| It has been hypothesized that subtypes of breast cancer defined by estrogen and progesterone receptor (ER/PR) status may have different etiologies. However, the findings on this topic have been inconsistent, especially in younger women.;This dissertation presents the first meta-analysis of results from epidemiological studies that have investigated associations of parity, age at first birth, breastfeeding and age at menarche with risk of ER+PR+ and ER-PR- breast cancer. Each birth reduced the risk of ER+PR+ cancer by 11% (relative risk [RR] = 0.89, 95% confidence interval [CI] = 0.84-0.94). Women who were in the highest age at first birth category had, on average, 27% higher risk of ER+PR+ cancer than women who were in the youngest age at first birth category (RR = 1.27, 95% CI = 1.07-1.50). Neither parity nor age at first birth was associated with the risk of ER-PR- cancer. Breastfeeding and late age at menarche were associated with decreased risk of both receptor subtypes of breast cancer.;The relationship between hormone-related factors and breast cancer risk overall and by ER/PR status was examined in a case-control study of women under age 50 years. The findings for parity, age at first birth, breastfeeding and age at menarche are consistent with the results from the meta-analysis. In addition, recent average alcohol consumption was associated with an increased risk of ER+PR+ cancer (Ptrend = 0.03), but not with risk of ER-PR- cancer (Ptrend = 0.42).;The relationship between low-dose medical radiation and breast cancer risk overall and by ER/PR status was examined in the same case-control study. An elevated breast cancer risk was observed among women who reported having had multiple chest X-rays (Ptrend = 0.0007), 7 or more mammograms (OR = 1.80, 95% CI = 0.95-3.42), or dental X-rays without lead apron protection before age 20 years (OR = 1.81, 95% CI = 1.13-2.90). Age at first exposure, parity, and breast or ovarian cancer family history appeared to modify the radiation effect on breast cancer risk, but we found no effect modification by ER/PR status. |
