One-fifth of all newly diagnosed
breast cancer cases are
ductal carcinoma in situ (
DCIS), but little is known about
DCIS risk factors. Recent studies suggest that some subtypes of
DCIS (high grade or comedo) share histopathologic and epidemiologic characteristics with invasive disease, whereas others (medium or low grade or non-comedo) show different patterns. To investigate whether reproductive and hormonal risk factors differ among comedo and non-comedo types of
DCIS and invasive
breast cancer (IBC), we used a population-based case-control study of 1,808 invasive and 446
DCIS breast cancer cases and their age and race frequency-matched controls (1,564 invasive and 458
DCIS). Three or more full-term pregnancies showed a strong inverse association with comedo-type
DCIS [odds ratio (OR), 0.53; 95% confidence interval (95% CI), 0.30-0.95] and a weaker inverse association for non-comedo
DCIS (OR, 0.73; 95% CI, 0.42-1.27). Several risk factors (age at first full-term pregnancy, breast-feeding, and age at menopause) showed similar associations for comedo-type
DCIS and IBC but different associations for non-comedo
DCIS. Ten or more years of
oral contraceptive showed a positive association with comedo-type
DCIS (OR, 1.31; 95% CI, 0.70-2.47) and IBC (OR, 2.33; 95% CI, 1.06-5.09) but an inverse association for non-comedo
DCIS (OR, 0.51; 95% CI, 0.25-1.04). Our results support the theory that comedo-type
DCIS may share hormonal and reproductive risk factors with IBC, whereas the etiology of non-comedo
DCIS deserves further investigation.