Background: While most studies have found no association between oral contraceptive use and breast cancer, several studies of younger women have reported an association with long-term oral contraceptive use. PURPOSE. We studied the relationship of patterns of oral contraceptive use to breast cancer risk among younger women. These women have had oral contraceptives available their entire reproductive lives and are now entering the breast cancer-prone years.
Methods: A population-based, case-control study of breast cancer was conducted in three counties in western Washington State among women born in 1945 or later, ages 21-45. Case patients were 747 women with breast cancer diagnosed in 1983-1990 and identified through the Seattle-Puget Sound Surveillance, Epidemiology, and End Results cancer registry. Control subjects were 961 women identified by random-digit telephone dialing. Subjects were interviewed in person, using pictures of brands of oral contraceptives and calendars of life events as recall aids.
Results: There was no increased incidence of breast cancer associated with ever having used oral contraceptives. Because only 8% of this cohort had never used oral contraceptives, short-term users (< 1 year) were combined with never users as the reference group for further analyses. A small increased risk of breast cancer was associated with long duration of oral contraceptive use (odds ratio for > or = 10 years = 1.3; 95% confidence interval [CI] = 0.9-1.9; P for trend = .03), particularly among women aged 35 years or younger (odds ratio for > or = 10 years = 1.7; 95% CI = 0.9-3.1). Breast cancer was also modestly related to oral contraceptive use early in reproductive life (odds ratio for use within 5 years of menarche = 1.3; 95% CI = 1.0-1.8; P for trend = .04) and to use of high-progestin-potency oral contraceptives for at least 1 year (odds ratio = 1.5; 95% CI = 1.1-2.1). These associations were adjusted for age, age at menarche, term pregnancy, induced abortion, and family history of breast cancer. The associations were not further confounded by case-control differences in education, religion, breast feeding of offspring, or infertility; in oral contraceptive contraindications, indications, or complications; or in measures of breast cancer detection such as mammography or breast biopsy.
Conclusions: Long-term oral contraceptive use among young women or use beginning near menarche may be associated with a small excess breast cancer risk, possibly due to susceptibility to genetic damage in breast epithelial cells at ages of high breast cell proliferative activity.
Implications: Future studies should investigate whether the patterns of risk we reported are present as this cohort ages.
PIP: A case control study was conducted in Washington among 21-45 year old white women from King, Pierce, and Snohomish counties (i.e., Seattle metropolitan area) to examine the relationship between oral contraceptive (OC) use and breast cancer. The 747 cases were diagnosed with invasive breast cancer between January 1983 and April 1990. The researchers combined short term OC users with never users since just 8% of all subjects had never used OCs. They controlled for age, age at menarche, term pregnancy, induced abortion, and family history of breast cancer. Longterm use (i.e., =or 10 years) of OCs was associated with a small increased risk of breast cancer (odds ratio [OR] = 1.3; p for trend = 0.03), especially among women not older than 35 years (OR = 1.7). This finding was consistent with results of other studies. OC use early in reproductive life (i.e., within 5 years of menarche) was also associated with a moderate increase in breast cancer (OR = 1.3; p for trend = 0.04). Breast cancer risk was also elevated among women who used high progestin potency OCs (as defined by the Dickey method for classifying OC potency) for at least 1 year (OR = 1.5). Case control differences in education, religion, breast feeding of children, or infertility; in OC contraindications, indications, complications; or in measures of breast cancer detection (e.g., mammography or breast biopsy) did not confound the associations. An association between breast cancer and long term OC use among young women and OC use beginning close to menarche suggest that puberty, a time when breast epithelial cells are undergoing considerable proliferative activity, are susceptible to genetic damage. Further research is needed to determine whether the aforementioned patterns of breast cancer risk continues as the cohort becomes older.