’ Health Study 1980§NHS80US1980-198286,60846161998351
Nurses’ Health Study 1996§NHS96US1996-199867,53062142010417
Nurses’ Health Study IINHSIIUS1989-1990111,80035202011215
New York University Women’s Health StudyNYUUS1984-199112,42749242012129
Netherlands Cohort Study on Diet and CancerNLCSNetherlands19862,75762172003448
Prostate, Lung, Colorectal and Ovarian Cancer Screening TrialPLCOUS1993-200260,19162122009358
Singapore Chinese Health StudySCHSSingapore1993-199931,9395614201195
Sister StudySSUS2003-200939,195555201239
Swedish Mammography Cohort StudySMCSweden199734,42760142011161
Vitamins and Lifestyle CohortVITALUS2000-200228,33160102011130
Women’s Lifestyle and Health StudyWLHSSweden1991-199249,08740212012201
Women’s Health StudyWHSUS1993-199633,54853182012204

Abbreviation: NIH, National Institutes of Health.

*After exclusions for baseline cancers and women with bilateral oophorectomy.

†These cohorts were included as a case-cohort design, which reflected a total cohort population of 39,618 women for the CSDLH and 62,573 women for the NLCS. Appropriate weights for subcohort selection were applied in all analyses.

‡Included only white women.

§The Nurses’ Health Study was broken into two study periods (1980 to June 1996 and July 1996 to 2010) because the follow-up was nearly twice as long as any other study. We updated the exposures in 1996 for that follow-up period.

Exposure Definitions

Full baseline cohort data (19 studies) or case-cohort data sets with weights for subcohort members (two studies) were harmonized centrally. Exposures included parity (ever versus never; number of births: per one birth and one, two, three, four or more births), OC use (ever versus never; duration of use: per 5 years of use and never, ≤ 1, > 1 to 5, > 5 to 10, > 10 years), duration of breastfeeding (per 1 year among parous women), age at menarche (per 1 year and ≤ 11, 12, 13, 14, ≥ 15 years), age at natural menopause (postmenopausal women only: per 5 years and ≤ 40, > 40 to 45, > 45 to 50, > 50 to 55, > 55 years), menopausal HT use (ever versus never; duration of use: per 1 year and never, ≤ 5, > 5 years), tubal ligation (ever versus never), hysterectomy (ever versus never), endometriosis (ever versus never), first-degree family history of breast cancer (ever versus never), first-degree family history of ovarian cancer (ever versus never), BMI (per 5 kg/m2 and < 20, 20 to < 25, 25 to < 30, 30 to < 35, ≥ 35 kg/m2), height (per 0.05 m and < 1.60, 1.60 to < 1.65, 1.65 to 1.70, ≥ 1.70 m), and smoking (ever versus never; per 20 pack-years and ≤ 10, > 10 to 20, > 20 to 35, > 35 pack-years). Studies that did not collect information on a specific risk factor were excluded from the analysis of that factor (Appendix Table A1, online only), which led to different samples sizes for each variable (Appendix Table A2, online only).

Outcome Definitions

Epithelial ovarian or peritoneal cancer cases were confirmed through cancer registries or medical record review (International Classification of Diseases [9th revision codes 183 and 158 and 10th revision code C56]). Ascertainment of incident cancers was ≥ 90% for all studies and ≥ 95% for 17 studies. We evaluated associations of risk factors with all invasive epithelial cancers combined (n = 5,584). Next, we evaluated associations with the four most common histologic types of invasive epithelial ovarian cancers (n = 4,584): serous/poorly differentiated, endometrioid, mucinous, and clear cell. One thousand cases had another histology or were missing histology information. Serous tumors were further divided by grade (well-, moderately, or poorly differentiated or unknown).

Statistical Methods

Women with a history of cancer (other than nonmelanoma skin cancer), with bilateral oophorectomy before study entry, or with missing age at baseline were excluded. We calculated hazard ratios (HRs) and 95% CIs by using competing-risks Cox proportional hazards regression to evaluate associations between exposures and ovarian cancer end points.18 Follow-up time was time between study entry and date of ovarian cancer diagnosis, death, or end of follow-up, whichever occurred first. Survivor function plots for exposures showed parallel curves, which suggest no relevant deviation from proportional hazards. In primary analyses, we pooled data from all cohorts and stratified by year of birth and cohort to account for potential differences in baseline hazards by these factors. Statistical heterogeneity of associations across subtypes was assessed through a likelihood ratio test that compared a model that allowed for the association for the risk factor of interest to vary by histology versus one that did not allow for the association to vary.16 We also used random-effects meta-analysis to combine cohort-specific estimates and to assess between-study heterogeneity. All models were adjusted for age at entry, number of children, and duration of OC use, unless the exposure of interest was collinear with one of these factors. Hysterectomy analyses were also adjusted for HT use. For missing data in covariates, we included a missing indicator in the model. The Sister Study was excluded from analyses of family history because all participants had a family history of breast or ovarian cancer. To evaluate whether our primary models sufficiently accounted for confounding, we performed a model that adjusted for all exposures together (by using missing indicators when needed). In 17 studies, grade was available for at least some serous cases. We conducted similar analyses among serous tumors by comparing risk factors for well-, moderately, and poorly differentiated tumors and unknown grade. We performed unsupervised hierarchical clustering of the four subtypes (with and without separating serous tumors by grade) with β-estimates for all exposures, except duration of breastfeeding (not significantly associated with any of the four subtypes), by using complete linkage and uncentered correlation (Pearson coefficient). SAS 9.3 software (SAS Institute, Cary, NC) was used to conduct the analyses. P < .05 was considered statistically significant. As a sensitivity analysis, we corrected for multiple comparisons for the test of heterogeneity by using an adjusted α of .004 (.05/13 exposures).

Study Population

Among 1,284,586 participants (1,381,275 with the inclusion of full cohort size for case-cohort studies), 5,584 invasive epithelial ovarian cancers were identified during follow-up. Case numbers ranged from 1,281 for breastfeeding to 5,523 for OC use (Appendix Table A2). There were 3,378 (73.7% of cases with known histology) serous, 606 (13.2%) endometrioid, 331 (7.2%) mucinous, and 269 (5.9%) clear cell carcinomas. Fifteen of 21 cohorts were based in North America, five in Europe, and one in Asia (Table 1); approximately one half of the cohorts started enrollment in the 1990s. The median age at diagnosis was 67.0 years for serous, 63.0 years for endometrioid, 64.0 years for mucinous, and 61.3 years for clear cell carcinomas and 68.9 years for cases of unknown histology.

Associations of Hormonal and Reproductive Factors

Most reproductive and hormonal risk factors, except for breastfeeding, were associated with ovarian cancer risk overall (Table 2). Parous versus nulliparous women had a reduced risk of all ovarian cancer subtypes, with significant heterogeneity by subtype (P value for heterogeneity [P-het] < .001). The strongest risk reduction was observed for clear cell carcinomas (relative risk [RR], 0.35; 95% CI, 0.27 to 0.47), whereas serous cancers had the least risk reduction (RR, 0.81; 95% CI, 0.73 to 0.90). Similar patterns were observed for number of children (P-het < .001). In subtype-specific analyses, a 5-year increase in duration of OC use was associated with a significant 14% to 15% lower risk of serous, endometrioid, and clear cell carcinomas but not with mucinous tumors (P-het = .04). Similarly, OC use for > 10 years was associated with a 36% to 49% reduction in risk for serous, endometrioid, and clear cell tumors.

Table

Table 2. Associations of Hormonal and Reproductive Factors With Invasive Epithelial Ovarian Cancer Overall and by Subtypes in the Ovarian Cancer Cohort Consortium

Table 2. Associations of Hormonal and Reproductive Factors With Invasive Epithelial Ovarian Cancer Overall and by Subtypes in the Ovarian Cancer Cohort Consortium

ExposureAll Invasive
RR (95% CI)Serous
RR (95% CI)Endometrioid 
RR (95% CI)Mucinous
RR (95% CI)Clear Cell
RR (95% CI)P-het (between histologic types)*
No. of patients5,5843,378606331269
Parity
 Ever/never0.69 (0.64 to 0.74)0.81 (0.73 to 0.90)0.48 (0.39 to 0.58)0.56 (0.42 to 0.74)0.35 (0.27 to 0.47)< .001
 No. of children, per one  child0.90 (0.89 to 0.92)0.93 (0.92 to 0.95)0.78 (0.74 to 0.83)0.91 (0.84 to 0.99)0.68 (0.61 to 0.76)< .001
 No. of children
  01.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)< .001
  10.82 (0.43 to 0.91)0.86 (0.75 to 1.00)0.78 (0.60 to 1.03)0.59 (0.38 to 0.92)0.67 (0.46 to 0.98)
  20.74 (0.68 to 0.81)0.87 (0.78 to 0.97)0.49 (0.39 to 0.62)0.61 (0.44 to 0.86)0.38 (0.27 to 0.53)
  30.67 (0.62 to 0.74)0.82 (0.73 to 0.92)0.41 (0.32 to 0.54)0.52 (0.36 to 0.74)0.29 (0.19 to 0.43)
  ≥ 40.58 (0.53 to 0.64)0.72 (0.63 to 0.81)0.34 (0.25 to 0.45)0.55 (0.38 to 0.80)0.14 (0.08 to 0.25)
Oral contraceptive use
 Ever/never0.84 (0.79 to 0.89)0.82 (0.76 to 0.89)0.89 (0.73 to 1.07)1.02 (0.80 to 1.31)0.72 (0.55 to 0.94).25
 Duration of use, per  5-year increase0.87 (0.84 to 0.90)0.85 (0.81 to 0.89)0.86 (0.77 to 0.95)1.54 (0.93 to 1.19)0.86 (0.74 to 1.00).04
 Duration of use, years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference).35
  ≤ 10.98 (0.87 to 1.05)0.99 (0.88 to 1.12)1.01 (0.76 to 1.35)0.98 (0.66 to 1.45)0.68 (0.42 to 1.09)
  > 1-50.86 (0.78 to 0.92)0.85 (0.77 to 0.95)0.82 (0.64 to 1.05)0.84 (0.58 to 1.21)0.88 (0.62 to 1.24)
  > 5-100.77 (0.67 to 0.84)0.72 (0.64 to 0.83)0.85 (0.64 to 1.13)0.91 (0.61 to 1.37)0.80 (0.54 to 1.20)
  > 100.67 (0.58 to 0.75)0.64 (0.54 to 0.74)0.64 (0.44 to 0.93)1.18 (0.77 to 1.81)0.51 (0.29 to 0.87)
Duration of breastfeeding,  per 1 year0.96 (0.89 to 1.03)0.94 (0.86 to 1.03)0.85 (0.69 to 1.05)0.88 (0.63 to 1.23)1.03 (0.81 to 1.33).64
Age at menarche
 Per 1-year increase0.99 (0.97 to 1.00)0.99 (0.97 to 1.02)1.00 (0.94 to 1.05)1.00 (0.93 to 1.07)0.92 (0.85 to 0.99).31
 Age, years
  ≤ 111.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference.).66
  120.92 (0.84 to 1.00)0.95 (0.85 to 1.05)1.02 (0.80 to 1.31)1.15 (0.81 to 1.65)0.78 (0.54 to 1.12)
  130.94 (0.87 to 1.02)0.99 (0.90 to 1.09)0.97 (0.79 to 1.22)1.06 (0.76 to 1.48)0.79 (0.56 to 1.11)
  140.93 (0.85 to 1.03)0.99 (0.88 to 1.12)0.84 (0.62 to 1.13)0.97 (0.64 to 1.47)0.80 (0.52 to 1.23)
  ≥ 150.88 (0.80 to 0.97)0.92 (0.81 to 1.05)0.98 (0.73 to 1.31)1.13 (0.76 to 1.66)0.55 (0.34 to 0.90)
Age at menopause§
 Per 5-year increase1.06 (1.02 to 1.10)1.05 (1.01 to 1.10)1.19 (1.05 to 1.34)0.95 (0.81 to 1.11)1.37 (1.15 to 1.64).009
 Age, years
  ≤ 400.89 (0.77 to 1.03)0.87 (0.73 to 1.04)0.59 (0.34 to 1.00)1.31 (0.78 to 2.20)0.15 (0.03 to 0.71).11
  > 40-450.80 (0.70 to 0.91)0.85 (0.73 to 1.00)0.76 (0.51 to 1.14)0.77 (0.44 to 1.33)0.43 (0.20 to 0.94)
  > 45-500.93 (0.86 to 1.00)0.96 (0.87 to 1.06)0.86 (0.67 to 1.09)0.95 (0.68 to 1.31)0.95 (0.64 to 1.39)
  > 50-551.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  > 551.02 (0.89 to 1.17)1.01 (0.85 to 1.21)1.19 (0.78 to 1.80)0.91 (0.49 to 1.68)1.03 (0.50 to 2.09)
Hormone therapy use§
 Ever/never1.36 (1.28 to 1.46)1.41 (1.30 to 1.53)1.67 (1.36 to 2.05)1.00 (0.75 to 1.34)0.90 (0.64 to 1.28).004
 Duration of use, per  5-year increase1.20 (1.16 to 1.23)1.21 (1.17 to 1.25)1.25 (1.15 to 1.36)1.09 (0.94 to 1.26)0.69 (0.52 to 0.92)< .001
 Duration of use, years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)< .001
  ≤ 51.17 (1.07 to 1.27)1.22 (1.09 to 1.36)1.46 (1.11 to 1.91)1.13 (0.78 to 1.63)0.94 (0.61 to 1.44)
  > 51.60 (1.47 to 1.74)1.75 (1.58 to 1.94)1.90 (1.44 to 2.51)1.06 (0.69 to 1.65)0.51 (0.27 to 0.96)
Tubal ligation, ever/never0.82 (0.73 to 0.93)0.91 (0.79 to 1.06)0.60 (0.41 to 0.88)1.01 (0.60 to 1.71)0.35 (0.18 to 0.69).005
Hysterectomy, ever/never0.96 (0.89 to 1.03)1.03 (0.94 to 1.13)0.84 (0.66 to 1.07)0.72 (0.51 to 1.02)0.57 (0.36 to 0.88).006
Endometriosis, ever/never1.35 (1.07 to 1.71)1.03 (0.74 to 1.46)2.32 (1.36 to 3.95)1.62 (0.58 to 4.51)2.87 (1.53 to 5.39).01

NOTE. Associations stratified by birth year and cohort and adjusted for age at study entry, parity, and duration of oral contraceptive use (except when parity or oral contraceptive use was the primary exposure of interest, and then we adjusted only for the other risk factor) by using pooled analyses of all cohorts combined. Abbreviations: P-het, P value for heterogeneity; RR, relative risk.

*Assessed by using a likelihood ratio test that compared a Cox proportional hazards competing-risks model to allow for the association to vary by histologic subtype to a model that forced the association to be the same across subtypes.

†Significant at a Bonferroni threshold.

‡Parous women only.

§Postmenopausal women only.

‖Also adjusted for duration of hormone therapy use.

A 5-year later menopause was associated with endometrioid and clear cell carcinomas (RR, 1.19 [95% CI, 1.05 to 1.34] and 1.37 [95% CI, 1.15 to 1.64], respectively), with no association for serous and mucinous carcinomas (P-het = .009). A 5-year increase in menopausal HT use was associated with an increased risk of serous (RR, 1.21; 95% CI, 1.17 to 1.25) and endometrioid (RR, 1.25; 95% CI, 1.15 to 1.36) carcinomas but a reduced risk of clear cell carcinoma (RR, 0.69; 95% CI, 0.52 to 0.92; P-het < .001). Tubal ligation was only associated with reduced risk of endometrioid (RR, 0.60; 95% CI, 0.41 to 0.88) and clear cell (RR, 0.35; 95% CI, 0.18 to 0.69; P-het < .001) carcinomas, whereas hysterectomy was associated with decreased risk of clear cell carcinomas (RR, 0.57; 95% CI, 0.36 to 0.88; P-het = .006). Self-reported endometriosis was significantly associated only with endometrioid (RR, 2.32; 95% CI, 1.36 to 3.95) and clear cell (RR, 2.87; 95% CI, 1.53 to 5.39; P-het = .01) carcinomas. There was no significant heterogeneity in associations by histology for breastfeeding or age at menarche, although the latter was significantly inversely associated with clear cell carcinomas.

Associations of Other Risk Factors

Height and family history of both breast and ovarian cancer, but not smoking or BMI, were significantly associated with ovarian cancer risk overall (Table 3). A first-degree family history of breast and ovarian cancer was associated with an increased risk of serous tumors (RR, 1.13 [95% CI, 1.02 to 1.26; P-het = .008] and 1.61 [95% CI, 1.32 to 1.97; P-het = .31], respectively). Family history of breast cancer was also associated with endometrioid carcinomas (RR, 1.47; 95% CI, 1.15 to 1.87). BMI was not significantly associated with ovarian carcinomas overall or with any subtype, although there was a borderline association with endometrioid carcinomas (RR per 5 kg/m2, 1.07; 95% CI, 0.99 to 1.16). Ever smoking was associated with mucinous carcinomas only (RR, 1.27; 95% CI, 1.01 to 1.59); each 20 pack-years of smoking was associated with an increased risk of mucinous and a decreased risk of clear cell carcinomas (P-het = .002).

Table

Table 3. Associations of Family History, Demographic, and Lifestyle Factors With Invasive Epithelial Ovarian Cancer Overall and by Subtypes in the Ovarian Cancer Cohort Consortium

Table 3. Associations of Family History, Demographic, and Lifestyle Factors With Invasive Epithelial Ovarian Cancer Overall and by Subtypes in the Ovarian Cancer Cohort Consortium

ExposureAll Invasive 
RR (95% CI)Serous 
RR (95% CI)Endometrioid 
RR (95% CI)Mucinous 
RR (95% CI)Clear Cell 
RR (95% CI)P-het (between histologic types)*
No. of patients5,5843,378606331269
First-degree family history of  breast cancer, ever/never1.09 (1.00 to 1.19)1.13 (1.02 to 1.26)1.47 (1.15 to 1.87)0.73 (0.47 to 1.13)0.75 (0.46 to 1.22).008
First-degree family history of  ovarian cancer, ever/never1.48 (1.26 to 1.75)1.61 (1.32 to 1.97)0.97 (0.52 to 1.82)1.33 (0.59 to 3.00)0.96 (0.36 to 2.57).31
Body mass index
 Per 5 kg/m21.01 (0.98 to 1.04)0.97 (0.93 to 1.01)1.07 (0.99 to 1.16)1.08 (0.96 to 1.20)1.04 (0.92 to 1.17).06
 Categorical, kg/m2
  < 201.02 (0.91 to 1.13)1.06 (0.92 to 1.21)0.85 (0.60 to 1.19)1.36 (0.90 to 2.04)0.96 (0.60 to 1.53).10
  20 to < 251.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  25 to < 300.97 (0.91 to 1.03)0.91 (0.84 to 0.99)0.97 (0.80 to 1.18)1.42 (1.10 to 1.83)1.21 (0.91 to 1.61)
  30 to < 350.99 (0.90 to 1.08)0.92 (0.82 to 1.04)1.09 (0.83 to 1.43)1.23 (0.83 to 1.82)0.97 (0.62 to 1.51)
  ≥ 351.09 (0.97 to 1.24)0.97 (0.83 to 1.14)1.26 (0.88 to 1.80)1.24 (0.69 to 2.21)1.23 (0.70 to 2.15)
Height
 Per 0.5 m1.06 (1.04 to 1.08)1.06 (1.03 to 1.09)1.06 (1.00 to 1.13)1.04 (0.95 to 1.13)1.08 (0.98 to 1.19).94
 Categorical, m
  < 1.600.89 (0.83 to 0.96)0.86 (0.78 to 0.95)1.03 (0.82 to 1.29)0.87 (0.64 to 1.18)0.92 (0.65 to 1.30).27
  1.60 to < 1.651.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  1.65 to < 1.701.02 (0.95 to 1.10)1.04 (0.95 to 1.14)0.93 (0.74 to 1.17)0.83 (0.61 to 1.13)0.97 (0.70 to 1.36)
  ≥ 1.701.12 (1.03 to 1.21)1.06 (0.96 to 1.17)1.27 (1.01 to 1.60)1.12 (0.82 to 1.52)1.24 (0.88 to 1.73)
Smoking
 Ever/never0.99 (0.94 to 1.05)0.99 (0.92 to 1.06)0.93 (0.79 to 1.09)1.27 (1.01 to 1.59)0.95 (0.74 to 1.21).14
 Per 20 pack-years0.98 (0.94 to 1.02)1.01 (0.96 to 1.06)0.92 (0.80 to 1.06)1.20 (1.04 to 1.39)0.68 (0.53 to 0.89).002
 Categorical, pack-years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference).09
  ≤ 101.07 (0.97 to 1.17)1.07 (0.96 to 1.21)1.02 (0.78 to 1.32)1.14 (0.78 to 1.68)0.95 (0.64 to 1.40)
  > 10 to 201.02 (0.90 to 1.15)1.04 (0.89 to 1.21)0.72 (0.49 to 1.07)1.40 (0.89 to 2.20)0.88 (0.52 to 1.48)
  > 20 to 350.96 (0.85 to 1.08)0.99 (0.85 to 1.15)0.92 (0.65 to 1.30)1.16 (0.72 to 1.88)0.44 (0.22 to 0.91)
  > 350.99 (0.88 to 1.12)1.08 (0.93 to 1.24)0.85 (0.57 to 1.26)1.60 (1.02 to 2.51)0.42 (0.18 to 0.94)

NOTE. Associations stratified by birth year and cohort and adjusted for age at study entry, parity, and duration of oral contraceptive use (except when parity or oral contraceptive use was the primary exposure of interest, and then we adjusted only for the other risk factor) by using a pooled analysis of all cohorts combined. Abbreviations: P-het, P value for heterogeneity; RR, relative risk.

*Assessed by using a likelihood ratio test that compared a Cox proportional hazards competing-risks model to allow for the association to vary by histologic subtype to a model that forced the association to be the same across subtypes.

†Significant at a Bonferroni threshold.

Associations by Subtype of Serous Carcinomas

Among serous tumors, moderately and poorly differentiated carcinomas had similar associations, whereas associations for well-differentiated carcinomas were qualitatively different. However, the heterogeneity was not significant for most individual factors (Table 4; Appendix Table A3, online only) for high-/moderate- versus low-grade serous carcinomas. For example, endometriosis was significantly associated with well-differentiated carcinomas (RR, 3.77; 95% CI, 1.24 to 11.48) but not poorly differentiated carcinomas (RR, 1.11; 95% CI, 0.70 to 1.74; P-het = .12). Similarly, > 5 years of HT use versus never use was associated with a 2.9-fold higher risk of well-differentiated carcinomas but only an 80% higher risk of poorly differentiated carcinomas (P-het = .45).

Table

Table 4. Associations of Risk Factors Among Serous Ovarian Carcinomas by Grade in the Ovarian Cancer Cohort Consortium

Table 4. Associations of Risk Factors Among Serous Ovarian Carcinomas by Grade in the Ovarian Cancer Cohort Consortium

ExposureGrade, RR (95% CI)P-het
Well Differentiated*Moderately DifferentiatedPoorly DifferentiatedUnknown
Parity
 Ever/never0.78 (0.47 to 1.29)0.77 (0.60 to 0.99)0.83 (0.72 to 0.96)0.88 (0.71 to 1.09).87
 No. of children, per one child0.89 (0.80 to 1.00)0.90 (0.85 to 0.95)0.94 (0.91 to 0.96)0.96 (0.93 to 1.01).20
 No. of children
  01.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  10.84 (0.41 to 1.73)0.90 (0.64 to 1.27)0.85 (0.69 to 1.05)0.94 (0.70 to 1.26)
  20.88 (0.50 to 1.55)0.86 (0.65 to 1.13)0.89 (0.76 to 1.05)0.89 (0.70 to 1.13).42
  30.88 (0.50 to 1.54)0.68 (0.51 to 0.91)0.87 (0.74 to 1.03)0.86 (0.67 to 1.10)
  ≥ 40.45 (0.22 to 0.91)0.68 (0.50 to 0.92)0.69 (0.58 to 0.82)0.89 (0.69 to 1.14)
Oral contraceptive use
 Ever/never1.11 (0.72 to 1.72)0.80 (0.65 to 0.98)0.85 (0.76 to 0.95)0.77 (0.66 to 0.90).36
 Duration of use, per 5-year increase0.79 (0.62 to 1.00)0.82 (0.73 to 0.92)0.90 (0.84 to 0.96)0.77 (0.69 to 0.87).09
 Duration of use, years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  ≤ 11.80 (0.98 to 3.30)0.90 (0.63 to 1.29)1.01 (0.84 to 1.20)0.96 (0.74 to 1.24)
  > 1-51.12 (0.65 to 1.94)0.95 (0.72 to 1.25)0.86 (0.74 to 1.00)0.85 (0.68 to 1.06).25
  > 5-100.94 (0.48 to 1.83)0.82 (0.60 to 1.13)0.77 (0.65 to 0.92)0.59 (0.44 to 0.79)
  > 100.56 (0.22 to 1.42)0.45 (0.28 to 0.73)0.76 (0.61 to 0.94)0.49 (0.34 to 0.71)
Duration of breastfeeding, per 1 year1.06 (0.68 to 1.66)0.93 (0.75 to 1.15)0.95 (0.83 to 1.08)0.89 (0.74 to 1.08).86
Age at menarche
 Per 1-year increase1.01 (0.91 to 1.11)1.00 (0.94 to 1.06)1.01 (0.98 to 1.04)0.95 (0.91 to 1.00).21
 Age, years
  ≤ 111.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  121.26 (0.70 to 2.28)0.86 (0.64 to 1.14)1.06 (0.91 to 1.23)0.86 (0.69 to 1.06)
  131.37 (0.83 to 2.28)0.94 (0.73 to 1.20)1.10 (0.96 to 1.26)0.76 (0.62 to 0.92).22
  141.20 (0.62 to 2.34)0.86 (0.62 to 1.18)1.16 (0.97 to 1.38)0.83 (0.65 to 1.05)
  ≥ 151.00 (0.49 to 2.05)0.99 (0.72 to 1.36)0.94 (0.78 to 1.14)0.80 (0.62 to 1.02)
Age at menopause
 Per 5-year increase1.54 (1.23 to 1.91)1.04 (0.93 to 1.16)1.03 (0.97 to 1.10)1.05 (0.95 to 1.16).06
 Age, years
  ≤ 450.20 (0.07 to 0.56)0.92 (0.66 to 1.28)0.91 (0.77 to 1.09)0.89 (0.69 to 1.17)
  > 45-500.49 (0.29 to 0.84)1.21 (0.94 to 1.56)0.96 (0.83 to 1.10)0.98 (0.80 to 1.21).02
  > 50-551.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  > 550.41 (0.13 to 1.32)1.16 (0.73 to 1.84)0.97 (0.75 to 1.24)1.23 (0.87 to 1.73)
Hormone therapy use§
 Ever/never1.80 (1.15 to 2.83)1.57 (1.27 to 1.95)1.49 (1.33 to 1.67)1.23 (1.04 to 1.45).15
 Duration of use, per 5-year increase1.35 (1.18 to 1.53)1.26 (1.17 to 1.36)1.21 (1.16 to 1.26)1.20 (1.12 to 1.29).54
 Duration of use, years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  ≤ 51.33 (0.71 to 2.48)1.26 (0.94 to 1.69)1.27 (1.09 to 1.48)1.12 (0.90 to 1.41).42
  > 52.91 (1.72 to 4.92)2.10 (1.60 to 2.76)1.80 (1.56 to 2.07)1.57 (1.27 to 1.95)
Tubal ligation, ever/never1.25 (0.66 to 2.36)1.05 (0.71 to 1.57)0.92 (0.76 to 1.11)0.62 (0.43 to 0.88).10
Hysterectomy, ever/never0.87 (0.53 to 1.43)1.05 (0.84 to 1.33)1.01 (0.89 to 1.14)1.04 (0.87 to 1.25).90
Endometriosis, yes/no3.77 (1.24 to 11.48)1.54 (0.72 to 3.30)1.11 (0.70 to 1.74)0.57 (0.18 to 1.80).12
First-degree family history of breast cancer, yes/no1.23 (0.71 to 2.15)1.20 (0.91 to 1.58)1.12 (0.97 to 1.30)0.96 (0.76 to 1.21).58
First-degree family history of ovarian cancer, yes/no0.90 (0.22 to 3.70)1.46 (0.83 to 2.54)1.63 (1.25 to 2.13)1.64 (1.08 to 2.47).82
Body mass index
 Per 5 kg/m20.92 (0.74 to 1.14)0.99 (0.90 to 1.08)0.92 (0.87 to 0.97)1.05 (0.97 to 1.13).03
 Categorical, kg/m2
  < 201.33 (0.67 to 2.62)0.78 (0.51 to 1.19)1.15 (0.95 to 1.39)1.11 (0.83 to 1.49)
  20 to < 251.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  25 to < 301.02 (0.65 to 1.59)1.08 (0.88 to 1.33)0.84 (0.74 to 0.94)0.89 (0.75 to 1.05).22
  30 to < 350.85 (0.44 to 1.66)0.98 (0.73 to 1.32)0.85 (0.72 to 1.00)1.04 (0.83 to 1.32)
  ≥ 351.15 (0.51 to 2.59)0.88 (0.56 to 1.39)0.88 (0.70 to 1.10)1.25 (0.92 to 1.70)
Height
 Per 0.5 m1.05 (0.93 to 1.18)1.06 (0.99 to 1.14)1.07 (1.03 to 1.11)1.03 (0.97 to 1.08).72
 Categorical, m
  < 1.600.83 (0.49 to 1.39)0.92 (0.72 to 1.17)0.82 (0.72 to 0.95)1.00 (0.82 to 1.21)
  1.60 to < 1.651.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference).70
  1.65 to < 1.701.21 (0.75 to 1.95)1.03 (0.81 to 1.30)1.03 (0.91 to 1.18)1.15 (0.95 to 1.39)
  ≥ 1.700.96 (0.55 to 1.69)1.08 (0.83 to 1.41)1.06 (0.92 to 1.22)0.96 (0.77 to 1.20)
Smoking
 Ever/never1.10 (0.85 to 1.41)0.95 (0.84 to 1.07)0.96 (0.90 to 1.03)1.04 (0.95 to 1.15).38
 Continuous pack-years, per 20 pack-years0.87 (0.59 to 1.26)1.00 (0.87 to 1.15)0.98 (0.92 to 1.05)1.07 (0.97 to 1.18).44
 Categorical, pack-years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  ≤ 201.20 (0.70 to 2.08)1.00 (0.76 to 1.32)1.08 (0.94 to 1.24)1.10 (0.88 to 1.36).91
  > 200.72 (0.34 to 1.52)0.97 (0.71 to 1.31)1.03 (0.89 to 1.21)1.09 (0.87 to 1.38)

NOTE. Associations stratified by birth year and cohort and adjusted for age at study entry, parity, and duration of oral contraceptive use (except when parity or oral contraceptive use was the primary exposure of interest, and then we adjusted only for the other risk factor) by using pooled analyses of all cohorts combined. Five cohorts with no information on grade for any ovarian cancer cases were excluded. Abbreviation: P-het, P value for heterogeneity; RR, relative risk.

*Number of cases ranged from 28 (breastfeeding) to 121 (oral contraceptive use) for well-differentiated, 113 (endometriosis) to 496 (oral contraceptive use) for moderately differentiated, 338 (breastfeeding) to 1,637 (oral contraceptive use) for poorly differentiated, and 141 (endometriosis) to 773 (oral contraceptive use) for unknown grade.

†Assessed by using a likelihood ratio test that compared a Cox proportional hazards competing-risks model to allow for the association to vary by grade to a model that forced the association to be the same across grades.

‡Parous women only.

§Postmenopausal women only.

‖Also adjusted for duration of hormone therapy use.

Meta-Analysis and Heterogeneity Across Studies

Results for meta-analyses were similar to the pooled analyses (Appendix Table A4, online only). We observed little heterogeneity in associations across studies (P < .01 for only 13 of 188 comparisons). All of these were for continuous variables, but the categorical associations did not show heterogeneity. Family history of ovarian cancer showed heterogeneity for all four subtypes across studies likely because of the small number of exposed cases in many studies. Results were similar when including women with a history of cancer at baseline or when all exposures were included in the model (data not shown).

Integrated Analysis of Risk Factors in Ovarian Cancer Subtypes

Each subtype had unique patterns of risk factor associations (Fig 1). The strongest associations for most factors were observed for endometrioid and clear cell tumors. Unsupervised clustering divided the four histologic subtypes into two major groups (Fig 1A). Serous carcinomas were separate from the other three subtypes (Pearson correlation, 0.19). Endometrioid and clear cell carcinomas had the most similar risk factor associations (Pearson correlation, 0.71). Serous cancers divided by grade (Fig 1B) were split into two distinct groups: well-differentiated serous carcinomas clustered with endometrioid carcinomas (Pearson correlation, 0.75), whereas moderately and poorly differentiated serous carcinomas clustered together (Pearson correlation, 0.90).

In a large pooled analysis of > 1.3 million women, we investigated 14 established or putative risk factors in ovarian cancer subtypes. Nine risk factors had significant heterogeneity across subtypes. Most reproductive and hormonal risk factors had stronger associations with endometrioid and clear cell carcinomas compared with the other types. Serous and poorly differentiated carcinomas, the most common and aggressive subtypes, had modest associations only with parity, OC use, menopausal HT use, and family history of breast cancer and stronger associations with family history of ovarian cancer.

The current analysis represents, to our knowledge, the largest comprehensive and prospective evaluation of ovarian cancer risk factors by histologic subtypes. The results are consistent with previous reports from individual prospective studies within the OC3 (ie, Nurses’ Health Study/Nurses’ Health Study II, National Institutes of Health-AARP Diet and Health Study, European Prospective Investigation Into Cancer).15-17 However, individually, these studies were underpowered to assess subtype-specific associations, particularly for rare types. Previously, other consortia, largely based on case-control studies, reported subtype-specific associations for individual risk factors12-14,19-21 similar to what we observed.

Models of ovarian carcinogenesis have separated epithelial tumors into major pathways with distinct cells of origin, carcinogenic pathways, and histology with different clinical behavior.8,10 An integrated evaluation of ovarian cancer risk factors by subtypes is important to understand factors that drive these etiologic pathways on the population level. Each subtype had a qualitatively unique pattern of associations, and serous carcinomas were clearly separated from endometrioid, clear cell, and mucinous carcinomas. Although endometrioid and clear cell carcinomas had qualitatively similar associations for 10 risk factors, they differed in associations related to HT use (which went in opposite directions), family history of breast cancer (associated with endometrioid only), as well as age at menarche and smoking (associated with clear cell only). Every reproductive/hormonal factor, except breastfeeding, was significantly associated with clear cell tumors.

The present results suggest that currently hypothesized, unifying mechanisms, such as incessant ovulation,4 do not apply equally to ovarian cancers. Several variables that determine a woman’s lifetime number of ovulations had significant heterogeneity across subtypes. Only parity and height were associated with all subtypes, which suggests a common biologic effect.22 Of note, mucinous tumors were not associated with any ovulation-related factors except parity, which suggests a more distinct etiology.

Ovarian cancer subtypes share some risk factors with other cancer sites. The inverse association between smoking and clear cell ovarian carcinomas is similar to that for endometrial cancer.23 Mucinous ovarian cancers share histologic appearance and an association with smoking with colorectal cancers.24 Serous ovarian cancers had weaker associations with most hormonal and reproductive factors compared with nonserous cancers (with the exception of OC use), which is similar to associations for hormone receptor–negative breast cancers.25 These similarities of risk factor associations across cancers mirror molecular data that showed that tumor subtypes from different organs may be more similar to one another on the molecular level compared with other subtypes at the same site (eg, high-grade serous ovarian cancer, basal-like breast cancer).26

Although the subtype-specific associations observed in the current study strongly corroborate the etiologic heterogeneity of ovarian cancers, a purely histology-based classification of end points may have limitations.27 Histologic evaluation is subjective, and pathology practice changes over time, which could affect subtype distributions by location and year of diagnosis. We observed heterogeneity among studies for four risk factors among mucinous tumors, which were possibly related to temporal and geographic differences in defining mucinous tumors. However, overall, we did not observe significant differences in subtype proportions across studies or over time (data not shown). Unsupervised clustering demonstrated that well-differentiated serous carcinomas were distinct from higher grade serous carcinomas and grouped with endometrioid carcinomas. This is important etiologically and further supports the differentiation of these two groups of serous carcinomas as proposed in models based on somatic mutations.8,9 However, in population-based studies, the grade reported on pathology reports may not be reliable, and low-grade serous carcinomas account for only approximately 5% of all serous cancers,28 which limits potential misclassification when associations for all serous carcinomas are considered together.29 Analyses by tumor aggressiveness and tumor dominance have also shown differences in risk factor associations, which indicates important biologic heterogeneity beyond histologic subtypes.30,31 Furthermore, additional molecular subgroups have been described within high-grade serous ovarian cancers,32,33 but thus far, based on small studies, these subtypes have shown only limited heterogeneity in risk factor associations.34

In summary, we conducted the largest integrated prospective analysis of ovarian cancer risk factors to date. Most factors showed heterogeneity across histologic subtypes, and each subtype had unique patterns of risk factor associations. The results have important implications with respect to etiology and prevention of ovarian cancers. OCs continue to be an important preventive factor for most types of ovarian cancer. Few other risk factors for ovarian cancer are modifiable, and those that are, such as smoking and obesity, did not show clear associations with serous carcinomas, the most common and fatal subtype. The substantial heterogeneity of individual risk factor associations across ovarian cancer subtypes supports that subtypes are indeed different diseases and underscores the importance of evaluating risk factors and biomarkers by ovarian cancer subtypes.35-37 Our work has implications for the development of risk prediction models, which generally consider ovarian cancer as a whole.38 Due to weaker associations observed for high-grade serous carcinomas, prediction of the clinically most important subtype may perform worse than for other types, which underscores the importance of finding better risk factors for serous carcinomas. Evaluation of subtype-specific risk factor associations is important to gain a better understanding of ovarian cancer etiology and for targeted development of novel prevention approaches; these analyses require pooling of data across many studies in consortia. To this end, future work in the OC3 will include an evaluation of circulating biomarkers, such as inflammation markers, by ovarian cancer subtypes and the development of risk prediction models that integrate risk factor information and genetic data that account for the heterogeneity of ovarian cancer. Furthermore, we and others should explore potential risk factors for high-grade serous cancers, which showed the weakest associations for most established ovarian cancer risk factors.

© 2016 by American Society of Clinical Oncology

Supported by Department of Defense Ovarian Cancer Research Program Grant No. W81XWH-12-1-0561. The UK Breakthrough Generations Study is supported by Breakthrough Breast Cancer and the Institute of Cancer Research. The Institute for Cancer Research is supported by National Health Service funding to the National Institute for Health Research Biomedical Research Centre. Also supported by K05 CA154337 from the National Cancer Institute (NCI) and Office of Dietary Supplements (VITAL [Vitamins and Lifestyle study cohort]); R01 CA39742 (Iowa Women’s Health Study); National Institutes of Health/NCI grant UM1 CA182876 (Singapore Chinese Health Study); CA047988, HL043851, HL080467, and HL099355 (Women’s Health Study); CA164973 (Multiethnic Cohort); R01 CA77398 and UM1 CA169417 (California Teachers Study); UM1 CA186107, P01 CA87969, UM1 CA176726, and R01 CA67262 (Nurses’ Health Study, Nurses’ Health Study II); grants from the Swedish Cancer Society and Swedish Research Council (Swedish Women’s Lifestyle and Health cohort study); and the Swedish Research Council (Swedish Mammography Cohort). All aspects of the Cancer Prevention Study II were funded by the Intramural Research Program of the American Cancer Society and by the NCI Intramural Research Program, Intramural Research Program of the National Institutes of Health, and National Institute of Environmental Health Sciences (Z01ES044005; Sister Study). The coordination of European Prospective Investigation Into Cancer (EPIC) is supported by the European Commission (DG-SANCO) and the International Agency for Research on Cancer. The national cohorts are supported by the Danish Cancer Society (Denmark); Ligue Contre le Cancer, Institut Gustave Roussy, Mutuelle Générale de l’Education Nationale, and Institut National de la Santé et de la Recherche Médicale (France); German Cancer Aid, German Cancer Research Center, and Federal Ministry of Education and Research (Germany); Hellenic Health Foundation (Greece); Associazione Italiana per la Ricerca sul Cancro-Italy and National Research Council (Italy); Dutch Ministry of Public Health, Welfare and Sports, Netherlands Cancer Registry, LK Research Funds, Dutch Prevention Funds, Dutch Zorg Onderzoek Nederland, World Cancer Research Fund, and Statistics Netherlands (the Netherlands); Nordic Centre of Excellence Programme on Food, Nutrition and Health (Norway); Health Research Fund, PI13/00061 to Granada, regional governments of Spain (Andalucía, Asturias, Basque Country, Murcia [No. 6236], and Navarra), and Instituto San Carlos III Las Redes Temáticas de Investigación Cooperativa en Salud (RD06/0020; Spain); Swedish Cancer Society, Swedish Scientific Council, and County Councils of Skåne and Västerbotten (Sweden); and Cancer Research UK (14136 to EPIC-Norfolk, C570/A16491 to EPIC-Oxford) and Medical Research Council (1000143 to EPIC-Norfolk; United Kingdom).

Authors’ disclosures of potential conflicts of interest are found in the article online at www.jco.org. Author contributions are found at the end of this article.

Conception and design: Nicolas Wentzensen, Elizabeth M. Poole, V. Wendy Setiawan, Kala Visvanathan, Patricia Hartge, Antonia Trichopoulou, Shelley S. Tworoger

Financial support: Shelley S. Tworoger

Administrative support: Louise K. Sjöholm, Shelley S. Tworoger

Provision of study materials: Nicolas Wentzensen, Leslie Bernstein, Eva Lundin, Kim Robien, Leo J. Schouten, Louise K. Sjöholm, Shelley S. Tworoger

Collection and assembly of data: Nicolas Wentzensen, Elizabeth M. Poole, Britton Trabert, Emily White, Alan A. Arslan, Alpa V. Patel, V. Wendy Setiawan, Kala Visvanathan, Elisabete Weiderpass, Hans-Olov Adami, Saioa Chamosa, Tess V. Clendenen, Annika Idahl, Victoria Kirsh, Woon-Puay Koh, Eva Lundin, Catherine Schairer, Leo J. Schouten, Louise K. Sjöholm, Anthony Swerdlow, Ruth Travis, Shelley S. Tworoger

Data analysis and interpretation: Nicolas Wentzensen, Elizabeth M. Poole; Britton Trabert, Emily White, Alpa V. Patel, V. Wendy Setiawan, Kala Visvanathan, Elisabete Weiderpass, Hans-Olov Adami, Amanda Black, Leslie Bernstein, Louise A. Brinton, Julie Buring, Lesley M. Butler, Laure Dossus, Renee Fortner, Susan M. Gapstur, Mia M. Gaudet, Inger T. Gram, Patricia Hartge, Judith Hoffman-Bolton, Annika Idahl, Michael Jones, Rudolf Kaaks, James V. Lacey Jr, I-Min Lee, Eva Lundin, Melissa A. Merritt, N. Charlotte Onland-Moret, Ulrike Peters, Jenny N. Poynter, Sabina Rinaldi, Kim Robien, Thomas Rohan, Dale P. Sandler, Leo J. Schouten, Sabina Sieri, Anna Tjonneland, Piet A. van den Brandt, Lynne Wilkens, Alicja Wolk, Hannah P. Yang, Anne Zeleniuch-Jacquotte, Shelley S. Tworoger

Manuscript writing: All authors

Final approval of manuscript: All authors

Ovarian Cancer Risk Factors by Histologic Subtype: An Analysis From the Ovarian Cancer Cohort Consortium

The following represents disclosure information provided by authors of this manuscript. All relationships are considered compensated. Relationships are self-held unless noted. I = Immediate Family Member, Inst = My Institution. Relationships may not relate to the subject matter of this manuscript. For more information about ASCO's conflict of interest policy, please refer to www.asco.org/rwc or jco.ascopubs.org/site/ifc.

Nicolas Wentzensen

No relationship to disclose

Elizabeth M. Poole

No relationship to disclose

Britton Trabert

No relationship to disclose

Emily White

No relationship to disclose

Alan A. Arslan

No relationship to disclose

Alpa V. Patel

No relationship to disclose

V. Wendy Setiawan

No relationship to disclose

Kala Visvanathan

Patents, Royalties, Other Intellectual Property: Licensing of patent with Cepheid (Inst)

Elisabete Weiderpass

No relationship to disclose

Hans-Olov Adami

No relationship to disclose

Amanda Black

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Leslie Bernstein

No relationship to disclose

Louise A. Brinton

No relationship to disclose

Julie Buring

No relationship to disclose

Lesley M. Butler

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Saioa Chamosa

No relationship to disclose

Tess V. Clendenen

No relationship to disclose

Laure Dossus

No relationship to disclose

Renee Fortner

No relationship to disclose

Susan M. Gapstur

No relationship to disclose

Mia M. Gaudet

No relationship to disclose

Inger T. Gram

No relationship to disclose

Patricia Hartge

No relationship to disclose

Judith Hoffman-Bolton

No relationship to disclose

Annika Idahl

No relationship to disclose

Michael Jones

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Rudolf Kaaks

No relationship to disclose

Victoria Kirsh

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Woon-Puay Koh

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James V. Lacey Jr

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I-Min Lee

No relationship to disclose

Eva Lundin

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Melissa A. Merritt

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N. Charlotte Onland-Moret

No relationship to disclose

Ulrike Peters

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Jenny N. Poynter

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Sabina Rinaldi

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Kim Robien

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Thomas Rohan

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Dale P. Sandler

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Catherine Schairer

No relationship to disclose

Leo J. Schouten

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Louise K. Sjöholm

No relationship to disclose

Sabina Sieri

No relationship to disclose

Anthony Swerdlow

Stock or Other Ownership: GlaxoSmithKline (I)

Anna Tjonneland

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Ruth Travis

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Antonia Trichopoulou

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Piet A. van den Brandt

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Lynne Wilkens

No relationship to disclose

Alicja Wolk

No relationship to disclose

Hannah P. Yang

No relationship to disclose

Anne Zeleniuch-Jacquotte

No relationship to disclose

Shelley S. Tworoger

No relationship to disclose

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Table

Table A1. Studies in the Ovarian Cancer Cohort Consortium That Contributed to Each Exposure Analysis

Table A1. Studies in the Ovarian Cancer Cohort Consortium That Contributed to Each Exposure Analysis

VariableStudies
Ever/never parousAARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
No. of children (continuous or categorical)AARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SMC, SS, VITAL, WHS, WLHS
Ever/never OC useAARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
Duration of OC use (continuous or categorical)AARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
Duration of breastfeeding (continuous)BGS, CTS, EPIC, NHS, NHSII, SS, WLHS
Age at menarche (continuous or categorical)AARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
Age at menopause (continuous and categorical)AARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS
Ever use of HTAARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
Duration of HT use (continuous and categorical)AARP, BCDDP, BGS, CPSII-NC, CSDLH, EPIC, IWHS, MEC, NHS, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS
Tubal ligationCPSII-NC, CTS, EPIC, MEC, NHS, NHSII, NLCS, NYU, PLCO, SMC, SS, VITAL, WHS
HysterectomyAARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS
EndometriosisBGS, CTS, IWHS, NHSII, PLCO, SS
Family history of breast cancerAARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, VITAL, WHS
Family history of ovarian cancerAARP, BCDDP, BGS, CLUEII, CPSII-NC, CTS, IWHS, MEC, NHS, NHSII, NLCS, PLCO, SCHS, SS, VITAL, WHS
BMI (continuous and categorical)AARP, BCDDP, BGS, CLUE, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
Height (continuous and categorical)AARP, BCDDP, BGS, CLUE, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
Ever/never smokerAARP, BCDDP, BGS, CLUEII, CPSII-NC, CSDLH, CTS, EPIC, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS, WLHS
Pack-years of smoking (continuous and categorical)BCDDP, BGS, CPSII-NC, CSDLH, IWHS, MEC, NHS, NHSII, NLCS, NYU, PLCO, SCHS, SMC, SS, VITAL, WHS

Abbreviations: AARP, National Institutes of Health-AARP Diet and Health Study; BCDDP, Breast Cancer Detection Demonstration Project Follow-Up Study; BGS, Breakthrough Generations Study; BMI, body mass index; CSDLH, Canadian Study of Diet, Lifestyle, and Health; CLUEII, Campaign Against Cancer and Stroke; CPSII-NC, Cancer Prevention Study II Nutrition Cohort; CTS, California Teachers Study; EPIC, European Prospective Investigation Into Cancer and Nutrition Study; HT, hormone therapy; IWHS, Iowa Women’s Health Study; MEC, Multiethnic/Minority Cohort Study; NHS, Nurses’ Health Study; NHSII, Nurses’ Health Study II; NYU, New York University Women’s Health Study; NLCS, Netherlands Cohort Study on Diet and Cancer; OC, oral contraceptive; PLCO, Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial; SCHS, Singapore Chinese Health Study; SS, Sister Study; SMC, Swedish Mammography Cohort Study; VITAL, Vitamins and Lifestyle Cohort; WLHS, Women’s Lifestyle and Health Study; WHS, Women’s Health Study.

Table

Table A2. Number of Invasive Epithelial Ovarian Cancer Cases Overall and by Histologic Subtype for each Exposure

Table A2. Number of Invasive Epithelial Ovarian Cancer Cases Overall and by Histologic Subtype for each Exposure

ExposureNo. of Cases for Each Exposure
SerousEndometrioidMucinousClear CellAll Invasive
Parity
 Ever/never3,3005983182545,429
 No. of children (continuous or categorical)3,2685873032415,351
Oral contraceptive use
 Ever/never3,3476043262655,523
 Duration of use (continuous or categorical)3,2875873182635,418
Duration of breastfeeding83115770631,281
Age at menarche (continuous or categorical)3,3316023272665,489
Age at menopause (postmenopausal only; continuous or categorical)2,1623452071323,494
HT use (postmenopausal only)
 Ever/never2,6824112381574,319
 Duration of use (continuous or categorical)2,3943472161383,802
Tubal ligation2,3874352131933,914
Hysterectomy3,1465503012305,486
Endometriosis90016973861,503
First-degree family history of breast cancer3,2915893162625,383
First-degree family history of ovarian cancer2,6344592382054,332
Body mass index (continuous or categorical)3,2345783192625,354
Height (continuous or categorical)3,2775923222675,433
Smoking
 Ever/never3,3356053282685,514
 Pack-years (continuous or categorical)2,2574162231914,690

Abbreviation: HT, hormone therapy.

Table

Table A3. Associations of Risk Factors for Low- and High-Grade Serous Ovarian Carcinomas in the Ovarian Cancer Cohort Consortium

Table A3. Associations of Risk Factors for Low- and High-Grade Serous Ovarian Carcinomas in the Ovarian Cancer Cohort Consortium

ExposureLow-Grade Serous*High-Grade SerousP-het
Parity
 Ever/never0.78 (0.47 to 1.28)0.81 (0.72 to 0.92).87
 No. of children, per one child0.90 (0.80 to 1.00)0.93 (0.90 to 0.95).58
 No. of children
  01.00 (reference)1.00 (reference)
  10.83 (0.41 to 1.65)0.85 (0.72 to 1.01)
  20.87 (0.51 to 1.50)0.87 (0.76 to 1.00).66
  30.87 (0.51 to 1.49)0.81 (0.71 to 0.93)
  ≥ 40.45 (0.23 to 0.89)0.67 (0.58 to 0.78)
Oral contraceptive use
 Ever/never1.12 (0.72 to 1.72)0.84 (0.76 to 0.93).19
 Duration of use, per 5-year increase0.79 (0.62 to 1.00)0.88 (0.83 to 0.94).40
 Duration of use, years
  Never1.00 (reference)1.00 (reference)
  ≤ 11.80 (0.98 to 3.31)0.99 (0.84 to 1.16)
  > 1 to 51.13 (0.65 to 1.94)0.88 (0.77 to 1.01).36
  > 5 to 100.94 (0.48 to 1.83)0.78 (0.67 to 0.92)
  > 100.56 (0.22 to 1.42)0.68 (0.56 to 0.83)
Duration of breastfeeding, per 1-year increase1.07 (0.69 to 1.66)0.95 (0.85 to 1.06).55
Age at menarche
 Per 1-year increase1.01 (0.91 to 1.11)1.01 (0.98 to 1.03).98
 Age, years
  ≤ 111.00 (reference)1.00 (reference)
  121.26 (0.70 to 2.28)1.01 (0.88 to 1.15)
  131.38 (0.83 to 2.28)1.06 (0.94 to 1.19).86
  141.21 (0.62 to 2.34)1.08 (0.93 to 1.26)
  ≥ 151.00 (0.49 to 2.05)0.96 (0.82 to 1.13)
Age at menopause
 Per 5-year increase1.54 (1.23 to 1.92)1.03 (0.98 to 1.09).006
 Age, years
  ≤ 450.20 (0.07 to 0.56)0.91 (0.78 to 1.07)
  > 45 to 500.49 (0.29 to 0.84)1.01 (0.90 to 1.14).001
  > 50 to 551.00 (reference)1.00 (reference)
  > 550.41 (0.13 to 1.33)1.01 (0.81 to 1.25)
Hormone therapy use§
 Ever/never1.87 (1.17 to 2.97)1.48 (1.34 to 1.65).36
 Duration of use, per 5-year increase1.34 (1.18 to 1.53)1.22 (1.18 to 1.27).26
 Duration of use, years
  Never1.00 (reference)1.00 (reference)
  ≤ 51.27 (0.68 to 2.37)1.27 (1.11 to 1.46).43
  > 52.67 (1.57 to 4.55)1.86 (1.64 to 2.11)
Tubal ligation, ever/never1.30 (0.69 to 2.46)0.98 (0.83 to 1.15).42
Hysterectomy, ever/never0.87 (0.53 to 1.43)1.02 (0.91 to 1.14).55
Endometriosis, yes/no3.74 (1.23 to 11.38)1.19 (0.80 to 1.76).08
First-degree family history of breast cancer, yes/no1.23 (0.71 to 2.14)1.13 (1.00 to 1.29).78
First-degree family history of ovarian cancer, yes/no0.90 (0.22 to 3.71)1.60 (1.26 to 2.03).38
Body mass index
 Per 5 kg/m20.92 (0.74 to 1.14)0.94 (0.89 to 0.98).85
 Categorical, kg/m2
  < 201.33 (0.67 to 2.62)1.07 (0.90 to 1.27)
  20 to < 251.00 (reference)1.00 (reference)
  25 to < 301.02 (0.65 to 1.59)0.89 (0.81 to 0.99).93
  30 to < 350.86 (0.44 to 1.67)0.88 (0.76 to 1.02)
  ≥ 351.16 (0.52 to 2.60)0.89 (0.73 to 1.09)
Height
 Per 0.5 m1.05 (0.93 to 1.18)1.06 (1.03 to 1.10).81
 Categorical, m
  < 1.600.83 (0.49 to 1.39)0.85 (0.75 to 0.96)
  1.60 to < 1.651.00 (reference)1.00 (reference).81
  1.65 to < 1.701.21 (0.75 to 1.95)1.03 (0.92 to 1.16)
  ≥ 1.700.96 (0.55 to 1.69)1.07 (0.94 to 1.21)
Smoking
 Ever/never1.11 (0.76 to 1.63)0.95 (0.87 to 1.04).42
 Continuous, per 20 pack-years0.86 (0.59 to 1.26)0.98 (0.93 to 1.04).45
 Categorical, pack-years
  Never1.00 (reference)1.00 (reference)
  ≤ 201.20 (0.69 to 2.07)1.06 (0.94 to 1.20).49
  > 200.72 (0.34 to 1.51)1.01 (0.88 to 1.16)

NOTE. Associations stratified by birth year and cohort and adjusted for age at study entry, parity, and duration of oral contraceptive use (except when parity or oral contraceptive use was the primary exposure of interest, and then we adjusted only for the other risk factor) by using pooled analyses of all cohorts combined. Five cohorts with no information on grade for any ovarian cancer cases were excluded.

Abbreviation: P-het, P value for heterogeneity.

*Number of cases ranged from 28 (breastfeeding) to 121 (oral contraceptive use) for low-grade serous and 460 (breastfeeding) to 2,133 (oral contraceptive use) for high-grade serous carcinomas; serous cases with unknown grade were excluded.

†Assessed by a likelihood ratio test that compared a Cox proportional hazards competing-risks model to allow for the association to vary by grade to a model that forced the association to be the same across grades.

‡Parous women only.

§Postmenopausal women only.

‖Also adjusted for duration of hormone therapy use.

Table

Table A4. Associations of Risk Factors With Ovarian Cancer Subtypes Based on Meta-analysis by Pooling the Results of Individual Studies in the Ovarian Cancer Cohort Consortium

Table A4. Associations of Risk Factors With Ovarian Cancer Subtypes Based on Meta-analysis by Pooling the Results of Individual Studies in the Ovarian Cancer Cohort Consortium

ExposureSerousEndometrioidMucinousClear cell
Parity
 Ever/never0.80 (0.73 to 0.89)0.44 (0.36 to 0.55)0.45 (0.31 to 0.64)0.32 (0.24 to 0.43)
 No. of children, per one child0.94 (0.92 to 0.96)0.78 (0.72 to 0.84)0.84 (0.75 to 0.95)*0.65 (0.57 to 0.73)
 No. of children
  01.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  10.87 (0.74 to 1.02)0.79 (0.58 to 1.07)0.83 (0.48 to 1.45)0.57 (0.36 to 0.91)
  20.87 (0.77 to 0.97)0.47 (0.37 to 0.59)0.52 (0.33 to 0.82)0.41 (0.27 to 0.63)
  30.80 (0.71 to 0.90)0.41 (0.32 to 0.54)0.53 (0.34 to 0.80)0.32 (0.19 to 0.52)
  ≥ 40.72 (0.63 to 0.83)0.33 (0.24 to 0.46)0.60 (0.39 to 0.91)0.31 (0.14 to 0.67)
Oral contraceptive use
 Ever/never0.82 (0.75 to 0.89)0.88 (0.73 to 1.05)1.04 (0.81 to 1.34)0.74 (0.54 to 1.01)
 Duration of use, per 5-year increase0.84 (0.78 to 0.90)0.89 (0.77 to 1.02)1.19 (0.99 to 1.43)0.96 (0.82 to 1.12)
 Duration of use, years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  ≤ 11.01 (0.88 to 1.16)1.15 (0.86 to 1.55)1.22 (0.77 to 1.91)1.24 (0.74 to 2.06)
  > 1 to 50.88 (0.78 to 0.99)0.95 (0.74 to 1.23)1.15 (0.77 to 1.71)1.25 (0.78 to 2.01)
  > 5 to 100.76 (0.65 to 0.89)0.90 (0.67 to 1.21)1.28 (0.84 to 1.95)1.06 (0.67 to 1.68)
  > 100.67 (0.57 to 0.79)0.75 (0.97 to 1.16)1.67 (1.06 to 2.64)0.73 (0.36 to 1.45)
Duration of breastfeeding, per 1 year1.01 (0.87 to 1.18)*0.93 (0.78 to 1.11)0.94 (0.68 to 1.31)1.13 (0.93 to 1.36)
Age at menarche
 Per 1-year increase0.99 (0.96 to 1.02)1.00 (0.95 to 1.05)1.00 (0.94 to 1.07)0.94 (0.87 to 1.02)
 Age, years
  ≤ 111.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  120.90 (0.75 to 1.08)0.97 (0.74 to 1.27)1.13 (0.75 to 1.70)0.81 (0.54 to 1.22)
  130.99 (0.88 to 1.10)1.00 (0.75 to 1.33)1.05 (0.74 to 1.49)0.84 (0.47 to 1.49)
  140.97 (0.85 to 1.12)0.88 (0.63 to 1.23)1.05 (0.65 to 1.68)0.77 (0.46 to 1.27)
  ≥ 150.91 (0.79 to 1.05)1.02 (0.73 to 1.42)1.37 (0.87 to 2.17)0.80 (0.46 to 1.40)
Age at menopause
 Per 5-year increase1.05 (1.00 to 1.10)1.44 (1.08 to 1.93)*1.04 (0.80 to 1.37)*1.96 (1.37 to 2.81)*
 Age, years
  ≤ 401.02 (0.82 to 1.27)0.79 (0.45 to 1.40)2.02 (0.67 to 6.04)0.64 (0.14 to 2.89)
  > 40 to 450.88 (0.75 to 1.04)1.03 (0.64 to 1.66)1.10 (0.54 to 2.25)0.95 (0.37 to 2.48)
  > 45 to 500.96 (0.86 to 1.06)0.86 (0.65 to 1.13)0.96 (0.68 to 1.35)1.06 (0.69 to 1.63)
  > 50 to 551.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  > 551.05 (0.88 to 1.25)1.35 (0.88 to 2.08)1.66 (0.83 to 3.34)1.93 (0.88 to 4.23)
Hormone therapy use
 Ever/never1.40 (1.27 to 1.55)1.81 (1.41 to 2.32)1.04 (0.77 to 1.41)0.90 (0.57 to 1.42)
 Duration of use, per 5-year increase1.22 (1.15 to 1.29)1.33 (1.17 to 1.51)1.08 (0.86 to 1.36)0.69 (0.49 to 0.98)*
 Duration of use, years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  ≤ 51.24 (1.11 to 1.38)1.71 (1.20 to 2.43)1.27 (0.87 to 1.85)1.06 (0.63 to 1.75)
  > 51.75 (1.55 to 1.98)2.32 (1.59 to 3.38)1.43 (0.89 to 2.30)0.83 (0.55 to 1.25)
Tubal ligation, ever/never0.97 (0.81 to 1.16)0.79 (0.53 to 1.18)1.43 (0.80 to 2.56)0.63 (0.27 to 1.46)
Hysterectomy, ever/never§0.99 (0.88 to 1.12)0.90 (0.70 to 1.16)0.82 (0.57 to 1.16)0.89 (0.54 to 1.46)
Endometriosis, yes/no1.14 (0.81 to 1.61)2.84 (1.56 to 5.18)5.06 (1.51 to 16.9)3.43 (1.52 to 7.75)
First-degree family history of breast cancer, yes/no1.19 (1.02 to 1.39)1.56 (1.22 to 1.99)1.04 (0.67 to 1.61)1.29 (0.78 to 2.13)
First-degree family history of ovarian cancer, yes/no1.16 (0.43 to 3.18)*0.29 (0.01 to 5.89)*0.01 (0.00 to 1.13)*0.02 (0.00 to 1.68)*
Body mass index
 Per 5 kg/m20.97 (0.93 to 1.01)1.03 (0.92 to 1.15)*1.08 (0.97 to 1.20)0.95 (0.80 to 1.14)*
 Categorical, kg/m2
  < 201.08 (0.94 to 1.24)1.18 (0.83 to 1.67)1.97 (1.28 to 3.02)1.50 (0.92 to 2.44)
  20 to < 251.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  25 to < 300.93 (0.84 to 1.03)1.00 (0.82 to 1.23)1.44 (1.11 to 1.87)1.37 (1.01 to 1.84)
  30 to < 350.94 (0.83 to 1.06)1.28 (0.97 to 1.70)1.86 (1.22 to 2.86)1.77 (1.04 to 3.00)
  ≥ 351.07 (0.84 to 1.35)1.73 (1.20 to 2.50)2.18 (1.09 to 4.36)2.26 (1.19 to 4.29)
Height
 Per 0.5m1.06 (1.03 to 1.10)1.06 (0.99 to 1.13)1.08 (0.96 to 1.19)*1.08 (0.98 to 1.17)
 Categorical, m
  < 1.600.87 (0.79 to 0.96)1.05 (0.83 to 1.32)0.98 (0.71 to 1.34)1.02 (0.71 to 1.46)
  1.60 to < 1.651.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  1.65 to < 1.701.05 (0.94 to 1.19)1.00 (0.79 to 1.27)1.02 (0.73 to 1.41)1.02 (0.67 to 1.58)
  ≥ 1.701.06 (0.96 to 1.17)1.28 (1.01 to 1.63)1.23 (0.88 to 1.71)1.23 (0.85 to 1.78)
Smoking
 Ever/never1.02 (0.92 to 1.12)0.95 (0.80 to 1.12)1.25 (0.99 to 1.57)0.92 (0.70 to 1.21)
 Continuous, per 20 pack-years1.03 (0.97 to 1.10)0.98 (0.84 to 1.15)1.21 (1.04 to 1.40)0.79 (0.59 to 1.05)
 Categorical, pack-years
  Never1.00 (reference)1.00 (reference)1.00 (reference)1.00 (reference)
  ≤ 101.12 (0.99 to 1.27)1.21 (0.91 to 1.59)1.29 (0.86 to 1.93)1.04 (0.67 to 1.63)
  > 10 to 201.09 (0.92 to 1.28)0.91 (0.61 to 1.37)1.62 (0.96 to 2.72)1.25 (0.66 to 2.37)
  > 20 to 351.08 (0.87 to 1.32)1.12 (0.77 to 1.63)1.53 (0.89 to 2.61)0.94 (0.42 to 2.11)
  > 351.13 (0.94 to 1.35)1.20 (0.78 to 1.85)2.13 (1.27 to 3.55)0.98 (0.40 to 2.40)

NOTE. Associations stratified by birth year and adjusted for age at study entry, parity, and duration of oral contraceptive use (except when parity or oral contraceptive use was the primary exposure of interest, and then we adjusted only for the other risk factor).

*Meta-analysis P value for heterogeneity across studies < .01 by using the q statistic from a random-effects meta-analysis.

†Parous women only.

‡Postmenopausal women only.

§Also adjusted for duration of hormone therapy use.

Acknowledgment

The UK Breakthrough Generations Study thanks the study participants; study staff; physicians, nurses, and other health care staff; and data providers who contributed to the study. We thank the participants and staff of the Nurses’ Health Study and Nurses’ Health Study II for their valuable contributions as well as the following state cancer registries for their help: Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Virginia, Washington, and Wyoming.

COMPANION ARTICLES

No companion articles

ARTICLE CITATION

DOI: 10.1200/JCO.2016.66.8178 Journal of Clinical Oncology 34, no. 24 (August 20, 2016) 2888-2898.

Published online June 20, 2016.

PMID: 27325851

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