Incidence rates for breast carcinoma in situ (CIS) have increased markedly over the past 20 years. Breast CIS, detected primarily on mammography, now represents 30-45% of all screened detected breast cancers. We conducted a large population-based case-control study to evaluate the impact of reproductive factors and lactation on breast CIS risk. Case subjects were newly diagnosed with breast CIS at ages 35-64 years between March 1, 1995 and May 31, 1998 (n = 567), resided in Los Angeles County and were born in the United States. Control subjects (n = 614), identified through random digit dialing, fulfilled the same eligibility criteria and were required to have had at least one screening mammogram in the 2-year period before their interview. Women with a positive family history of breast cancer had a 2-fold increase in breast CIS risk. Parous women were at reduced risk relative to nulligravid women (odds ratio [OR] = 0.67, 95% confidence interval [CI] = 0.46-1.00). Among nulliparous women, pregnancy was unrelated to breast CIS risk. Among parous women, risk declined with each additional term pregnancy (p-trend = 0.003). No associations were found with age at first term pregnancy, induced abortion or miscarriage. Long duration of breast-feeding (> or =24 months) was associated with increased risk (OR = 2.00, 95% CI = 1.11-3.60). The observed effects of family history and pregnancy on breast CIS risk are consistent with those observed for invasive breast cancer. The results for breast-feeding are contrary to what has been observed in studies of invasive breast cancer.
Copyright 2004 Wiley-Liss, Inc.