Reproductive and menopausal factors and risk of second primary breast cancer after in situ breast carcinoma.

PURPOSE: In situ breast cancer patients have a higher risk of developing a second primary breast cancer than women in the general population have of developing breast cancer. We have limited understanding of why some women with a previous in situ breast cancer develop second primary breast cancers while others do not.

METHODS: In this population-based nested case-control study, we evaluated the association between reproductive and menopausal factors and risk of developing a second primary breast cancer among women with a previous in situ breast cancer. Using conditional logistic regression, these associations were evaluated in 552 cases and 1032 individually matched controls.

RESULTS: Older age at menarche was associated with risk of second primary breast cancer among women with a previous in situ breast cancer (compared to age < 12, age 13: OR 0.60 (0.42, 0.85); age ≥ 14: OR 0.69 (0.47, 1.00); Ptrend  = 0.07). Breastfeeding for > 12 months was associated with a decreased risk of developing a second primary breast cancer (OR 0.62 (0.39, 0.98)). No associations were observed for other reproductive or menopausal factors evaluated.

CONCLUSIONS: Results from this study suggest that reproductive factors may play a role in development of a second primary breast cancer after diagnosis of in situ breast carcinoma.