title: Reproductive factors and subtypes of breast cancer defined by hormone receptor and histology.
abstract: Reproductive factors are associated with reduced risk of breast cancer, but less is known about whether there is differential protection against subtypes of breast cancer. Assuming reproductive factors act through hormonal mechanisms they should protect predominantly against cancers expressing oestrogen (ER) and progesterone (PR) receptors. We examined the effect of reproductive factors on subgroups of tumours defined by hormone receptor status as well as histology using data from the NIHCD Women's Contraceptive and Reproductive Experiences (CARE) Study, a multicenter case-control study of breast cancer. We estimated odds ratios (ORs) and 95% confidence intervals (CIs) as measures of relative risk using multivariate unconditional logistic regression methods. Multiparity and early age at first birth were associated with reduced relative risk of ER + PR + tumours (P for trend=0.0001 and 0.01, respectively), but not of ER - PR - tumours (P for trend=0.27 and 0.85), whereas duration of breastfeeding was associated with lower relative risk of both receptor-positive (P for trend=0.0002) and receptor-negative tumours (P=0.0004). Our results were consistent across subgroups of women based on age and ethnicity. We found few significant differences by histologic subtype, although the strongest protective effect of multiparity was seen for mixed ductolobular tumours. Our results indicate that parity and age at first birth are associated with reduced risk of receptor-positive tumours only, while lactation is associated with reduced risk of both receptor-positive and -negative tumours. This suggests that parity and lactation act through different mechanisms. This study also suggests that reproductive factors have similar protective effects on breast tumours of lobular and ductal origin.
authors.affiliation: Department of Preventive Medicine, Keck School of Medicine, University of Southern California/Norris Comprehensive Cancer Center, Los Angeles, CA, USA. gursin@usc.edu
authors.forename: G
authors.initials: G
authors.lastname: Ursin
authors.forename: L
authors.initials: L
authors.lastname: Bernstein
authors.forename: S J
authors.initials: SJ
authors.lastname: Lord
authors.forename: R
authors.initials: R
authors.lastname: Karim
authors.forename: D
authors.initials: D
authors.lastname: Deapen
authors.forename: M F
authors.initials: MF
authors.lastname: Press
authors.forename: J R
authors.initials: JR
authors.lastname: Daling
authors.forename: S A
authors.initials: SA
authors.lastname: Norman
authors.forename: J M
authors.initials: JM
authors.lastname: Liff
authors.forename: P A
authors.initials: PA
authors.lastname: Marchbanks
authors.forename: S G
authors.initials: SG
authors.lastname: Folger
authors.forename: M S
authors.initials: MS
authors.lastname: Simon
authors.forename: B L
authors.initials: BL
authors.lastname: Strom
authors.forename: R T
authors.initials: RT
authors.lastname: Burkman
authors.forename: L K
authors.initials: LK
authors.lastname: Weiss
authors.forename: R
authors.initials: R
authors.lastname: Spirtas
mesh_terms: D000328:Adult; D000367:Age Factors; D001942:Breast Feeding; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000378:metabolism; D016022:Case-Control Studies; D005260:Female; D020073:Gravidity; D006801:Humans; D008875:Middle Aged; D010298:Parity; D011960:Receptors, Estrogen; D011980:Receptors, Progesterone; D012307:Risk Factors; D013997:Time Factors
chemical_list: D011960:Receptors, Estrogen; D011980:Receptors, Progesterone