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A comparison of reproductive risk factors for CIS lesions and invasive breast cancer

A differential effect of reproductive factors on the incidence of carcinoma in situ of the breast (CIS) and invasive cancer may indicate that hormonal factors related to reproductive history not only influence the initial steps towards breast cancer but also preinvasive malignant lesions. A comparison of reproductive factors was performed using a population‐based cohort of 1.5 million Danish women born between 1935 and 1978. Between 1983 and 1998, 15,590 cases of invasive breast cancer and 871 cases of CIS were identified using a database with extensive clinical information. Number of births and age at first birth were similarly associated with the risk of being diagnosed with ductal carcinoma in situ (DCIS) compared to invasive breast cancer [RR(DCIS)per birth/RR(invasive)per birth = 1.03(0.93–1.14), RR(DCIS)per 5yr /RR(invasive)per 5yr = 1.06(0.96–1.17)]. Also, the short‐term risk the first 10 years after birth was similar for DCIS and invasive cancer [RR(DCIS) /RR(invasive) = 0.90 (0.74–1.09)]. Additional analyses were performed according to characteristics of the DCIS lesion (size, malignancy grade, noncomedo or comedo type). In conclusion, our observations do not support the theory that reproductive history is associated with progression from noninvasive to invasive breast cancer. © 2003 Wiley‐Liss, Inc.